Recently in the Diseases Category

Michelle Grabowski, University of Minnesota Extension

I always recommend that gardeners thoroughly inspect every plant for disease problems before they purchase it. Examine both the upper and lower surfaces of the leaves, especially those leaves closest to the soil. Look at the stems and even the roots. Any spots, streaks, dark discolored or soft, mushy tissue is a warning sign that your new plant may come with a disease problem. Once a new pathogen is introduced into a garden, it may cause problems for years to come. In addition, nursery plants may move across the country before they make it to a local garden center. As a result infected plants can bring new plant diseases to Minnesota.

Boxwood Blight

One disease of concern is boxwood blight, a fungal disease caused by Cylindrocladium buxicola (syn C. pseudonaviculatum). Boxwood blight was first identified in the eastern United States in 2011. It has since been found in 8 eastern states as well as in Oregon. To date boxwood blight is known to infect all species of boxwood (Buxus spp.) and Pachysandra sp.

Boxwoods infected with the boxwood blight fungus first develop tan leaf spots with a dark brown border. Chalky white spores form on the lower surface of the leaf just below leaf spots, under humid conditions. Leaf spots grow large enough to blight the entire leaf. Infected leaves fall off, resulting in bare branches throughout the shrub. The fungus also infects branches anywhere from the soil line to the tip of the branch. These infections start out as dark brown to black short lines, and grow to completely encircle and girdle the stem. Roots of the boxwood shrub are not infected by the fungus and remain healthy.

Boxwood blight has not yet been identified in Minnesota. If you suspect your plant is infected with boxwood blight first use the University of Minnesota Extension's online diagnostic tool for Boxwood to determine if the problem might be a more common look alike. If Boxwood blight appears to be the most likely suspect, please contact the Minnesota Department of Agriculture by sending an email with photos of the plant to arrest.the.pest@state.mn.us or leave a voice message at 1- 888-545-6884. A MDA employee will respond in 1 to 2 days.

Downy Mildew
A second invasive disease to be on the look out for this year is downy mildew of basil. This disease is caused by the fungus-like organism Peronospora belbahrii. Downy mildew on basil was first found in the United States in Florida in 2007. It was seen throughout the eastern states in 2008 and was identified in Wisconsin in 2010. Downy mildew of basil has not been officially identified in Minnesota.

Only basil plants (Ocimum sp.) are susceptible to this species of downy mildew, although other species of downy mildew can infect other common garden plants. The commonly grown sweet basil, Ocimum basilicum, is highly susceptible and growers have reported 100% crop loss due to the pathogen. Infected plants first display yellowing on the lower leaves. As the disease progresses these leaves develop dark brown to black spots. If the lower surface of infected leaves is examined, gardeners will notice that the leaves look dirty, or are covered with a thin layer of gray to brown fungal growth. This dirty looking fungal growth includes spores and spore producing structures of the downy mildew pathogen.

The downy mildew pathogen can move into a garden on infected seed, transplants or on wind currents. Unlike other downy mildew pathogens that prefer cool weather, basil downy mildew will tolerate cool weather but thrives in warm conditions. This means that the pathogen is actively growing and spreading for most of Minnesota's growing season.

At the end of April, University of Wisconsin plant pathologist Amanda Gevens reported identification of basil transplants infected with downy mildew, available for sale at a local store in Wisconsin. This early arrival of the pathogen in the Midwest could mean that more severe disease is found in the region this year as the pathogen will have time to grow and spread.

Avoid downy mildew of basil by purchasing only healthy transplants and spacing plants to provide good air movement and quick drying of leaves after rain or dew. The basil varieties 'Spice' 'Blue Spice' and 'Blue Spice Fil' have been reported to have good resistance to downy mildew. 'Genovese', 'Martina', 'Italian Large Leaf' and 'Superbo' are all highly susceptible to the disease. 'Red Leaf' 'Red Rubin' 'Lemon' 'Lemon Mrs. Burns' 'Lime' and 'Sweet Dani Lemon Basil' were susceptible to downy mildew but had fewer symptoms than the most susceptible varieties.

If you suspect your basil plants are infected with downy mildew, please contact the Minnesota Department of Agriculture by sending an email with photos of the plant to arrest.the.pest@state.mn.us or leave a voice message at 1- 888-545-6884. A MDA employee will respond in 1 to 2 days.

Michelle Grabowski, UMN Extension

Cool wet spring weather stimulates galls of several rust fungi (Gymnosporangium spp.) to produce bright orange gelatinous spore producing structures that readily catch a gardeners eye. Several species of Gymnosporangium rust fungi are native to Minnesota and infect eastern red cedar trees, Juniperus virginiana during part of their lifecycle and trees and shrubs from the Rosaceae family during a different part of their lifecycle. Native Gymnosporangium rusts include cedar apple rust, quince rust, hawthorn rust and juniper broom rust. Photos and descriptions of these plant diseases can be seen at the UMN Extension online plant diagnostic tool What's wrong with my plant?

A new Gymnosporangium rust has recently been found in the United States but not yet in Minnesota. Japanese apple rust is caused by Gymnosporangium yamadae. This fungus does not infect Minnesota's native red cedar trees but does infect Juniperus chinensis and J. squamata. Both of these species of juniper are sold as ornamental trees or shrubs here in Minnesota. Although Japanese apple rust is unlikely to cause any serious damage to junipers, this fungus also infects apple trees. At this time, it is unknown how the apple varieties grown in Minnesota will respond to Japanese apple rust.

To identify Japanese apple rust in juniper, look for round woody galls that are 1/2 to 1 inch in diameter. When wet, these galls become covered in a 1/4 inch thick layer of orange gelatinous goo. As this spore filled goo dries, it will become apparent that they arise from short orange projections, like small shelves sticking out from the side of the gall. In contrast, the native cedar apple rust galls produce orange gelatinous tentacles that swell to 1-2 inches long when wet.

If you suspect a juniper in your area has Japanese apple rust, please contact the Minnesota Department of Agriculture by sending an email to arrest.the.pest@state.mn.us or leave a voice message at 888-545-6884. The MDA will contact you with further information in 1-2 days.

Michelle Grabowski, UMN Extension

If you grew impatiens last summer, you may have noticed that some plants were stunted, turned yellow, and soon became barren stalks with perhaps one or two small yellow leaves clinging to them. These symptoms are caused by downy mildew, a disease caused by the water mold Plasmopara obducens. Had you turned a leaf over, you would have seen a fluffy white fungal 'down' covering the lower surface of the leaf.

Downy mildew of impatiens has been observed in the United States since 2004, but became widespread and highly destructive in 2011. Cool wet weather likely contributed to the disease epidemic of 2011 since the downy mildew pathogen thrives under these conditions.

Plasmopara obducens produces tough survival spores called oospores that can overwinter in soil and plant debris. As a result if downy mildew was present in your garden last year, it is likely to show up again in 2012. Downy mildew can also move in on wind blown spores. Although it is not known how far the downy mildew pathogen can travel by wind, a close relative, downy mildew of cucurbits, has been shown to move over 600 miles in 48 hrs. Plasmopara obducens does not move on seed, but it could move into a garden on infected transplants. Inspect all new impatiens for yellowing of leaves, stunting and white fluffy growth on the lower leaf surface. Nurseries producing impatiens are well aware of the threat of downy mildew and are scouting regularly to find and control the pathogen.

Once the impatiens are in the ground for 2012, inspect the plants weekly for symptoms of downy mildew. Yellowing and stunting of the plants are typically the first symptoms observed. Downy white growth on the lower leaf surface confirms the disease. If downy mildew shows up, promptly bag and remove infected plants to reduce spread to neighboring plants. Space plants to allow air movement between plants. This will help reduce humidity and leaf moisture. If beds were infected in 2011, consider choosing a different annual plant this year. Downy mildew of impatiens only infects Impatiens walleriana, the standard impatiens. New Guinea impatiens, impatiens hawkerii are highly tolerant of the disease. No other plants are infected by this pathogen, so Begonias, Caladiums and other shade tolerant ornamentals are good choices as well.

M. Grabowski, University of Minnesota Extension

Damping off is a fungal disease caused most commonly by the fungi Rhizoctonia spp., Pythium spp., and Fusarium spp. All three of these fungi survive quite well in soil and plant debris. Since the tissue of young seedlings is soft and easy to infect, these pathogens can attack a wide variety of flowers and vegetables when they are seedlings. Damping off fungi can kill the seed before it emerges from the soil or it can attack the young stem and new leaves, resulting in tan mushy spots, pinched, rotted stems, and often complete collapse of the seedling. Once an infection has begun, the damping off fungi can move through the potting mix to infect nearby seedlings. Quite often a large section or an entire tray of seedlings is killed by damping off, resulting in few or no surviving seedlings to grow into mature plants.

Damping off is only a disease of seedlings. Once plants have mature leaves and a well developed root system, they are better able to naturally resist the damping off fungi. There is a critical period of growth where special care needs to be taken to protect sensitive seedlings. This period begins before the seeds are ever planted. One of the most import strategies of controlling damping off is preventing it in the first place. Since the damping off fungi typically survive on plant debris, soil, or in contaminated water, all pots, trays, potting mixes and other planting equipment must be sterilized prior to planting. New trays, pots, and potting mix are typically sterile. If you are reusing last year's pots and trays, they should be soaked in a 10% bleach solution for about 30 minutes. It is best to use new unopened potting mix to start seeds.

Once seeds are planted, it is important to maintain a good growing environment for the seed. Potting mix should be moist but not soggy. The damping off fungi thrive in wet cool conditions. Many types of seeds, like tomatoes, need warm soil to germinate and grow well. Warming mats designed specially for heating seedling trays from below are sold through many garden catalogues and nurseries. These may be very helpful in preventing damping off especially if seeds are being grown in cool areas like a basement or garage.

Careful attention should be paid to watering seedling trays. Seeds need moisture to germinate, but too much water will encourage damping off. All pots and trays should have drainage holes to allow excess water to drain away. Seedlings should be watered with lukewarm clean water (like tap or other drinking water). Watering with cold water will slow seedling growth and favor the damping off fungi. Hoses and watering heads should be kept off the floor where they could come into contact with infected soil or plant debris. An oscillating fan can be used to increase air flow around the seedlings, but care should be taken to avoid over drying of the sensitive new plants.

Cultural practices that result in tall, thin, weak plants like growing plants under low light conditions or over fertilizing with nitrogen will result in increased damping off problems. Provide seedlings with just enough of what they truly need.

Michelle Grabowski, UMN Extension Educator

A healthy bur oak will drop all of it's leaves in the fall. Leaves that are infected with the fungal pathogen (Tubakia sp.) that causes Bur Oak Blight (BOB) remain attached to the tree into the winter. As a result, now is a good time to examine landscape bur oaks for possible infection with BOB.

Bur Oak Blight causes leaves of bur oak trees to develop brown wedge shaped lesions in July and August. This fungal disease often starts in the lower canopy and progresses up the tree in following years. Some bur oak trees are highly susceptible to BOB. After several years of infection, the entire canopy can appear brown and scorched. These severely infected trees are weakened and often fall prey to secondary pests like two lined chestnut borer and Armillaria root rot. It is possible for bur oaks to be killed by this combination of fungal and insect attackers.

Bur Oak Blight was first identified in Minnesota in 2010. Since then BOB has been found in 20 Minnesota counties including Mille Lacs, Sherburne, Hennepin, Ramsey, Washington, Anoka, Wright, Dakota, Carver, Pennington, Beltrami, Pope, Lac Qui Parle, Ottertail, Stearns, Polk, Marshall, Mower, McLeod, and Morrison.

If you suspect your bur oak tree is infected with BOB, contact the University of Minnesota Plant Disease Clinic about how to submit a sample for diagnosis. For more information about BOB, read the USDA Forest Service Pest Alert about BOB.

Michelle Grabowski, UMN Extension Educator

New research published in Plant Health Progress provides Minnesota grape growers with more information about disease resistance of cold hardy grapes. Canadian researchers tested several cold hardy cultivars of wine grape for resistance to Anthracnose. Anthracnose, a disease caused by the fungus Elsinoe ampelina, can infect leaves, tendrils, shoots, and immature berries of grape vines. Leaves have dark brown to black spots. As leaf spots grow, the center of the spot turns gray to white and eventually falls out. Leaves may appear peppered with small shot holes. Anthracnose lesions on stems and petioles are sunken oval spots that almost look like hail damage, but the edges Anthracnose spots will always be black. Berries infected with anthracnose have brown to black spots with a pale white center. These spots are often described as 'bird's eye' spots.

Anthracnose thrives in warm, wet weather. In Minnesota, some vineyards see Anthracnose every year, and others rarely have a problem says University of Minnesota Grape Breeder Dr. Jim Luby. The Canadian researchers found Frontenac and Frontenac Gris to be resistant to anthracnose, Frontenac Blanc and La Crescent to be susceptible, and Marquette to be highly susceptible. Growers interested in trying new wine grape cultivars should learn about disease resistance to several grape diseases in addition to anthracnose. Downy mildew, black rot, powdery mildew and Botrytis can all be problematic in Minnesota vineyards. More information about disease resistance and culture of cold hardy grapes can be found at the University of Minnesota Cold Hardy Grape webpage.

Karl Foord, UMN Extension Educator

The goal of IPM is to prevent unacceptable levels of pest damage through the use of pest biology and environmental information. It seeks a solution that poses the least possible risk to people and the environment.

