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September 2010 Archives

Contents: September 15, 2010

In this issue of the Yard and Garden News:

  • Pumpkins are Too Cool!
  • What Naked Crabapple Trees Tell About Apple Scab - This Year and in the Coming Year
  • Nuisance Insects in Fall
  • Midseason Apple Tasting Video

Pumpkins are Too Cool!

Karl Foord, UMN Extension Educator

The word pumpkin originates from the Greek word pepon or "large melon". The French changed this to pompon, the British to pumpion, and the American colonists to "pumpkin".

Pumpkins (cucurbits) originated in Mesoamerica, and many of the wild species are found in the area south of Mexico City to the Guatemalan border. There are three species of interest: Cucurbita pepo, C. maxima, and C. moschata. The terms squash and pumpkin have no botanical meaning because as you will see each species has produced both squash and pumpkins. This article will mention the squash but focus on what we traditionally call pumpkins.

All species are monoecious having separate male and female flowers. The pollen is heavy and must be transferred by a pollinator. Two bee genera evolved to become efficient pollinators of cucurbits (squash bees, Peponapis spp. and Xenoglossa spp.). Interestingly enough pollination seems to be a morning phenomenon as breeders have found that the percentage of successful pollinations is greater in the early morning and then decreases gradually until noon.

There is great variation for size, color, skin type, and shape both within and among pumpkin species. The species that contains most of the varieties that we would encounter at a market or pick your own come from Cucurbita pepo. The size categories break down as follows: 1. Miniatures (< 1 lb.) like 'Wee-B-Little', 2. Baby pumpkins (1 - 3 lbs) like 'Summer Ball', 3. Small pie pumpkins (4 - 7 lbs) like 'Baby Pam', 4. Jack-o'Lantern types (7 - 30 lbs.) like the white 'Moonshine', the wart skinned 'Knuckle Head' , American Tondo, and the traditional 'Howden Biggie'. Other commonly encountered Cucurbita pepo members include: most summer squashes, Gourds, Pattypan Summer squash, Crookneck squash, Scallop Summer Squash, and Zucchini.

The species Cucurbita maxima as expected given the name the large giant pumpkins like 'Dill's Atlantic Giant' which grown normally produces 50 - 100 lb pumpkins, but when given special attention can produce 2 - 300 lb pumpkins, and is the variety that holds the record for the largest pumpkin at > 1,600 lbs. This species also has some pumpkins with interesting characteristics such as the 'Rouge Vif D'Etampes' (rouge vif meaning "vivid red") whose shape served as the model for Cinderella's carriage pumpkin. Also quite different is the variety 'Marina Di Chioggia' with its green color and wart like banded skin. Other commonly encountered Cucurbita maxima members include: Hubbard squash and most winter squashes.

The final species is Cucurbita moschata whose main contribution is the 'Libby's Select Dickinson' field pumpkin. Libby's owns close to 90 percent of the canned pumpkin market in North American. In the early 1800's the Dickinson family moved from Kentucky to Illinois and started a canning facility using what is now called the 'Libby's Select Dickinson' pumpkin. Libby's purchased both the canning plant and variety rights to the pumpkin in 1929. Some other interesting Cucurbita moschata varieties are 'Musque de Provence' and 'Naples Long' a peanut shaped squash that is considered an Italian heirloom variety. Other Cucurbita moschata members include butternut squash.

The variation within these Cucurbita species is really quite remarkable and we haven't even touched on the variability among the squashes.

All photo credits Johnny's Selected Seeds.

A special thanks to Johnny's Selected Seeds of Winslow, Maine for graciously allowing us to use their photos in this article; some very nice people up there in the State of Maine. Seed can be obtained for all varieties pictured from Johnny's Selected Seeds.







Michelle Grabowski, University of Minnesota Extension Educator

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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.
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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.

Nuisance Insects In Fall

Jeffrey Hahn, Asst. UMN Extension Entomologist

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Photo 1: Boxelder bug. Jeff Hahn.