The great appeal of IPM is the understanding of the biological systems at play in the garden and the degree one needs to understand them in order to effectively use IPM strategies. This requires a certain knowledge and skill set. One needs to be able to identify the key insect and disease pests and the types of damage they inflict. One needs to understand the biology of these key pests and how climate influences their behavior. It is also important to understand the natural balances that exist in your garden ecology and to be able to identify beneficial organisms that are a part of that balance. The last item is to understand the use of various chemicals and their effects both direct and indirect. This is understandably quite a challenge. Among the many appealing aspects of gardening, one of them must be its challenges.

Most organisms living in your garden are benign in terms of our perspective of fruit or vegetable production. A few are labeled pests because they conflict with our goals, and a few are labeled beneficials because they tend to attack the pest organisms or because they aid in pollination and assure fruit set. Beneficial fungi and bacteria help plants absorb nutrients from the soil in the same way certain strains of E. coli help in the digestion of our food. However, consider the dilemma of labeling a yellow jacket that preys on caterpillars in the summer and feeds on ripe fruit in the fall.

IPM strategy begins with avoidance of the pest problem itself through use of pest-resistant varieties and cultural systems. For example, most fungi require leaf surfaces to be wet for a certain amount of time at a certain temperature for their spores to germinate. Cultural systems that reduce the opportunity for fungal populations to get established include: 1) proper selection of planting site, and 2) planting systems to reduce the time that leaf surfaces remain wet. Most fungal spores are omnipresent waiting for the right climatic conditions to grow, and by eliminating those microclimate conditions in our gardens we are using IPM strategies.

Some situations cannot be avoided by cultural systems and require monitoring of the plants by scouting for the presence of insects and disease. IPM recognizes that the garden exists within an ecosystem and as such there is a dynamic flux between predator and prey insect species, as well as a flux of fungal and bacterial presence based on temperature and moisture conditions.

The elimination of all insects through the use of a broad spectrum insecticide provides the opportunity for the fast reproducing prey species to bounce back and become an even bigger problem, or requires implementation of a time based spray schedule. The IPM strategy is to monitor insect levels and tolerate the presence of pest species as long as it remains below a threshold level. In commercial systems this threshold level is an economic level based on the cost of control materials and their application. For the home gardener this is probably not an economic number but rather an acceptable control point based on the expectations of harvest quality and quantity.

An insect example

Tarnished Plant Bug (Lygus lineolaris) is an insect whose nymph stage feeding can cause significant damage to strawberry fruit creating misshaped "button berry" fruit. In a commercial setting the nymphs would be sampled by walking through a field at 5% bloom and tapping blossom clusters against a white pan looking for the small green nymphs moving across the pan. If 25% of the 20 or more blooms tested have nymphs, the commercial action threshold has been reached and the grower should take action. This sampling would be conducted every few days to keep a close view of the pest situation.

For the home grower depending on the size of your planting, a pan of soapy water could be used instead of a white pan and all blossoms could be tapped with bugs falling into water and drowning. This would certainly reduce the population of tarnished plant bug and might reduce it below your threshold level. Also the plants could be examined for predators of the tarnished plant bug such as other true bugs "damsel bugs" or nabids (Family Nabidae), and big-eyed bugs (Geocorids), ladybird beetles, spiders, and parasitic wasps. Seethe following URL with descriptions of beneficial insects3: http://www.mda.state.mn.us/plants/pestmanagement/ipm/strawberry-guide.aspx
This would be an example of physical removal of the pest as opposed to chemical.

A Disease Example

The use of a fungicide is based on weather conditions and the fungus in question. Leather Rot (Phytophthora cactorum) is a fruit disease of strawberries that is best managed by judicious use of straw mulch. "Straw mulch can reduce fruit diseases better than fungicides."1 Both Gray Mold (Botrytis cinerea) and Anthracnose (Colletotrichum spp.) overwinter on strawberry leaf litter and spores are transferred to flowers by splashing dispersal in heavy rains. If the temperatures are optimal the spores will germinate. Gray mold is a problem when plants are flowering whereas Anthracnose is a problem when the plants are fruiting. The IPM approach to these diseases involves use of straw mulch to reduce splash effects, removal of leaf litter as a source of disease material mostly in the renovation process2, and use of fungicides if weather conditions are optimal for fungal development.

The more you look into IPM, the more the world seems to expand.

References:
Integrated Pest Management Manual for Minnesota Strawberry Fields Minnesota, Department of Agriculture, September, 2007.

Strawberries for the Home Garden

Field Guide for Identification of Pest Insects, Diseases, and Beneficial Organisms in Minnesota Strawberry Fields.

Further references

Managing Pests in Landscapes and Homes - A Homeowner's Guide to IPM in Minnesota

Michelle Grabowksi, UMN Extension Educator

This time of year the beautiful display of yellow flowers put on by Black Eyed Susan plants (Rudbeckia sp.) is often ruined by the plant's leaves turning partly or completely black. The leaf discoloration is caused by the fungal pathogen Septoria rudbeckiae. This pathogen causes dark brown to black leaf spots much earlier in the season. The disease often begins on the lower leaves of the plant and may go unnoticed. As the season progresses, so does the disease. By September, plants may not have a single green leaf remaining. Septoria rudbeckiae will survive in plant debris, so it is best to remove infected stems and leaves at the end of the season. These should be discarded in a backyard compost that gets hot or at a municipal composting site. Next year, thin plants and remove volunteer seedlings to provide good air movement around plants. Water with drip irrigation or early in the day so leaves dry quickly in the sun. Look for leaf spots early in the season and pinch off infected leaves. Never remove more than 1/3rd of a plants foliage. For more information read Diseases of Rudbeckia.

Michelle Grabowski, UMN Extension Educator

Several shrubs and shade trees exhibiting symptoms of verticillium wilt have been recently observed in Minnesota. Verticillium wilt is caused by the fungus Verticillium dahliae. This pathogen infects through roots and moves into the vascular system of the plant. Infected trees and shrubs may have small pale leaves or leaves with scorched edges in chronic infections. In severe infections, leaves may be completely discolored yellow to red, curl, wilt and die. Often symptoms of verticillium wilt appear on one to a few branches in the canopy. If you suspect Verticillium wilt may be a problem in a shade tree or shrub, peel back the bark on an infected branch and look for grayish streaking in the sapwood. To learn more about Verticillium Wilt visit the UMN Extension web publication Verticilium Wilt of Trees and Shrubs.

Michelle Grabowski, UMN Extension Educator

Bright orange powdery spots on red or purple raspberry leaves are symptoms of late leaf rust, a fungal disease caused by Pucciniastrum americanum. This pathogen can also infect individual druplets in the fruit, turning them into small bright orange powdery masses on an otherwise delicious looking fruit. Late leaf rust needs to alternate between raspberry and white spruce trees. It does not survive on raspberry plants from year to year. Removing nearby white spruce trees is not an effective way to control this disease, however, as spore can travel long distances on the wind. High humidity in Minnesota this summer has favored infection with late leaf rust.

Michelle Grabowski, UMN Extension Educator

Gray Mold is a common disease of flowers and even some vegetables. The fungus Botrytis cinerea is the pathogen that causes gray mold. This fungus thrives in wet weather. Recent rains have resulted in increased sightings of gray mold. Gardeners should look for brown spots on leaves with concentric brown rings like a bull's eye. Flowers can be blighted by gray mold at any stage. Brown blossoms or petals can all be caused by gray mold. A fine fuzzy gray mold can be seen on infected plant parts when moisture sits on the plants. Early morning is a good time to check for this pathogen as dew often encourages the fungus to produce spores.

Read more about gray mold.

Michelle Grabowski, University of Minnesota Extension

Photo 1: Witches' brooms on willow M. Grabowski UMN Extension.

As spring slowly unfolds in Minnesota this year, many gardeners are anxiously watching the buds on their favorite landscape shrubs open up for the season. Some shrubs have one to several clusters of thin weak twigs arising from one point on a larger branch. These unusual growths are known as witches' brooms and they can form at the tip of the branch or lower down the stem. Many different problems can result in a witches' broom. Mites, fungi, aphids, salt or other damage to buds can all result in a proliferation of small branches, where only one should have emerged.

Another less known and little understood pathogen causes witches' brooms in Minnesota landscapes. Phytoplasmas are single celled organisms in a group known as fastidious bacteria. These tiny pathogens were unknown to scientists prior to 1967. They are very difficult and often impossible to grow in a laboratory and can only be seen with a powerful electron microscope.

As a result, little is known about phytoplasmas. Many of them do not even have proper scientific names (a genus and species) because they have not been properly described. Often they are categorized into groups of phytoplasmas that are believed to be related to one another. To truly identify a phytoplasma in a landscape shrub, DNA analysis needs to be done.

Phytoplasmas live in plant sap and interfere with photosynthesis, plant growth, development and seed production. Infected shrubs often have yellow, curled or distorted leaves.

Photo 2: Yellow leaves & dieback in a phytoplasma infected lilac M. Grabowski UMN Extension.

Branches are often stunted or unusually thin. Witches' brooms are very common in phytoplasma infected shrubs. The entire plant may be stunted and in very severe cases killed.

Phytoplasmas are carried from plant to plant by insects that feed on the sap. They can survive from year to year within the insect or the host plant. Once a shrub is infected, it cannot be cured. Some shrubs tolerate the infection; others decline for several years and then die. The diseases below are phytoplasma diseases that can occur in Minnesota.

Willow Witches' Broom

Witches' brooms in willows can be seen in willows growing alongside roads as well as in landscapes throughout the state. Black willow (Salix nigra) and pussy willow (S. discolor) are both susceptible to a phytoplasma in the aster yellows group. Willows infected with this phytoplasma have witches' brooms on one or more branches. These brooms often have stunted yellow leaves and may die during winter months. Exact identification of a phytoplasma requires a costly DNA analysis. Although the willows with witches' brooms in Minnesota have not undergone this analysis, phytoplasma is the most likely cause of the problem.

Photo 3: Brooms on phytoplasma infected lilac M.Grabowski UMN Extension.

Willows are considered tolerant of the disease and often continue to grow vigorously despite infection. In fact, it is believed that the willow is able to isolate the pathogen in the infected branch. The brooms themselves have a very high concentration of phytoplasmas, where as other branches in the same shrub are often completely free of the pathogen. Gardeners with infected willows should prune out any branches with witches' brooms.

Lilac Witches' Broom

Lilac witches' broom is caused by Candidatus Phytoplasma fraxini, a specific member of the aster yellows phytoplasma group that only infects lilac and ash trees. Over 20 species of Syringa are susceptible to lilac witches' broom. Common lilac (S. vulgaris) is tolerant and often shows no symptoms other than slower growth and shorter twigs. Japanese tree lilac (S. reticulata) and many hybrid lilacs are very susceptible to lilac witches' broom. These lilacs have yellow, distorted small leaves that often scorch brown on the edges by midsummer. Many tiny thin shoots form in clumps at the base of the plant. The shrub declines and is often killed a few years after the first witches brooms appear.

Dogwood Witches' Broom and Stunt

Witches' brooms caused by phytoplasmas have been identified on silky dogwood (Cornus amomum), gray dogwood (C. racemosa) and red osier dogwood (C. sericea). It is believed that the phytoplasma responsible for witches' brooms in dogwoods is a member of the aster yellows group. It is unknown how commonly this disease occurs in Minnesota.

Michelle Grabowski, UMN Extension Educator

Photo 1: Container production in the western United States M.Grabowski, UMN Extension.

Many Minnesota gardeners are anxiously awaiting the melting of snow and the warming of soil. As you decide on what new plants to add to your garden this year, consider carefully where those plants are coming from and if there is a risk that they might not be coming alone.

As the world becomes more internationally connected, common garden plants are being shipped across the country and in some cases across the ocean. This is not a problem unless there are plant pathogens hitching a ride on these garden plants. In 2009, tomato transplants from the south eastern United States were shipped to garden centers from Maine to Ohio. Unfortunately these tomato transplants were infected with the pathogen responsible for causing a devastating disease of tomato and potato known as late blight. The resulting disease epidemic spread from home gardens to commercial tomato and potato fields; from the east coast through Wisconsin.

Devastating diseases such as White Pine Blister Rust and Chestnut Blight are believed to have been brought into the United States on infected nursery stock. Other invasive pest problems, like Dutch Elm Disease and Emerald Ash Borer, are believed to have been moved across the U.S. on infected firewood. As responsible gardeners and plant lovers, it is important to choose garden plants wisely.

Photo 2: Minnesota grown basil transplants M.Grabowski UMN Extension .

Where are pathogen stowaways hitching a ride?
· Nursery plants
· Seeds
· Potting soil
· Wood or other plant products

How can you avoid stowaway pathogens?
Buy locally grown plants
Why?
· Locally grown plants have local plant disease problems, plants shipped from distant states or other countries may contain pathogens from those areas that were not previously found in MN.
· Vegetable and annual flower transplants grown from seed in MN greenhouses are grown March through June. Very few pathogens are active this time of year. The same plants grown in warmer climates may be exposed to many different fungi and bacteria before being shipped north.

Photo 3: New seedling trays, provide seedlings a clean start in life M.Grabowski, UMN Extension.

How?
· Shop for vegetable and flower transplants at local farmers markets.
· Ask at the garden center where transplants are coming from.
· Read plant labels and choose plants from local nurseries.

Start plants from seed
Why?
· Only a few pathogens can travel on seed. Many more can be carried in infected potted plants on roots, stems, leaves or in potting soil.

How?
· Purchase seed from a reputable producer to avoid problems with contaminated seed.
· Purchase certified disease free seed when possible. This is more common with fleshy 'seed' like potatoes or bulbs than with true seeds.
· Plant seeds in new potting soil with new or sterilized pots and trays. For more information about starting plants from seeds, read the UMN Extension publication 'Starting Seeds Indoors'

Select container grown plants wisely
Why?
· Potted plants can harbor plant pathogens on roots, leaves, stems, flowers or in potting soil.

Photo 4: Healthy plants from a reputable nursery are a good place to start a healthy garden M.Grabowski, UMN Extension.

How?
· Purchase plants from a reputable nursery.
· Choose locally grown plants whenever possible.
· Purchase certified disease free plants if possible. This is common with some plants like certified virus free roses, raspberries and hostas, but not all plants have a grower certification process.
· Inspect all plants prior to purchase. Reject any plants that have dark, soft or sunken spots on roots, leaves or stems. Reject any plants that have diseased neighbors in the garden center as disease spreads easily from one potted plant to another.
· Report diseased plants to the garden center management.

Do not move firewood
Why?
· Invasive insects and pathogens can be carried on wood from infected trees.

How?
· Burn firewood in the same area from which the tree was harvested.

Learn more about the plant diseases in your garden and report suspected invasive pests
How?
· Use the UMN Extension Online diagnostic tool 'What's wrong with my plant?' to identify pest problems
· Information about invasive pests can be found at the Minnesota Department of Agriculture web site

Michelle Grabowski, UMN Extension Educator

Although very few plant diseases are active in the landscape during the cold winter months of Minnesota, winter is a good time to check trees and shrubs for branch infections like cankers and galls.

Photo 1: Black knot gall M.Grabowski UMN Extesnion.

One common gall forming disease of Prunus species is black knot. This fungal infection causes swollen lumpy wooden black growths to form along the length of infected branches. In the winter, with no leaves to hide them black knot galls stand out and can be easily found within the canopy of infected Prunus trees.

Black knot is caused by the fungus Apiosporina morbosa, and infects over 25 species of Prunus. This includes fruit bearing trees like sour cherry (P. cerasus), European plum (P. domestica) and American plum (P. americana), as well as wild Prunus species like pin cherry (P. pensylvanica) and choke cherry (P. virginiana). Ornamental trees and shrubs like flowering almond (P. triloba) and purple leafed plums (P.cerasifera) can also be infected. There are a few disease resistant fruit tree varieties, but unfortunately most are not hardy in Minnesota.

Photo 2: Black knot infection on a trunk M.Grabowski, UMN Extension.

Black knot galls are elongate, knobby growths that grow parallel to the length of the stem. When young, galls may appear olive to brown. They then turn black when mature, and may crack and crumble with age. Galls are a combination of fungal tissue and wood, so that galls will be hard.

The black knot fungus initially infects young growing shoots or wounded stems in spring. Spores are released from mature black knot galls and splashed by rain or blown by the wind to infect new branches. Infected branches do not begin to swell until late summer or early the next spring, and it will be one more growing season before those galls are mature and capable of releasing spores themselves.

Photo 2: Branch killed by black knot gall M.Grabowski UMN Extesnion.

The damage done by black knot can vary greatly depending on the species, variety and intended use of the tree. Some ornamental Prunus trees can have a canopy full of galls and still produce a beautiful flower show in the spring and a healthy flush of foliage in the summer. In other trees, galls can distort twig growth and eventually girdle the twig, killing all leaves and branches beyond the gall. Multiple infections of this type eventually reduce the vigor of the tree and send the tree into decline. Occasionally black knot infections can be found on the main trunk of the tree. These infections are swollen and covered with lumpy black gall like growth. They often crack and ooze sap. Although the black knot fungus will not cause trunk decay itself, the cracks formed by a trunk infection can provide an entry point for other wood rotting fungi.

March is the best time to prune out and destroy black knot galls. Make the pruning cut 4 inches below the visible gall. This will insure that all of the fungal infection has been removed. Galls should not be left near the tree, since spores can be produced even on pruned branches. Instead remove or destroy the infected branches.

Michelle Grabowski, UMN Extension Educator

Storage vegetables are a great way to keep eating healthy and local throughout Minnesota's long winter. A wide variety of vegetables will keep for one to several months with minimal preparation. Common storage vegetables include carrots, beets, parsnips, turnips, cabbage, kohlrabi, potatoes, onions, garlic, winter squash and celeriac.

Under ideal conditions winter squash can keep for 2-6 months (depending on variety), onions keep up to 4 months, potatoes keep up to 6 months, and carrots keep up to 8 months. It is important to remember that each vegetable has specific moisture and temperature requirements to maintain them in good condition during storage. In her publication 'Harvesting and Storing Home Garden Vegetables', Dr. Cindy Tong of the University of Minnesota Department of Horticulture, provides a detailed table on what environmental conditions each vegetable requires. In addition, Dr. Tong talks about different places in the home that these conditions could be found or created.

Photo 1: Dark, soft, sunken spots on this pumpkin are the early stages of storage rot IM.Grabowski, UMN Extesnion.

Whether you grow vegetables in your own garden, receive them as part of a CSA, or purchase them from the farmers market, always select undamaged produce for storage. Small wounds are easy entry points for storage rot fungi and bacteria. Produce with even a small area of rot will continue to rot and can spread the pathogen to neighboring produce in the refrigerator or on the pantry shelf.

Storage rot fungi often infect vegetables while they are growing in the field, but symptoms may not show up until weeks later in storage. The 2010 growing season had regular rains and warm weather. These are ideal conditions for fungal plant pathogens. Many common garden vegetables suffered from severe leaf spot and fruit rot diseases in the field. It is likely that even healthy looking vegetables put into storage contained some unseen latent infections.

It is important to inspect storage vegetables regularly through the winter. At this time of year, fungal rot can frequently be found in storage vegetables, particularly those in less than ideal conditions. Look for soft sunken spots or dark discolored areas on the surface of the vegetable. Fluffy white cottony growth is an indication of fungal activity and can often be found at the stem end or base of the vegetables, where two vegetables touch together, or areas where humidity is high. Powdery black, blue green or even pink fungal spores may been present.

Photo 2: Fusarium basal rot on shallot M.Grabowski, UMN Extension.

If early signs of rot are found, these vegetables should be immediately removed from the storage bin. If caught early enough, rotted areas can be cut out and the remaining healthy tissue can be used for dinner. If left, however, these minor infections can grow until the vegetable completely collapses from rot. In many cases the disease will spread to infect and rot neighboring vegetables. It is possible for an entire bin of vegetables to succumb to rot started on one fruit.

If you do not have ideal vegetable storage facilities in your home, look for stores that carry local Minnesota storage vegetables. Often these vegetables are labeled with the Minnesota Grown label.

Grace Anderson, MAg, UM Department of Plant Pathology
Hennepin County Master Gardener

Photo 1: Hosta 'Sum and Substance' infected with Hosta Virus X. Note the crinkled tissue between the leaf veins. Grace Anderson.

HVX is a plant virus in the Potexvirus group first identified in 1996 by Dr. Ben Lockhart: Plant Virologist, Department of Plant Pathology, University of Minnesota. It is thought to be host specific and is not transmitted by traditional insect fungi or nematode vectors, or via seed or pollen. It is transmitted mechanically through wounds created during propagation or transplanting, or any time sap to sap contact is made through dividing or trimming plants. Vegetative propagation of infected plants, whether by tissue culture or division, will produce infected plants. Once a plant has HVX, there is no cure and it must be destroyed.

HVX reduces plant vigor and destroys foliage appearance through leaf distortion, color bleeding, and necrosis. Symptoms vary among cultivars and may take years to surface, however the likely result is an unattractive and unacceptable foliage plant (Photos 1 - 3). See http://www.americanhostasociety.org/PDF/HostaViruses.pdf for additional pictures of these symptoms.

The disease appeared to be widespread and yet little was known about procedures to identify the virus, its methods of transfer, or the existence of resistant varieties. In order to address these issues The American Hosta Society under the leadership of Cynthia Wilhoite (VP Genus Hosta, Indianapolis, Indiana) initiated an effort to obtain research-based, empirical data on the nature and transmission of the virus. Funds were obtained and the research effort was led by Dr. Ben Lockhart.

Photo 2: Note the dark green discoloration on the leaf margin Grace Anderson.

Experiments were designed to replicate the actions taken by gardeners and growers in maintaining and propagating Hosta. The goals of the experiment were to answer the following questions. Results of the experiment follow each question.

Can HVX be transmitted during normal cultivation?
Virus transmission was accomplished through the use of contaminated tools and by planting in soil containing pieces of infected plant material.

If it can be transmitted in this way, how long is it infective on tools and soil?
Infected plant material kept in the refrigerator remained infective for more than nine weeks. Fresh infected plant material was always infective. Soil with HVX infected plant debris and root material was infective for more than two years.

What practical measure can be taken to eliminate the virus if it can be transmitted by my tools or via infected soil?
All three tested methods of decontamination including, household detergent (Dawn), 70% alcohol solution, and 10% household bleach solution were effective at eliminating infectivity of the virus. The decontamination process required intense scrubbing and cleaning of tools, hands, and pots to remove plant material, soil and sap. It is not enough to simply dip tools in cleaning solution. Tools must be scrubbed free of all dirt and debris.

Photo 3: Hosta 'Sum and Substance' - Note the lack of crinkling and the uniform color in a healthy leaf. Enter Caption Description Here Karl Foord.

Is HVX transmission dose dependent?
No difference in infectivity or speed of infectivity was related to dosage of the virus isolate. A significant difference in the rate of infectivity was dependent on the stage of plant growth. The virus was most easily transmitted prior to flowering and when the plant was rapidly growing the spring. We were not able to transmit the virus while the plants were flowering in late summer/fall or dormant.

Are all isolates of the virus transmissible?
Yes, we collected and mechanically transmitted 15 isolates of HVX. The success of transmission was not only dependent on mechanical injury but also on the season in which the contact occurred.

Are there resistant varieties of HVX?
This warrants further study. We were able to infect all Hosta tested under the proper conditions in the field, home garden or greenhouse. At this time, we believe all hosta are susceptible to the virus.

Photo 4: Author Grace Anderson working in the greenhouse.

What is the best way to test for the virus?
ELISA testing through Plant Disease Clinics and certified labs is the most reliable method of testing. The new rapid test strips are accurate, reliable, and portable. The strips can be used in the field or greenhouse and will work with leaf or root material.

Strips can be obtained from Agdia, Inc.

What is the best protection against HVX?
Know whether the original sources of plants you buy tests for HVX. Don't be afraid to ask your retail or wholesale source for this information.

For further information on this topic please visit the American Hosta Society.

Grace Anderson recently completed her 23rd year as a Master Gardener in Hennepin County. She is a scientist in the Soybean Pathology Project at UMN and recently received the Master of Agriculture degree in Horticulture at the University of Minnesota. Conducting this research was one of the requirements for receiving this degree.

Karl Foord, UMN Extension Educator
Dimitre Mollov - Director of Diagnostic Services, UMN Plant Disease Clinic

Dimitre Mollov at his dissecting scope. Karl Foord.

Every year brings its unique weather but 2010, with its unseasonably warm April and our cool and wet late May and June, presented some ideal conditions for disease. To explore this further I visited with Dimitre Mollov, the Director of Diagnostic Services of the Plant Disease Clinic of the University of Minnesota. Dimitre took over leadership of the lab some three years ago and to date has analyzed some 6,500 samples from 22 states. These samples have included some 1000 pathogens on some 300 hosts. Eighty plus percent of the samples come from commercial entities where control decisions have greater financial impact, but there may be times when it might be worth it for a homeowner to send a sample to the clinic. How are samples processed and how are the results analyzed at the clinic?

The challenge in diagnosis

You can see the challenge in diagnosis when someone hands you three leaves and asks what is wrong. Sometimes this can be easy with clearly diagnostic insect chewing or piercing damage or pathogens with characteristic necrotic lesions. However there are times when the evidence is asymptomatic, or confounded by more than one organism, or saprophytes who have followed wounds created by other means or organisms. This is why the information sheets submitted with the sample are so important; the more that is known about the specific situation the more information there is to work with in complicated situations.

The importance of information beyond the sample

A well supported sample has information about the plant's symptoms and what parts of the plant were affected. Is there a pattern observed such as the problems began at the top of the plant and worked their way down or started at the bottom and worked their way up. Were other plants in the area affected? When were the symptoms first observed? Other information about the site such as slope, or predominant compass direction of exposure is helpful. The soil type and drainage of the site as well as other chemical inputs to the situation are useful to the diagnostician.

Karl Foord.

The importance of soil analysis

As an example, a sample that had just been received was a three year old Fir tree sent in by a Christmas tree grower. The small tree was about two feet tall and the length of annual growth nodes had been decreasing for the last two years (Exhibit 1). In this case there was no evidence of the presence of an insect or disease pathogen. Given this one would have to expect abiotic factors. Also this stunted growth was not uniformly distributed throughout the field. It is possible that the tree is experiencing problems associated with a high pH soil. See the Climate and Site Requirements section of the publication entitled Choosing Landscape Evergreens. http://www.extension.umn.edu/distribution/horticulture/dg1430.html However, in this case a soil analysis had not been performed and the diagnosis could not be definitive. Dimitre recommends having a soil test done before sending in samples for pathological analysis. Having such basic information is a good base from which to continue diagnosis. But what if the sample does show other symptoms?

A systematic approach to diagnosis

The approach I learned from Jeff Hahn and Michelle Grabowski is to first look for insect damage which is typically at a macro level and can be viewed with the naked eye. The next step is to look for signs of fungal or bacterial pathogens. Dimitre begins this at a macro level with a dissecting scope (picture) and confirms identification with a microscope (picture). He will look for characteristic fungal structures such as spores or mycelia or evidence of the presence of bacteria from cell breakdown or lysis. Should these forms of identification not be present Dimitre has the ability to perform laboratory tests for viruses. Correct diagnosis of viral diseases normally requires laboratory tests because symptoms induced by viruses can also occur due to adverse environmental conditions. Common laboratory tests include identification of specific proteins of the virus by ELISA (enzyme-linked immunoasorbent assay) or DNA of the virus by PCR (polymerase chain reaction).

Some of these tests for viruses are now available to the commercial grower or passionate homeowner. One supplier is Biobest who make Flashkits for viral detection.

The lesson I took from my visit to Dimitre is as follows: Understand your limitations as a diagnostician and the environmental impacts of your decision. If you cannot identify the pathogen with certainty, avoid application of environmentally potentially harmful chemicals that may have little impact on the problem. To help in diagnosis you can use the diagnostics section of the extension website.

If you want more information about the clinic or to obtain sample submission instructions and forms, please go to the UMN Plant Disease Clinic website.

Bob Mugaas, UMN Extension Educator

Photo 1: Areas of rust infection on Kentucky bluegrass lawn. Bob Mugaas.

Rust diseases of our lawn grasses have been on the increase throughout much of this fall period but became especially evident during the drier conditions of October. Rust infestations usually show up as areas of yellow to orange-yellow grass blades, see Picture 1.

Upon closer examination of the grass blade, one will usually see orange colored, tiny tuft-like pustules breaking through the grass leaf surface, see Picture 2. It is these pustules that produce massive numbers of individual spores. These are the same spores that can become air-borne and cover our shoes or lawn mowers in an orange 'powder' as we walk through rust infected areas of the lawn. They can also re-infect other grass plants that in turn can produce more of the same spore producing rust pustules thus carrying on the infection cycle.

What is a rust disease?

Rust diseases have very complex life cycles that include as many as five different stages during a single year. In addition, it is often necessary for various species of rust to spend a portion of their life cycle on one plant species and the other portion on an entirely different plant, often referred to as an alternate host. Such is the case with the specific rust disease known as crown rust (Puccinia coronata) of grass. This disease completes part of its lifecycle on its alternate host, common buckthorn (Rhamnus cathartica) or glossy buckthorn (Frangula alnus, formerly Rhamnus frangula), and the second portion of its lifecycle on some of our lawn grasses, especially perennial ryegrass and tall fescue. Other rust species including Puccinia graminis (Stem Rust) and Puccinia striiformis (Stripe Rust) can also affect Kentucky bluegrass, along with many other grass species.

Photo 2: Spore producing rust pustles pushing through grass leaf surface. Bob Mugaas.

Rust on turfgrass can overwinter in plant debris but it will need to infect its alternate host before returning to the grass plant. It should also be noted that the disease causing spores can develop in warmer climates to our south and be blown up here during the summer and serve as a source of infection from summer into the middle of fall. This can be an important source of rust infection in this area. In a typical year and under favorable conditions, crown rust will usually start to show up toward the end of June while stem rust can be a bit later. Once begun, the rust infection cycle can continue throughout much of the growing season so long as favorable rust infection conditions persist.

Why rust and why now?

Slow growing lawn grasses are a prime target for rust disease attack. It is usually the combination of warm daytime temperatures, dry weather and heavy amounts of overnight dew production on the grass foliage that creates a favorable environment for rust spores to germinate and infect the foliage. When these common weather conditions are combined with low levels of available nitrogen, an element responsible for active, vigorous growth of our grasses, you have very favorable conditions for a rust outbreak. Shadier areas often experience greater incidence of rust. Note the lighter yellow to orange areas scattered around the lawn underneath the spruce trees in Picture 3.

Rust disease started showing up more frequently around the Twin Cities during late August to early September. However, it wasn't until the very dry conditions lasting nearly the entire month of October that significantly increased the occurrence of rust in our lawns and other turfgrass areas. Frequent enough rainfall combined with an occasional supplemental watering kept our lawn grasses actively growing and utilizing available nitrogen throughout much of the summer period.

Photo 3: Rust infection on a partially shaded lawn area. Bob Mugaas.

Nitrogen can also be lost when it is carried with water down through the soil and beyond the reach of grass roots, a process known as leaching. Thus, due to relatively continuous grass growth during the summer and nitrogen losses due to leaching, it is quite likely that the amount of available soil nitrogen was in short supply by late summer, a time of year when our lawn grasses naturally resume active shoot and root growth. That condition along with the prevailing weather conditions during late September and much of October has contributed to a much higher than usual amount of rust disease on our lawns and other turfgrass areas.

So, what should I do now?

With the rains of the last few days of October, we have improved our previously dry soil conditions. That will be a big help in improving the growing conditions for lawn grasses. While it's late to be putting down nitrogen for this year, it would be a good idea to plan on applying some next spring as our lawns are beginning to show active growth. For the most part, we try to manage rust diseases by changes in our cultural practices. There are fungicides that can be applied in severe cases. However, at this late date in the season, both the rust fungi and the turfgrasses are preparing for winter survival and dormancy. Thus, fungicide applications at this time of year will be of no benefit. Use of protective fungicides can be reevaluated next year should serious rust problems begin to develop.

Where disease levels were quite high and there was some thinning of the lawn, one should be prepared to do some reseeding of those areas as needed. Some overseeding could be done yet this fall in a process known as dormant seeding. Normally this would be done once the ground is cold enough to prohibit germination with the seed remaining in the ground until next spring when it will sprout and grow. One could also wait until early next spring to do some seeding.

For some additional information on rust diseases of lawns, check out the following link to our Gardening Information page, What's Wrong with My Plant?

The author would like to gratefully acknowledge the input and review of Dr. Eric Watkins, Assistant Professor-Turfgrass Science, University of Minnesota Department of Horticultural Science and Michelle Grabowski, Extension Educator - Horticulture & Plant Pathology, University of Minnesota Extension, in the preparation of this article.

Michelle Grabowski, UMN Extension Educator

Photo 1: Snow mold damage on a lawn in spring 2010Photo by T.Burnes.

In the spring of 2010 snow melted away from yards and landscapes to reveal round dinner plate sized patches of tan or gray matted turf. Two fungi, known as snow molds, were responsible for the damage. Many gardeners were dismayed to learn that little could be done in the spring to cure snow mold. Rather they had to wait for the weather to change and the grass to recover. This is because the time to prevent snow mold is not in the spring. The time is now.

Snow mold is caused by two different fungi, Microdochium nivale and Typhula sp. Both of these fungi thrive at temperatures just above or below freezing with high levels of moisture. Although snow cover is not a requirement for the growth for snow mold, snow cover provides ideal conditions for the fungi. Of course, gardeners cannot control how much snow Minnesota will receive this winter or how long that snow will stay. Gardeners can prepare their lawns for winter in a way that provides the best chance of a healthy spring.

Photo 2: Fluffy white mycelia of snow mold can often be seen in moist conditions Photo by T.Burnes.

Several factors can increase the chances of snow mold the following spring. Long turf that is bent over by snow creates a canopy that traps humidity at the base of the turf plant. Piles of leaves or other debris on the lawn have a similar effect. Snow mold thrives in these humid microclimates. Fertilizing lawns in late September or early October can cause the grass to produce a flush of young succulent growth. This new succulent turf often does not have time to harden off before winter comes and is an easy meal for the snow mold fungi.

Several simple steps can be followed to prepare turf for the winter and reduce the risk of snow mold the following spring.

1. Rake up leaves and any other debris on the lawn.
2. Continue mowing until the grass goes dormant. A height of 2 inches will help the turf remain upright and facilitate air movement and drying of the turf.
3. Do not fertilize lawns until next year. A late season application of fertilizer can be done around Labor Day, but there is little benefit of fertilizing beyond that date.

Michelle Grabowski, UMN Extension Educator

A new fungal leaf blight of bur oak (Quercus macrocarpa) trees has been confirmed in Minnesota. Bur Oak Blight, also known as BOB, was collected by Jill Pokorny of the USDA Forest Service in several central Minnesota counties and confirmed by Dr. Harrington of Iowa State University.

How to Recognize BOB
• Bur oak blight only infects bur oak trees. Leaf spots or blights on other types of oak tree are caused by different pathogens.
• On bur oaks, symptoms first appear in July or August.
• Infected leaves have brown angular or wedge shaped lesions. Leaf veins turn brown and small black spots (fungal spore producing structures) can be seen with a magnifying glass within the brown lesions.
• Infected leaves are often found first on the lower branches. After many years of infection, however, the entire canopy can be infected.
• Healthy bur oak trees typically drop all of their leaves in the fall. Trees infected with BOB, however, often have many leaves that remain attached throughout the winter.

Will BOB Hurt my Oak Tree?
Bur Oak Blight affects leaves late in the season. Because the leaves have had several months to do photosynthesis before disease becomes severe, the tree has had an opportunity to store up energy. One year of infection, therefore, will not hurt the tree.

Unfortunately trees that are infected one year are likely to have the disease in following years as well. Over several years, the disease becomes more severe, slowly affecting more and more of the canopy. This repeat infection puts stress on the tree, weakening it and making it more susceptible to secondary pests like two lined chestnut borer and Armillaria root rot. Both of these secondary pathogens can cause severe damage to the tree.

Where Did BOB Come From?
Reports of damage to bur oaks similar to BOB were reported in Minnesota as early as the 1990's. Before this time, the problem had not been noticed on bur oaks. It is only recently that the fungus that causes BOB was identified as a new species of Tubakia. It is possible that this fungus was present in Minnesota but never noticed, or perhaps was present but the environment was not conducive to disease prior to the 1990's.

On individual trees, spores of Tubakia sp. overwinter on infected twigs, leaves and petioles that remain attached in the canopy. Rain splashes spores from these old infections to the new emerging leaves, resulting in year after year of disease. New trees can be infected by spores splashing onto leaves from a neighboring infected tree. Overall the spread of the disease within a canopy and within a group of trees is surprisingly slow given the large number of spores produced by the fungus.

What Can I Do To Protect my Tree from BOB?
There are currently no fungicides recommended for treatment of BOB, but research is being conducted to determine if a fungicide injection or spray could protect uninfected oak trees and reduce disease in trees already suffering from BOB.

For now the only thing gardeners can do is reduce stress on infected trees as much as possible. Removing turf grass from below a tree and replacing it with wood chip mulch will help to keep the soil moist. In years of low rainfall, trees should be watered to reduce drought stress. Care should be taken not to wound trees with lawn mowers or other yard equipment. Avoid driving cars or other heavy vehicles over the root zone of mature trees. This can compact the soil and cause root damage.

For more information about bur oak blight read 'BOB in Minnesota' by Jill Pokorny, USDA Forest Service.

Michelle Grabowski, University of Minnesota Extension Educator

Photo 1: crab apple infected with apple scabM.Grabowski, UMN Extension.

Across Minnesota, crab apple trees are nearly leafless due to high levels of apple scab this year. Apple scab is a common fungal disease caused by Venturia inaequalis. In the landscape, apple scab can infect flowering crab apple (Malus sp.), hawthorn (Crataegus sp.) and mountain ash (Sorbus sp.) trees.

Apple scab infects leaves and fruit of susceptible trees. Leaf infections are olive green to black spots with feathered edges. Severely infected leaves or leaves with an infected petiole, yellow and fall off prematurely. This early defoliation not only reduces the ornamental value of the tree in the landscape but weakens the tree. Severely defoliated trees are likely to have fewer blooms the following year. Repeated years of defoliation can predispose a tree to winter damage.

Although many Minnesotans don't notice apple scab until July when leaves start to fall from the tree, the disease actually starts early spring just as the buds are opening. In fact the first infections occur before the leaves have completely spread out. New infections peak in the time period between when pink flower buds are visible on the tree and when the petals finally fall from the blossoms. The apple scab fungi need warm wet weather to start new infections and this year provided ideal conditions for disease.

Photo 2: Scab infected leaves M.Grabowski, UMN Extension.

Although we cannot predict what the weather will be next spring, we do know that there will be an abundance of apple scab spores to start the disease all over again. The apple scab fungi survive from one season to the next in infected leaves that have fallen from the tree. The high level of disease this year means that there are many infected leaves going into the winter. Gardeners can reduce the amount of fungi surviving to next year with several simple strategies. All of the leaves should be raked up and removed from below the tree. These leaves can be composted, buried, or burned depending on city regulations. Alternatively the leaves can be mulched with a mulching lawn mower to speed up breakdown. Applying a nitrogen fertilizer to the leaf litter has been shown to further speed up leaf break down and reduce disease the following year.

Michelle Grabowski, University of Minnesota Extension Educator

Two types of mildew can be seen on pumpkins, squash and cucumbers in Minnesota this August. Powdery mildew and downy mildew have both been reported. Despite the similarities in their names, these two diseases are caused by very different pathogens and have very different symptoms and control strategies.

Powdery Mildew

Photo 1: Powdery mildew on squash M.Grabowski, UMN Extension.

Powdery mildew is caused by the fungus Podosphaera xanthii. As its name implies, this disease can be recognized by the powdery white fungal growth on leaves and stems. Infections often start as a few white powdery spots but can quickly grow to cover the entire leaf in a layer of powdery white fungal spores and mycelia.

Although the powdery mildew fungi do not commonly attack squash fruit, yield of infected plants can be reduced because infected plants have less energy to invest in fruit. Often severely infected plants produce fewer and smaller squash, cucumbers or pumpkins.

Spores of the powdery mildew fungus blow in on the wind and are impossible to keep out of the garden. The best way to prevent powdery mildew is by planting disease resistant varieties. Many powdery mildew resistant varieties of pumpkin, squash and cucumber are available. Home gardeners can also use a fungicide with sulfur as the active ingredient to protect susceptible plants. In order for fungicides to be effective, they must be applied when the first small spot of powdery mildew is observed and the fungicide must be sprayed to cover both the upper and lower leaf surfaces. Perhaps the simplest solution for the home gardener is to include a few extra plants in the garden, to make up for the yield lost to powdery mildew.

Downy Mildew

Photo 2: Downy Mildew on Cucumber D.Moen, UMN Extension.

Downy mildew is caused by Pseudoperonospora cubensis, a fungus like organism, often called a water mold because it thrives in moist conditions. This disease can be recognized by the almost square yellow to brown spots that appear on leaves. Leaves infected with Downy Mildew often look like a patchwork quilt of yellows, greens and brown. On the underside of infected leaves, a purplish gray fuzzy mold can be seen. Under warm wet conditions, downy mildew can spread rapidly. Many leaf spots grow together, turning infected leaves brown so quickly; they almost appear to have been hit by frost.

The downy mildew pathogen cannot survive Minnesota's harsh winters. Each year new spores must move into the state on moist wind from areas to the south. This means that gardeners experiencing problems with downy mildew this year may not see it at all next year.

Although there are several varieties of cucumber that are resistant to downy mildew, gardeners may not be able to find squash or pumpkin varieties with good resistance. Once downy mildew has started an infection, it is very difficult to control. In a home garden, the best solution is to remove infected plants as soon as symptoms appear to reduce the spread of the disease to other cucurbits in the garden.

Photo 1: Stinkhorn fungi Photo by M.Grabowski UMN Extension.

Frequent summer rains, wet soils and humid conditions have created a favorable environment for a wide variety of fungi this summer. Gardeners are noticing mushrooms, shelf fungi, and other odd and interesting fungal spore producing structures of all shapes, sizes and colors sprouting in the landscape. The question then arises, which of these fungi should a gardener be concerned about?

Although it is true that more plant diseases are caused by fungi than any other type of pathogen, only 11% of all fungi are capable of causing disease in plants at all. The grand majority of fungi are saprophytes. That is they survive by breaking down organic matter and absorbing nutrients from it. In many cases, the mushrooms sprouting in the woodchip mulch or pushing up through the lawn are not harming the nearby plants, but are working on breaking down woodchips, plant debris or other organic matter.

Two saprophytic fungi commonly found in Minnesota landscapes are stinkhorns and birds nest fungi.

Stinkhorns
Several kinds of stinkhorns can be found in Minnesota. These fungi start as round to oval egg like structures. When mature a spongy looking stalk with a slime covered cap emerges. Often the remains of the 'egg' can be seen at the base of the mature stinkhorn. Stinkhorns get their name from the smelly sticky slime that caps the mushroom. This slime is full of fungal spores. The smell attracts flies, which will carry the spores to new locations. Stinkhorns can be found in mulched beds, under trees and shrubs and occasionally in lawns.

Bird's nest fungi
There are two common genera of bird's nest fungi found in Minnesota; Cyathus sp. and Crucibulum sp. These two fungi form tiny cup shaped structures with small round disks inside known as peridioles. The cups are designed to catch rain drops and send the peridioles (full of fungal spores) flying to a new location. These hard dark disks can often be found nearby clinging to plants, siding, or whatever else is in the way. Bird's nest fungi commonly grow on woodchip mulch in landscapes.

Photo 2: Cyanthus striatus - Bird's nest fungi Photo by M.Grabowski UMN Extension.

Photo 3: Crucibulum laeve - Bird's nest fungi Photo by M.Grabowski UMN Extension.

Mushrooms and Shelf Fungi of Plant Pathogens
Mushrooms or shelf fungi that are growing directly on the trunk of the tree, out of the root flare or right at the base of the tree indicate that the tree is suffering from heart rot, root rot or butt rot. Trees suffering from internal wood rot may or may not have symptoms in the canopy. For example a honey locust with Ganoderma root rot may have a shiny brown shelf fungi growing at the base of the tree and several dead or wilting branches within the canopy. These branches have died because the trees rotted roots were no longer able to provide the nutrients and water they needed. In contrast a cottonwood tree with heart rot may have a full healthy green canopy despite internal rot in the trunk of the tree. Regardless of how healthy the canopy appears, all three types of rot can greatly weaken the tree due to internal decay.

Photo 4: Car trapped below a fallen branch from a cottonwood suffering from heart rot Photo by M.Grabowski, UMN Extension.

This year's strong winds and thunderstorms have resulted in many broken branches from trees weakened by decay. In severe cases, the entire trunk may break or the tree may fall over. If you suspect a tree on your property has been weakened by a decay causing fungi, consult a certified arborist (www.treesaregood.org) as soon as possible. These professionals can help determine the structural stability of the tree and recommend appropriate action.

Michelle Grabowski, UMN Extension Educator

Recent wet weather has provided perfect conditions for a common fungal disease of flowering annuals known as gray mold. Gray mold is caused by the fungus Botrytis cinerea

Photo 1: Gray mold from an infected flower moves to leaves M.Grabowski UMN Extension.

and can occur in a wide variety of annual flowers including impatiens, zinnias, geraniums and many more.

Photo 2: Brown leaf spot from gray mold infection M.Grabowski UMN Extension.

Often called Botrytis blight, gray mold causes a dark brown to black blight of flowers, buds, leaves and stems. Flower petals are especially susceptible to infection by the gray mold fungus. Brown spots may be seen on petals or the entire flower may turn brown. As flowers age, they fall off onto healthy leaves below. The gray mold fungus then infects the leaves. Removal of these rotted petals often reveals a brown target shaped spot on the leaf which quickly grows to rot the entire leaf. With high humidity a cloud of fluffy gray spores forms on old infected leaves and petals. These spores are easily blown or splashed to new flowers to start the infection cycle all over again.

Photo 3: Zinnia seedling from the center of a nine pack is killed by Gray mold M. Grabowski UMN Extension.

Gray mold thrives in wet crowded conditions. This time of year it can easily be found in over crowed annuals. Gardeners may have flats of annuals waiting to be planted. As these plants outgrow their small containers, humidity builds, and gray mold takes off. It is not unusual for a gardener to find the center plant of a six or nine pack completely blighted by gray mold.

Photo 4: Gray spores of gray mold on rotted leaves and petals M.Grabowski UMN Extension.

The best management strategy to minimize gray mold in annuals is to space out plants so that leaves and petals dry out quickly after rain or irrigation. If the annuals cannot be planted in the garden due to weather conditions or other factors, take pots out of the flat and space them out to allow good air movement between plants. If plants in multipacks cannot be planted quickly, transplant them into larger pots where they will have room to grow and will not crowd one another. When planting in the garden, place plants to allow room for the mature plant. Gray mold can show up anytime wet weather occurs during the growing season.

Spent flowers and infected leaves should be pinched off and removed. Fungal spores are formed on these old rotted plant parts, so do not leave them lying in the garden. Rather collect all infected plant parts in a paper bag and dispose of them. It is ok to compost leaves and flowers infected with gray mold because Botrytis cinerea is commonly found in soil and old plant debris. With a little care most plants can recover from gray mold once warm dry conditions return.

Michelle Grabowski, UMN Extension Educator

Photo 1: Young crown gall on a rose Photo by FL dept. Ag and CS Bugwood.org.

As you purchase new plants for your garden or landscape this spring, one plant disease to look out for is crown gall. Crown gall is caused by the bacteria Agrobacterium tumefaciens. As its name implies, the crown bacteria causes a tumor like growth on the stems and roots of infected plants. Galls are round, rough textured growths. New galls are often light colored and may be smooth and slightly spongy. Older galls become hard and dry; often dark in color with many rough cracks and fissures. The most common place for galls to form is on the main stem, at the point where it enters the soil. Galls can also form on below ground roots. In some plant species, the bacteria travel through the plants vascular system and initiates rows of galls along branches.

The presence of galls on roots or the main stem of a plant may or may not affect the overall growth and productivity of the plant. Young plants with many galls and plants with a gall completely encircling the main stem are the most severely affected. Galls can restrict the flow of water and nutrients through the plant, resulting in reduced growth, low flower and fruit production, and in some cases wilting and death of leaves and stems. Plants with crown gall are more susceptible to drought stress, winter injury and secondary diseases like Armillaria root rot, that enter the plant through cracks in the gall. That being said many plants tolerate a few galls without showing any obvious above ground symptoms. Mature trees of some species have been found with many galls that appear to have little effect on the trees overall growth and productivity.

Photo 2: Crown gall on the trunk of a peach tree Photo by G.Felton UMN Extension.

Over 600 species of plants from over 90 different families can be infected with crown gall. This includes a wide variety of ornamental trees and shrubs, fruit trees, as well as several perennial flowers and vegetables. In Minnesota, the most commonly affected plants are roses, willow or poplar trees and fruit trees like apple, plum, cherry or apricot. All of these plants are especially susceptible to crown gall.

The crown gall bacteria are often brought into a yard or garden on infected plants or soil. A wound is necessary for the crown gall bacteria to infect a plant. Nursery activities like grafting, pruning and transplanting provide ample opportunity for the bacteria to enter and infect susceptible plant tissue. If the plant is actively growing at the time of infection, a gall can be seen in 2-4 weeks and hopefully the plant will be culled before sale. If the plant is dormant however, it may be a much longer time before the gall is visible and of course below ground galls may go completely undetected. If an infected plant is placed in the landscape, the crown gall bacteria can move into the soil and spread to other plants.

Photo 3: Older crown galls on rose stem and roots Photo by M. Grabowski UMN Extension.

As galls age, they begin to break down. Layers of the gall slough off, releasing the crown gall bacteria into the soil. Crown gall bacteria can survive for a long time by living freely in the soil or in association with the roots of a wide variety of plants. Once established in an area, it can be difficult to get rid of the crown gall bacteria, so prevention is the best method of control. Be sure to inspect all new plants prior to planting them in the yard or garden. Pay especially close attention to roses, fruit trees, and poplars or willows as these are known to be highly susceptible to crown gall. Do not plant any tree or shrub with galls on the roots or stem. If crown gall is found on a recently planted tree or shrub, dig up the plant along with the soil immediately around the roots and dispose of them. Large established trees have been shown to tolerate infection with crown gall. If an established tree or shrub is found in the garden, it can be left but care should be taken to sterilize pruning tools after use on these plants. In addition presence of an infected tree indicates presence of the crown gall bacteria. Gardeners with infected trees or shrubs should avoid planting the highly susceptible trees and shrubs mentioned above.

Michelle Grabowski, University of Minnesota Extension Educator

Regardless of the time and effort put into a garden, inevitably at some point a plant disease will show up. These spots, rots and wilts can be quite distressing to a gardener working to beautify their yard or hoping for a fresh crop of fruits or vegetables. Often the first response to disease problems is to spray a fungicide. Many gardeners however are looking for alternatives to this management strategy. Whether you are going organic, trying to reduce the number of pesticides used in your yard or looking for a simple inexpensive means to reduce plant diseases, cultural control practices can be a great strategy for keeping plants healthy. The US Department of Agriculture Organic rule states that preventive and cultural control practices must be a grower's first choice for pest control. In yards and gardens, preventative and cultural control practices are often effective at reducing disease problems to a level where they are no longer a concern or eliminating them altogether. Combining several of the cultural control practices below can keep gardens healthy without the use of pesticides.

Photo 1: Healthy basil plants.  M. Grabowski UMN Extension.

Purchase Healthy Plants - Do not accidentally bring plant pathogens into the garden on infected transplants or seed. Carefully inspect all transplants prior to purchase for disease symptoms like leaf spots, discolored areas on stems, leaves or roots. Above ground plant parts should be firm and green. Roots should be firm and light tan to white. Many root hairs should be present. Reject any plants with symptoms of disease. Purchase seed and transplants from a reputable source.

Disease Resistant Plants - Some plants are bred to be resistant to a specific disease. Whenever possible select varieties that have resistance to common diseases. Look for varieties that advertise resistance to specific disease problems like powdery mildew resistant pumpkins or apple scab resistant crab apple trees. General statements like 'good disease resistance' often imply a hardier plant, but these varieties may not include resistance to any specific disease problems.

Scouting and Diagnosis - Examine plants regularly throughout the growing season to find pest problems while they are still minor. Identify the pest causing the problem before taking action. Visit www.extension.umn.edu/gardeninfo/diagnostics for help in identifying unknown pest problems. Knowing the identity of the pest will allow you to choose management practices effective against that particular pest.

Sanitation - If a plant disease problem is identified on a few leaves, stems or fruit, these plant parts should be promptly removed from the garden. Fungal and bacterial plant pathogens reproduce on infected plant parts. Removing infected plant tissue will reduce the growth and spread of the pathogen within the garden. Remember, never remove more than 1/3rd of a plants leaves. In some cases it is worthwhile to completely remove one severely infected plant to prevent spread of the disease to its healthy neighbors. Infected plant tissue can also be removed from the garden at the end of the growing season to reduce the pathogens ability to survive from one season to the next.

Infected plant parts can be composted if the compost pile heats up to 160F. Otherwise infected plant parts can be buried, burned or disposed of in the trash. Follow local city or county regulations regarding disposal of plant material. Many cities offer municipal composting sites for yard materials.

Photo 2: Moisture favors growth of fungal and bacterial pathogens.  M. Grabowski UMN Extension.

Manage Moisture - Most fungal and bacterial plant pathogens thrive in moist environments. Moisture on the surface of leaves and stems allows these pathogens to infect, grow, reproduce and spread. Roots growing in heavy wet soils are prone to root rot. Create an environment that favors plant growth, not disease development, through proper water management.

Use drip irrigation or a soaker hose to water plants. This puts water in the soil, where roots can take it up, not on the leaves, where fungi and bacteria thrive. If using sprinkler irrigation, water early in the morning so leaves dry quickly in the sun. Avoid watering as the sun goes down. Wet leaves will remain wet for many hours in the night, providing excellent growing conditions for fungal and bacterial plant pathogens.

Water deeply and infrequently. This will encourage growth of deep plant roots and will allow soil to dry slightly between watering. Continuously wet soil favors the growth of some root rotting pathogens and can suffocate roots. Amend heavy soils with organic matter to improve soil drainage.

Photo 3: Soaker hose in the flower garden. M. Grabowski UMN Extension.

Mulch the soil with an organic mulch like wood chips or straw. This will keep moisture in the soil and reduce humidity in the plant canopy. Mulch also helps to reduce the spread of plant pathogens that splash from leaf debris in the soil onto the lower leaves of plants.

Space plants to allow good air movement through the garden. This will help leaves dry out quickly after rain and irrigation.

Tolerate a Non-threatening Disease - Remember not all plant diseases are deadly. In fact many common diseases in the yard and garden affect the aesthetics of the plant more than the health of the plant. Learn more about the plant disease you have encountered at www.extension.umn.edu/gardeninfo before deciding what level of disease control is necessary. Some diseases like oak wilt or Dutch elm disease require action. Others, like powdery mildew on lilac can be tolerated as they will cause no significant damage to the plant.

Michelle Grabowski, University of Minnesota Extension Educator and Jeff Hahn, University of Minnesota Asst. Extension Entomologist

Photo 1: Hackberry tree with witches brooms. W. Cranshaw, Bugwood.org.

As you look up at the trees this spring, watching for emerging buds or perhaps a returning song bird, you might notice many small clumps of short twigs scattered through the branches of some hackberry trees (Celtis occidentalis). These clumps of twigs are called witches' brooms. Although witches' brooms are present within the trees canopy throughout the year, they are most easily observed in the winter or early spring, before leaves emerge. Hackberry trees growing in open areas, like a yard or along a street, are more likely to have witches' brooms than hackberry trees in a forest. Often one hackberry tree will have many witches' brooms while its near neighbors have none.

Witches' brooms occur when the bud of the tree is injured or infected. Normally, a healthy bud opens to produce one shoot. However, when a bud is damaged or killed, multiple weak shoots may develop from the same point on the branch. Witches' brooms in trees can be caused by a variety of problems. Trees growing alongside roads where salt is applied in the winter may have buds damaged or killed by splashing salt. In some cases, infection of the tree by a fungus, a phytoplasma or even a parasitic plant like mistletoe can cause witches' brooms to form within the tree's canopy.

Photo 2: Witches broom on hackberry tree, up close. W. Cranshaw, Bugwood.org.

The exact cause of hackberry witches' broom remains unknown, although two organisms are consistently found within these twig clusters. The first is an eriophyid mite, Eriophes celtis. Eriophyid mites are tiny, measuring no more than 0.5 mm (1/50th inch) long. Even with magnification, people are unlikely to see these mites. Little is known about their life cycle. We do know that eggs are laid in May and mites cluster on the buds, developing until the end of the summer. The second organism is the powdery mildew fungus Podosphaera phytoptophila. The fungus may be seen as a white cobweb like coating growing on the young shoots and leaves within the witches' broom in spring or early summer. Throughout the year tiny brown to black round fungal resting structures can be found on infected buds, but these are best observed with the help of a magnifying glass. How these two pests interact with the hackberry tree is uncertain. One theory suggests that the eriophyid mite causes the witches' broom to form and the powdery mildew fungus takes advantage of the weakened plant and starts an infection secondarily. It is clear that hackberry witches' broom causes little damage to the health of the tree. Trees with numerous witches' brooms have been found to grow vigorously for years.

Photo 3: Several hackberry witches brooms, up close. Jeff Hahn. 

Since the true cause of the witches' broom remains uncertain, there is no known method to prevent or to control the problem. Gardeners who are concerned about the affect of the witches' brooms on the ornamental value of the tree can prune off severely infected branches. In most cases however, hackberry witches' broom is only an aesthetic problem in the winter months. The flush of new leaves soon to be produced will hide the witches' brooms, leaving only a beautiful green canopy to be seen by the casual observer.

Michelle Grabowski, University of Minnesota Extension Educator

Cyclamen are popular plants to brighten the home during the winter months. Flowers come in multiple shades of pink, red, lavender and white. When the blooms are spent cyclamen have interesting white patterns on their leaves, varying from an almost complete white horseshoe to regularly spaced white blotches depending on the cultivar. These leaf patterns are normal for cyclamen and make them an interesting foliage plant.

Photo 1: INSV symptoms on cyclamen. Photo by K. Snover-Clift, NPDN .

Patterns that indicate a problem

Gardeners should beware, however, of leaf patterns that occur on some leaves but not others. The natural white color patterns on cyclamen leaves should be fairly consistent on all of the plant's leaves. If you are noticing unusual color patterns on some leaves but not others, this may be a symptom of a common viral infection.

Cyclamen are one of many hosts to the plant virus Impatiens Necrotic Spot Virus (INSV). This virus was first discovered on impatiens plants showing dark colored ring spots on its leaves. Since then it has been discovered that INSV can infect over 300 species of plants. Many flowering house plants, annuals, vegetables and even weeds can be infected with INSV.

The symptoms of INSV vary from plant to plant and even between cultivars. Some plants have random brown dead spots on leaves or streaks on stems. Others are stunted, wilt and die. Many have ring spots on leaves. Cyclamen infected with INSV have random brown spots, often with one or more brown rings around them. In many cases, multiple yellow to brown rings form on infected leaves, looking almost like a fingerprint. These types of ring spots are characteristic of viral infection.

How did my plant get infected?

Photo 2: INSV symptoms on cyclamen. Photo by K. Snover-Clift, NPDN .

INSV is transferred from plant to plant by western flower thrips (Frankliniella occidentalis), a common insect pest found in greenhouses. Thrips larvae that feed on an INSV infected plant pick up the virus. The virus survives within the thrips and the adult thrips are able to transmit the virus to any plant that they feed on for 5-10 minutes. Once infected with INSV, thrips carry the virus with them for the rest of their lives. Infected house plants could have been infected at the time of purchase or thrips carrying the INSV virus could have been brought into the house on the cyclamen or other plant.

What can I do about INSV?
Unfortunately plants infected with INSV can never be cured. They will carry the virus with them for the rest of their lives and can pass it to other plants if western flower thrips are present. Therefore it is best to destroy infected plants to prevent the spread of the disease. Plants infected with INSV can be thrown into the compost pile because the virus will not survive without a live host plant. If thrips are a problem on this or other houseplants, steps should be taken to control them. Many cultural and chemical control strategies are available to manage thrips and can be learned by reading the UMN extension publication 'House Plant Insect Control'. Remember, only thrips that have fed on an INSV infected plants will be able to transmit the virus, so the presence of thrips alone does not mean that plants are infected with INSV.

The best management strategy is to avoid bringing home plants that are infected with INSV or western flower thrips. Before purchasing a cyclamen, inspect both the upper and lower surface of the leaves for unusual yellow to brown spots, especially ring spots. Thrips may be difficult to see without a hand lens since they are only 1/16th of an inch long and very thin. Tapping the leaves of a plant over a white piece of paper can knock off some insects that you will then be able to see moving across the sheet.

Unfortunately plants recently infected with INSV may not show symptoms for a week up to a month. It is therefore possible to purchase a healthy looking infected plant. To avoid future problems, keep the new plant separate from other plants in the house for about 2-3 weeks. This will allow time for symptoms of the virus or thrips feeding to develop without allowing the problem to spread to other house plants.

For more information about general cyclamen care, read the UMN extension publication 'Cyclamen care'.

Michelle Grabowski, University of Minnesota Extension Educator

Photo 1: Maple tree infected with many Phomopsis galls. M.Grabowski.

As leaves are lost this fall, some gardeners are noticing unsightly lumps and bumps on the bare branches of their trees. A variety of causes, both living and non living, can result in irregular tree growth. However if round rough balls of wood are clustered on the trunk, branches, and twigs of the tree this is likely phomopsis gall. Phomopsis gall is a fungal disease caused by several species of Phomopsis. In Minnesota, Phomopsis galls are most commonly seen on Hickory (Carya spp.) and Maple (Acer spp.) trees. This disease can also infect American elm (Ulmus americana), several species of oak (Quercus spp.), Viburnum spp., Forsythia sp., rhododendrons and azaleas (Rhododendron spp.).

Phomopsis galls are spherical wood balls growing on tree trunks or branches. Galls may be as small as a pea or up to 10 inches in diameter. In most trees, the galls look like a cluster of small nodules or bumps, all pushed together into one lump like a popcorn ball. In maple trees, however, galls often start as smooth round balls and become rough with age as the bark cracks. In all cases, the galls are made of hard disorganized wood that may be difficult to cut through. Examination of young growing gall tissue under a microscope reveals strands of fungal mycelia growing within the wood, but signs of the fungal pathogen are rarely visible to the naked eye.

It is common for a tree to have many galls scattered throughout the branches. Galls may occur individually on branches or be clustered together in small groups. Often one or a few trees in an area will be heavily infected with phomopsis galls and nearby neighbors of the same species will be completely healthy and gall free. Heavily infected trees may have slower growth than their uninfected neighbors. Small twigs that are girdled by one or more galls can be killed. Typically however trees with phomopsis galls are able to continue to grow despite their unusual appearance.

Photo 2: Phomopsis galls on maple. M.Grabowski.

Phomopsis gall is a mysterious disease because very little is known about its lifecycle. Spore producing structures of the fungus are almost never seen on galls. As a result, it is unknown how the fungus spreads from one tree to another and under what conditions infection occurs. No one knows why some trees are highly susceptible to this disease while a neighboring tree of the same species seems to be completely disease free. As a result of this gap in knowledge, few control strategies are available to manage Phomopsis galls. In most cases gardeners are encouraged to tolerate the galls; although heavily infected branches can be pruned off and disposed of. Reducing other stresses on infected trees by mulching the soil around the tree with woodchips or other organic matter, providing water during times of drought, and avoiding accidentally wounding the tree will help the tree to continue to grow at the best rate possible.

Michelle Grabowski, University of Minnesota Extension Educator

Photo 1 (left): Basidocarp of Climacodon septenrionalis. Michelle Grabowski.

With October's frequent rain, many gardeners have noticed fungi sprouting from some landscape trees. These fungi may be any number of shapes, sizes and colors. They may arise from the trunk itself, from the root flare of the tree or from the roots. In all cases, fungi growing directly on a live tree tell the tale of heart rot within.

What's in the Trunk?

In order to understand heart rot, gardeners must understand a little bit about the wood within a tree trunk. Trees can have several different kinds of wood within their trunk. Sapwood is composed of living cells with a number of jobs to do. Sapwood cells conduct sap through the tree, store extra energy, close off wounds, and actively fight invading microorganism. In all trees, sapwood occurs in the outer most rings of the tree. Some trees, like maple, birch, beech and poplar form only sapwood. Other trees also form a second type of wood, known as heartwood at the core of their trunk. Heartwood cells are dead cells that serve primarily to add structural support to the tree. Heartwood cells contain a number of toxic chemicals that protect the heartwood from wood decay fungi. How well these chemicals protect the heartwood varies from tree to tree. Trees like cedar and redwoods are so effective at defending against wood rotting fungi that their wood is highly valued for use in wood products like lumber.

Photo 2 (above): Internal decay shown on cut trunk.

Heart rot can cause decay in both heartwood and sapwood. Many different fungi from the phyla Basidiomycota, can cause heart rot. These fungi are often seen on rotting logs or dead trees as well as on living trees. The mushrooms, shelf fungi and other interesting fungal structures that emerge in wet weather are spore producing structures of the fungus, and are generally known as basidiocarps. If a basidiocarp is observed on a live tree, it indicates that there is rot within the tree. Basidiocarps can be used to identify the specific fungi causing rot as they are often unique to a specific genera or species of fungus. <

How the Invasion Begins

Heart rot fungi are not aggressive pathogens and are unable to infect a tree through intact bark. Instead heart rot fungi take advantage of wounds from lawn mowers, weed whips, fire scars, deer rubbing, rodent chewing, frost cracks, broken branches and other injuries to access the sapwood and heart wood. Some heart rot fungi can also enter a tree through an old branch stub or through the trees roots. Once inside the tree, the fungi use a variety of enzymes and other chemicals to breakdown the wood for food.

The Progress of Decay

White rot fungi breakdown all components of the wood including lignin, cellulose and hemicellulose. Trees suffering from white rot have soft white wood that is stringy, spongy, or crumbly to the touch. Brown rot fungi break down cellulose in the wood but not lignin. Trees suffering from brown rot will have brown decaying wood that breaks apart into cube like chunks. This type of rot is often called brown cubical rot. In both cases advanced decay can significantly reduce the strength of the tree resulting in fallen branches or trees that break or fall over. Trees often survive many years with heart rot. Internal rot often goes unnoticed for years because the tree can continue to grow with no symptoms of disease or decline in the canopy. Remember heartwood cells are not living cells and serve only to provide structural support for the tree. If basidiocarps are observed on the tree, it is likely that the tree has already been infected for several years. The decay caused by many heart rot fungi progresses very slowly, often at a rate of about 3 inches per year.

Assessing Damage in an Infected Tree

Photo 3 (above): Fungi, indicating butt rot on maple. Michelle Grabowki.

A tree with signs of heart rot does not need to be immediately removed, but it should be further examined to determine how structurally sound it is. Special care should be taken with trees that are located near people or property that could be damaged by fallen branches or trees. In natural areas, trees with heart rot serve a valuable function in providing nesting sites for birds and animals and should be left undisturbed if possible. The stability of a heart rot infected tree will vary depending on the type of tree that is infected, the extent of the decay, the type of rot, and if other defects like cracks or cavities are present. The USDA Forest Service (http://www.na.fs.fed.us/spfo/pubs/howtos/ht_haz/ht_haz.htm ) recommends that action be taken to stabilize or remove a tree if it has signs of a heart rotting fungi and one or both of the following:
 

•  a crack, open wound or other defects are also present
  
•  less than 1 inch of solid wood for every 6 inches of tree diameter.

The level of internal decay can be difficult to determine if an open cavity is not present. Gardeners concerned about the structural stability of a tree should contact the local city forester or a certified arborist (www.treesaregood.org). These professionals can run a variety of tests to help assess the stability of the tree.

 

Preventing Heart Rot in Trees

Gardeners can prevent heart rot by protecting the trees most important natural defense - its bark.
 

• Take care not to wound trees with lawn care equipment.
• Protect trees from deer rubbing and rodent damage with fencing.
• Use proper pruning cuts when pruning trees and never leave a branch stub (see: http://www.extension.umn.edu/yardandgarden/YGLNews/YGLNewsMar12007.html#pruningcuts).
  
•  If branches are broken during a storm, cut damaged branches with a proper pruning cut below the damaged area. The tree will be able to heal a smooth pruning cut more quickly than a rough jagged rip.

 

Michelle Grabowski, University of Minnesota Extension Educator

As the Minnesota growing season is winding down and first frost is just around the corner, many gardeners are starting to prepare their landscapes for the long winter ahead. Plants are making physiological changes that will allow them to survive Minnesota's cold winter and so are the plant pathogens that have plagued the garden this year. Many plant pathogens overwinter in infected leaves and herbaceous stems that will soon fall to the ground and become part of the leaf litter. Other pathogens survive in live plant tissue like perennial crowns and tree branches. Reducing the number of pathogens that survive from one season to the next is a great way to reduce potential disease problems in the next growing season. Sanitation, the removal of infected plant parts, is an important part of disease prevention. Although sanitation will not prevent all future disease (pathogens can often move in from long distances away), it can reduce the severity of disease or delay its appearance.

In the Flower Garden

Photo 1 (left): Peony with a fungal infection on leaves and stems. M. Grabowski.

A wide variety of fungal and bacterial plant pathogens cause leaf spot and blight diseases in annual and perennial flowers. After the first frost hits, these infected leaves, stems, and flowers fall to the ground. The pathogens spend their winter nicely insulated within this old plant tissue, covered by snow. In the spring, warming temperatures and moisture from spring rains and melting snow stimulate the pathogens to start reproducing. Fungal spores and bacterial pathogens are then soon being splashed or blown onto the newly emerging plant shoots, starting the disease cycle all over again.

What can a gardener do?

• Scout your gardens now and take note of any plants with leaf spot or blight diseases. Even mild cases are worth addressing now.
  

• Do not bother spraying fungicides at this time. It is too late. Fungicides are protective and preventative in nature. They may be useful early in the season to protect young growing tissue but are unnecessary as the plants go dormant.
  

• After the first hard frost, go back to each infected plant and collect all of the fallen leaves. Cut herbaceous stems back to the ground. Remove all of this infected plant material from the garden.
  

• Infected plant debris can be composted if the compost pile heats up to 160 degrees F. Don't forget to check with municipal compost facilities. These are often more intensely managed than a backyard compost pile and are often an acceptable place to take infected plant material. If composting is not an option, infected plant material can be buried to speed up decomposition or thrown away with the trash.
  

• If any perennials are found to be infected with a virus or with aster yellows, remove the plant. These pathogens survive in the live crown of the plant. Once infected, the plant will always carry the disease. Virus and aster yellows infected plant material can be composted since these pathogens do not survive without a live host plant.

Trees and Shrubs

As in the flower garden, fungal and bacterial pathogens that infect tree leaves, survive the winter on these same leaves once they have fallen to the ground. New fungal spores and bacterial pathogens will be produced in the following spring to infect the newly emerging leaf buds. Other pathogens survive within live branches, the trunk or roots of the tree. Depending on where the pathogen is living, the control strategy is different

Photo 2 (right): Black knot on Prunus. M.Grabowski
.

• Scout trees and shrubs for any signs of infection before the first hard frost. Make notes as to where the pathogen is found. In leaf spot diseases, the pathogen infects the leaves. With cankers or galls, the pathogen survives in the infected branches.
  

• After leaf drop, rake up and remove all leaves from trees suffering from any leaf spot diseases. If there are too many leaves to collect, use a mulching lawn mower to chop of the leaves and speed up their breakdown.
  

• Infected leaves can be composted, burned (if allowed in your city) or buried.
  

• If cankers and galls are found, mark them for later removal. The best time to prune out cankers and galls in Minnesota is in February and March. At this time pathogens and insect pests that might infect the pruning wound are not active. For more information about dormant season pruning, see: http://www.extension.umn.edu/projects/yardandgarden/YGLNews/YGLNewsMar12007.html
  

• It is also important to help trees prepare themselves for winter by providing adequate water until the ground freezes. The trees root system will continue to take up water even after leaf drop. Research has shown that trees stressed by dry soil conditions are more likely to suffer from frost cracks. For more information about frost cracks visit http://www.extension.umn.edu/yardandgarden/YGLNews/YGLNews-Oct0106.html#frost
  

• If the base of the tree is mulched, make sure that the mulch is not piled up against the tree trunk. Volcano mulch cones provide excellent hiding places for rodents in the winter and can result in damage from rodent feeding along the trunk of the tree. Rake any mulch away from the trunk so that an air space occurs between the trunk and the mulch.

Lawns

Snow mold is a common disease of Minnesota lawns. Although this disease most commonly causes problems in early spring, there are many cultural control practices that can be implemented now to reduce problems next year. The fungi that cause snow mold love moist cool conditions.

Photo 3 (left): Lawn with pink and gray snow mold  B.Mugaas

• Reduce mowing height to 2.0 - 2.5 inches. This will allow the turf to remain upright underneath the snow cover. Longer grass flops over creating moist pockets that favor snow mold growth.
  

• Remove all leaves from nearby trees and shrubs from the lawn. This can be done by using a mulching lawn mower to break up the leaves or by raking and removing the leaves. Clumps of leaves left on the grass, do not allow the grass leaves to dry properly and create conditions favorable to snow mold.
  

• If making a second application of fertilizer, wait until the grass blades have stopped actively growing.  This typically occurs about mid to late October in the Twin Cities. The roots and crowns of the grass plant will still be active and will use the fertilizer nutrients to build up the plant's food reserves. If fertilizer is applied very late in the season,  while the turf is still growing above ground, new leaves may not have time to harden off before winter. This soft succulent tissue is especially susceptible to snow mold.

Emily Tepe, Research Fellow, Department of Horticultural Science

If you walk through the St. Paul campus Display and Trial Gardens these days you're bound to see a lot of activity. No, I'm not talking about bees on the flowers (although there were a lot of those with the unusual warm weather in September), I'm talking about students. With the start of the fall semester comes a plethora of courses on plant identification, propagation, diseases and insects. The Display and Trial gardens offer a convenient and valuable living laboratory for these courses. In fact, throughout the year (save for a couple of months in the depths of winter) these gardens offer education to many people in the University community and beyond.

Photo 1 (left): Edible landscape portion of the University of Minnesota Display and Trial gardens. Emily Tepe

An Inspiring Outdoor Classroom

The Display and Trial gardens are comprised of various areas between Alderman Hall (home of the Department of Horticultural Science) and the Plant Growth Facilities on Gortner Avenue. Trees, shrubs, and hardscaping create the foundation for the gardens, and break it up into beds, each with their own theme. These themes change from year to year as new varieties are introduced, student projects are realized, and interesting gardening styles bring an opportunity to explore and experiment. The 2009 season brought some inspiring plantings and great educational opportunities.

These educational opportunities often get started while there is still snow on the ground, as students propose projects for the garden and begin designing beds and planting seeds in the greenhouse. Classes ,such as Professor, Neil Anderson's Floriculture Crop Production, research and schedule their assigned crops, working backwards from the planned finish date (mid-May), to assure their annual flowers are at the perfect stage for judging before being planted out in the gardens. Many of the varieties they grow are trials for major seed companies.

When spring arrives, students who have proposed projects for the gardens, begin breaking ground, laying out beds, sowing seeds, and eventually setting out transplants. They are responsible for maintaining their plantings throughout the season, keeping the beds watered, weeded and looking good. It's a great experience for students to take what they've learned in the classroom and put it all into practice. These projects bring the fresh ideas of students to the forefront, allowing them to experiment with new concepts and interesting designs, and even showcase some of their research.

By the time the gardens are in full swing, the St. Paul campus is pretty quiet. Most of the student body is gone for the summer, and the gardens become an inspirational outdoor venue for summer camps, youth enrichment programs, Master Gardener events, and horticulture industry field days. 

Photo 2 (right): Master Gardeners tour the Edible Landscape at the University of Minnesota State Master Gardener Conference. Emily Tepe

On any given summer day you are bound to find a group of high school students cutting flowers for a design and marketing program, or a flurry of youth in matching t-shirts tending a plot of vegetables; kept on task by their nurturing and enthusiastic mentors. Members of the local community often visit the gardens to view the new varieties released by the University, the vast array of annual flowers, and the creative ideas such as this year's Edible Landscape.

A Living Laboratory

Once classes start in September, University students begin spending a lot of time in the gardens. Many of the students in the introductory horticulture courses have never seen some of these plants before, and the gardens offer a close-up look at the topics they're studying. Tom Michaels, professor in the Department of Horticultural Science (teaching Plant Propagation this semester) said of the Edible Landscape portion of the gardens, "Students pass right by those beds every time they come to lab. They can't help but see examples of the food they buy in the produce department actually growing in front of them. It gives me the opportunity to talk about those foods and encourage them to stop by the beds and find examples of how chard differs from lettuce or dinosaur kale, or similarities and differences between beans and peas". The gardens are indispensable for the plant identification courses as well. Students find examples of hundreds of species, and with hand lenses and forceps, can scrutinize tiny flower parts to determine the plant family to which they belong.

Photo 3 (above): Plant pathology students observing symptoms of apple scab in the Display and Trial garden. Emily Tepe.

Horticulture students aren't the only ones spending time in the gardens. The Display and Trial Gardens provide a wonderful laboratory for plant pathology and entomology students as well. Todd Burnes, scientist in the Department of Plant Pathology, said numerous courses spend time in the gardens identifying and studying various plant diseases. While in the home garden, powdery mildew, leaf spot and white mold would likely prompt immediate action, here we aren't so hasty. The opportunity for students to observe the symptoms of diseases, collect samples and study them in the lab is worth a few ugly plants here and there at the end of the season. Entomology students roam the gardens, sweeping their longs white nets along the edge of the prairie strip, or carefully trapping unsuspecting insects on the zucchini flowers. Once back in the lab, they'll identify and study their specimens.

Photo 4 (below and right) : Powdery mildew on zinnias in the Display and Trial Gardens. Emily Tepe.

It is truly amazing the wealth of education that can be found in a garden. Here on the St. Paul campus, the Display and Trial gardens offer many people a chance to get up close and personal with flowers, grasses, trees, fruits and vegetables. And whether in class or just wandering through, there are countless opportunities to discover. Every garden offers such opportunities for young and old alike.

Michelle Grabowski, University of Minnesota Extension Educator

Many articles about the late blight epidemic of potatoes and tomatoes in the northeastern United States have been appearing in the news this summer. These reports are making some Minnesotans worried about late blight here in Minnesota. Although the extreme level of disease that is occurring in the northeast is not currently present here in Minnesota, late blight is present in the state and disease is possible. Gardeners should be aware of what late blight infected plants look like, what environmental conditions favor late blight, and what to do if late blight occurs. At this point, however, there is no need to panic. 

Photo 1: Late blight on leaves. H.Schwartz, Colorado State University, Bugwood.org.

About late blight

Late Blight is a disease caused by a fungus-like organism called Phytophthora infestans. Phytophthora literally means 'plant destroyer' in Latin. This pathogen has earned its name by causing several famous epidemics including the Irish potato famine of the 1840's that resulted in the death or emigration of millions of Irish people. Late blight is most severe on potatoes and tomatoes but can also infect related plants like petunias and nightshade. Olive brown spots on leaves or stems are often the first obvious symptoms of disease. These spots grow until the entire leaf is affected, progressing into petioles and stems. Eventually the entire plant is brown and wilted. Under very moist conditions, fine white cobweb like fungal growth may be visible on infected plant parts. Infected tomato fruit have a large greasy brown spot. Infected potato tubers have sunken brown lesions on the surface and reddish granular rot extending into the flesh of the potato. Rot can start in the field and continue in storage. 

Phytophthora infestans thrives in cool wet weather. Temperatures ranging from 60-80 degrees F are ideal. In addition, moisture on leaves and other plant parts is necessary for the pathogen to infect and spread. This moisture can come from rain, irrigation or dew. The late blight pathogen is so devastating because once an infection starts the pathogen reproduces and spreads very rapidly. One lesion can produce 100,000 to 300,000 spores in the right weather conditions. These spores are carried on wind or splashing rain to other plants, devastating an entire field in as little as a week's time.

Photo 2: Late blight causes olive brown lesions on stems. Photo credit: H.Schwartz, Colorado State University, Bugwood.org

2009 late blight epidemic in northeastern states

In the northeastern states this summer, several factors combined to create the perfect conditions for a late blight epidemic. First, Phytophthora infestans was brought into several states on infected tomato transplants. These infected plants were then sold at big box stores and distributed throughout the area in home gardens. This early season arrival of the pathogen, combined with unusually cool wet weather, allowed the disease to take hold and spread rapidly from gardens to farm fields.

2009 Minnesota late blight outlook

Until August, most of Minnesota was extremely dry with a few areas in moderate to severe drought conditions (US drought monitor - link to http://climate.umn.edu/doc/journal/drought_2009.htm ). These conditions are not favorable to late blight and it is not surprising that the disease was not found in Minnesota until August. 

With recent rains, the possibility of late blight on tomatoes and potatoes in Minnesota has increased. Although very few cases have been reported in Minnesota, late blight has been found in North Dakota, South Dakota and Wisconsin. Because the disease spreads so rapidly, there is still time for damage to be done. In large part, the spread and development of late blight in Minnesota will depend on the weather conditions, but spread and disease development also depends on growers and gardeners properly caring for plants that may become infected. Commercial potato growers are already vigilantly looking for new late blight infections and have been spraying fungicides to protect their crop

Photo 3: Late blight on tomato fruit. R.Wick, University of Massachusetts, Bugwood.org

Monitoring and controlling late blight

What can you do?

1. Monitor: First monitor tomatoes and potatoes in your garden at least once a week, and more frequently if rainy weather persists. Examine the lower and inner leaves where humidity is highest and disease is most likely to appear first. Use the UMN extension online diagnostic tool to identify the problem you are seeing on your tomato plant (www.extension.umn.edu/gardeninfo/diagnostics/ ) or send a sample to the UMN plant diagnostic clinic (http://pdc.umn.edu).
2. Remove diseased plants: If late blight appears, immediately remove the infected plant, place it in a tightly sealed plastic bag, and throw it in the trash. This will prevent the pathogen from spreading to nearby gardens and farms.
3. Apply fungicides: Some gardeners may choose to protect their tomatoes and potatoes with a fungicide spray. Chlorothalonil is the only product available to home gardeners that will provide adequate control of late blight. The vegetable being sprayed MUST be listed on the fungicide label, and all label instructions MUST be read and followed. Organic gardeners can use a copper based fungicide, but should be aware that it will only provide partial protection. Fungicides must be applied before the disease starts in order to control the disease.

Photo 4: Late blight on potato. S.Bauer, USDA ARS, Bugwood.org

 With luck, the season will pass without further disease development. The active management strategies already in place by Minnesota farmers will reduce the amount of fungal spores available to spread the disease. Gardeners can do their part by keeping a watchful eye over their plants and responding quickly if disease occurs.

Michelle Grabowski, University of Minnesota Extension Educator

This time of year gladiolus blossoms are a welcome sight, whether left in the garden or brought inside as a cut flower. Unfortunately not all gardeners will have gladiolus blossoms this year. A corm rot disease of gladiolus, known as Fusarium wilt has been found in Minnesota. This disease is caused by the fungus Fusarium oxysporum f.sp. gladioli. The last part of the fungus's name 'f.sp. gladioli' indicates that this fungus has a very narrow host range and will only cause disease in gladiolus and very closely related plants. This is good news for gardeners who have infected gladiolus plants mixed with other flowering annuals and perennials.

Gladiolus plants infected with Fusarium wilt are often stunted and do not produce flowers. The leaves first yellow, then turn completely brown and fall over. The fungus infects the corm of the plant, often starting at the basal plate where the roots attach. Gardeners may see dark spots on the outside of infected corms. When cut open, a reddish brown rot can be seen within the corm.

Michelle Grabowski, University of Minnesota Extension Educator

Bacterial Leaf Scorch (BLS) is a disease of shade trees common in the southern states and known to occur as far north as Ontario and as far west as Texas. To date, however, it is unclear if this disease occurs in Minnesota and how common it might be. It may seem unusual that a disease of a large tree might go unnoticed for years, but many states, having never reported Bacterial Leaf Scorch previously, have found multiple infected trees once scientists began looking for the tiny bacterial pathogen. Recent reports show that BLS is becoming more common in northern states like New Jersey. Scientists now want to know, does BLS occur in the upper Midwest?

 

The reason that BLS can slip by unnoticed is that the symptoms it causes look very similar to the symptoms caused by several other diseases and by environmental stress. Scorch is the term used to describe leaves that have singed brown edges. In shade trees infected with BLS, the margins and tips of the leaves turn brown. Unfortunately leaf browning at the edges and tip of the leaf is also commonly seen in trees stressed from drought, compacted soil, restricted root growth, salt damage and many other factors. In addition, diseases like oak wilt and Dutch elm disease can cause browning of leaves that may be confused with BLS. In the end, scientists rely on several laboratory tests, like ELISA, PCR and electron microscopy, to determine if BLS is truly present or not.

 

Michelle Grabowski, University of Minnesota Extension Educator

Several kinds of blight are common on tomatoes grown in Minnesota. Almost every gardener has struggled with the fungal diseases Septoria leaf spot or early blight on their tomato plants at one time or another. This year bacteria are the primary pathogens being isolated from tomato leaf spots.

There are two bacteria that result in leaf spots on tomato; bacterial speck caused by Psuedomonas syringae pv. tomato and bacterial spot caused by Xanthomonas campestris pv. vesicatoria. Bacterial speck and spot can cause spots to form on the leaves, stems and fruit of tomato plants. The leaf spots caused by bacterial speck and spot look identical but the two pathogens can be distinguished by differing types of fruit spots that form later in the season.

Leaf spots are dark brown to black, small (about the size of a pencil tip) and have a yellow halo around them. Often the center of these leaf spots dry up and drop out leaving tiny holes throughout the leaf. If a leaf is infected with many leaf spots, the leaf may turn partially or completely yellow, and may even fall off. In past years, only minor infections from bacterial spec and spot have been observed in Minnesota. This year’s weather conditions seem to favor the pathogens, resulting in much more severe infections than previously seen.

Some varieties of tomato seem to be more sensitive to the bacterial pathogen than others. A few varieties, like Brandywine tomatoes, have been observed with larger leaf spots (up to 3/8th of an inch across). Infections with larger leaf spots could easily be confused with one of the common fungal leaf blight diseases. If gardener’s are unsure which pathogen they are dealing with, they should use the UMN Extension Online Diagnostic Tool ‘What’s wrong with my plant?’ before making any management decisions.

On the tomato fruit, bacterial speck infections are small (the size of a pencil tip) raised black spots. These spots start on green fruit and may develop a green halo as the fruit turns red. In contrast, bacterial spot fruit infections are larger (the size of the eraser end of the pencil) dark brown to black, raised and often corky appearing. Bacterial spot infections may be surrounded by a white halo. In both cases, the infection on the tomato fruits remains fairly superficial and does not result in fruit rot.

Both bacterial speck and spot can come into the garden on infected seed or transplants. The bacteria are then easily transferred from plant to plant through splashing water, strong winds or on a gardener’s hands and tools. The bacteria survive Minnesota’s harsh winters in plant debris.

Luckily in most cases infection with bacterial speck and spot do not result in significant yield loss. Although the fruit with raised corky bacterial spots would not be considered marketable at the grocery store, smart gardeners know the fruit can still be enjoyed once the superficial spots have been cut away. Infected tomatoes should not be used for canning, however, because the disease may have changed the pH of the fruit.

To reduce the spread of the disease, gardeners should avoid working in tomato plants when the leaves are wet. Under moist conditions the bacteria reproduce and easily stick to a gardener’s hands and tools. Waiting until plants are dry for chores like staking, pruning and weeding will reduce the spread of the bacteria. In addition, providing good air movement around the plants by staking or caging tomatoes, pulling weeds, and spacing plants far apart will allow leaves to dry quickly. Allow 2 years to pass before planting tomatoes or peppers in the same location. Do not save seed from infected plants.

Michelle Grabowski, University of Minnesota Extension Educator

Cool temperatures this spring seem to be encouraging rust fungi on roses. Several different species of Phragmidium (the rust fungus) can infect both wild and cultivated roses. Gardeners should keep an eye out for two different forms of this fungus.

Bright powdery orange spores, known as uredinia, are likely to catch a gardener’s eye. These spores form in raised pustules on the underside of infected leaves, stems, or petioles (central portion of the leaf that the individual leaflets are connected to). Yellow to brown leaf spots may be noticeable from the top surface of the leaf but may not form on all rose cultivars. Infected petioles and young green stems may actually become twisted and distorted around the site of the infection. Rust fungi can infect all plant parts except the roots and gardeners may notice bright orange pustules in unusual places!

Michelle Grabowski

Powdery orange golf balls (or larger spheres) have been spotted on pine trees across Minnesota. What are these strange growths? They are galls, a woody tumor like growth that is part plant, part fungus. In this case the pine trees have been infected with one of two different gall forming rust fungi: pine-pine rust (Endocronartium harknessii) or pine-oak rust (Cronartium quercuum). The diseases get their names from the trees they infect. Pine-pine rust only infects 2-3 needled pine trees like Jack pine, Scots pine and Ponderosa pine. Pine-Oak rust lives half of its life in galls on pine trees and the other half on the leaves of oaks like northern pin oak or bur oak. The galls are present year round. In the spring, both fungi release powdery yellowish orange spores, drawing attention to otherwise discrete brown woody galls.

Michelle Grabowski, University of Minnesota Extension Educator

As the season warms up and new leaves reach their full size a few gardeners are noticing unsightly spots in a wide variety of plants.

What are these spots?

Many of the spots that gardeners are seeing are caused by fungal or bacterial pathogens infecting the leaf tissue. There are a few look a likes, however, so gardeners should pay attention to the details. Fungal and bacterial leaf spots are typically randomly scattered across the leaf. There are often several sizes of leaf spots because the spots get bigger as the pathogen grows. In addition leaf spots caused by a pathogen start out on one leaf and eventually spread throughout the plant or to other plants. A gardener may notice fluffy white cobweb like fungal growth, powdery spores or other signs of the pathogen in the spots themselves.

Michelle Grabowski, University of Minnesota Extension Educator

Downy Mildew, a potentially devastating fungal disease of roses has been found on several roses for sale in Minnesota. The disease has been identified on Double Knock Out®, Pink Knock Out® and Europeana®, but all rose cultivars are susceptible to Downy Mildew. This disease results in irregular purplish red leaf spots that eventually turn tan in the center. Infected leaves often turn yellow and fall off. Under very humid conditions gray fuzzy fungal growth may be seen on the underside of infected leaves. Downy Mildew also causes purplish black streaks on rose stems. Downy Mildew thrives under cool humid conditions. It spreads easily on the wind. If infected plants are brought into the garden, the disease could easily spread to other roses and raspberries in the area. Do not purchase roses with dark leaf spots!

David C. Zlesak

The core or pith of plant stems are typically either hollow or contain loosely packed, spongy parenchyma cells.  The pith of this butternut or white walnut (Juglans cinerea) is distinctive in that it is chambered with dark bands of sclerenchyma plates separating hollow zones. Black walnut (Juglans nigra), a close relative, also has chambered pith and has bands that are typically a bit lighter in color. Parenchyma cells are relatively large, of variable shape, and have thin cell walls.  As developing stems elongate, parenchyma can tear and disintegrate in many plant species.  Another cell type found in plants is sclerenchyma. Sclerenchyma cells help provide support to plant tissue and have thickened, secondary cell walls containing cellulose and are often impregnated with lignin.  As stems of white and black walnuts grow, parenchyma cells eventually collapse leaving mainly sclerenchyma cells in distinctive plates.  

Michelle Grabowski, University of Minnesota Extension Educator

The word golden brings about images of wealth, vitality and prosperity. Unfortunately when this word is used to describe the branches of a pagoda dogwood, the connotation is quite the opposite. Golden canker is one of the most common diseases of pagoda dogwood small trees/large shrubs in Minnesota and it can be disfiguring and even deadly.

Photo 1: Yellow infected branch areas contrast sharply with purplish red healthy bark. Michelle Grabowski

In the early spring sunlight the infected branches are bright yellow to orange compared to the dark almost purplish red healthy bark. The striking contrast is almost pretty. Sadly any branch that has completely turned yellow is already dead and will not leaf out this spring. Close examination of these yellow branches will reveal that the branch is covered in tiny blister-like orange spots. A sharp line marks the border between healthy and diseased branch tissue.

Michelle Grabowski, University of Minnesota Extension Educator

Photo 1: Fusarium wilt resistant and susceptible tomato seedlings. Michelle Grabowski

Although winter still holds Minnesota in its icy grasp, smart gardeners are already pouring over seed catalogs and preparing for the season ahead. There are many factors to consider when choosing which vegetable variety to grow this season. One option available to gardeners is disease resistant varieties. These are varieties that have been specially bred or selected for their ability to remain healthy in the presence of a pathogen. Choosing disease resistant vegetable varieties can save the gardener time and money since they will likely not require fungicide sprays or other control measures to prevent the development of fruit rot, leaf spots or other disease problems.

Michelle Grabowski, University of Minnesota Extension Educator

Photo 1: Fire blight canker on a young apple branch. Michelle Grabowski

This past summer, many Minnesotans noticed dead brown wilted leaves on apple, crabapple and mountain ash trees caused by the bacterial disease known as fire blight (Read Midsummer Trouble for Trees & Shrubs) Although symptoms of fire blight are most apparent in spring and summer months, one of the best times to manage this disease is right now.