As the days get shorter and the days and nights get cooler, this is a cue to people that summer is ending and fall is upon us. That is also a sign for insects, signaling them that they need to prepare for winter. For some insects and related arthropods, this means finding sheltered places to overwinter which unfortunately can mean our homes.

Some insects, particularly boxelder bugs, multicolored Asian lady beetles, cluster flies, and hackberry psyllids, will fly to buildings and congregate on the outside, especially on the south and west facing exposures where it is the sunniest. As they find spaces and cracks to get inside, some end up in attics, wall voids, and other spaces (where they remain until a mild winter day or spring) while others find their way into the interior part of homes. Yet other arthropods, such as sowbugs, millipedes, and crickets, don't fly but crawl to buildings and find their way indoors at ground level.

Regardless of which nuisance invader you find in your home, the good news is that they do not reproduce indoors and are essentially harmless to people and property. They are short-lived indoors, although some, like boxelder bugs and multicolored Asian lady beetles, can successfully overwinter and can move into the inside of homes periodically during mild winter days and in spring. SowbugsUnderLog.jpg

Photo 2: Sowbugs under log. Jeff Hahn.

The best management of nuisance invaders is prevention; take steps to keep these insects and arthropods out of your home to begin with. First, examine the outside of your home for possible entry points that they may use to enter your home. Look particularly around windows, doors, where utility lines enter buildings, and areas of buildings where vertical and horizontal surfaces meet. If you are dealing with flying insects, concentrate your efforts on the south and west facing exposures. If you are dealing with crawling insects, check for mulch, leaves, and other possible debris close to the building that may provide harborage. Removing this will make it more challenging for them to get inside. It isn't possible to insect-proof your home so that nothing can get in, but it is possible to minimize the number of insects and other arthropods that can into your home.


Photo 3: Multicolored Asian lady beetles trapped indoors. Gail Felton.

You can supplement your pest proofing efforts by using an insecticide to treat areas where insects and other arthropods are most likely to enter homes. Treat just as you are starting to notice insects coming indoors. Examples of insecticides that you can use, include products that contain bifenthrin, cyfluthrin, deltamethrin, and permethrin. Be sure that the particular product you wish to use is specifically labeled for use around the exterior of buildings.

Once insects and other arthropods get inside, you do not have many options other than to physically remove them with a vacuum or broom and dust pan.

Midseason Apple Tasting

Yard and Garden News Editor: Karl Foord
Technical Editor: Bridget Barton

Contents: September 1, 2010 and Monarch Caterpillars

  • Knowing When it's Time to Say Goodbye
  • Home Lawn Care To-Do List
  • Dogwood Sawflies
  • Wooly Oak Gall
  • Mildew Aplenty on MN Pumpkins, Squash and Cucumbers
  • Winter Damage in Apples
  • The Taste of Minnesota Apples- An Adventure!

This just in! Monarch Caterpillars spotted by Karl Foord at the Minnesota Landscape Arboretum in Chaska.

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Monarch Caterpillar_2pptx [Read-Only].jpg

Yard and Garden News Editor: Karl Foord

Technical Editor: Bridget Barton

Knowing when it's time to say goodbye.

Gary Johnson, UMN Extension Professor, Urban and Community Forestry, Department of Forest Resources


Preventative cabling such as this dynamic system greatly reduces the likelihood of failure during wind, ice or snow loading events.
Gary Johnson.

No one willingly lets go of old friends, whether they're human, pets or (ahem) trees. But the reality of life is that there's an aging process that's inevitable and not all problems can be diagnosed and successfully treated. So that old friend that sheltered you from harsh winter winds, shaded you on hot summer afternoons when the heat index was over 100 degrees, greeted you in the spring with a bouquet of flowers...must eventually or prematurely be removed.

Four Questions Before Letting Go

Gardeners of all shapes and degrees tend to be incredibly hopeful. For example, when news comes along that there's a new cure for emerald ash borer (have you heard about the fruit juice and dish soap treatment yet?), the urge is to try it without considering four "critical" questions .

· Is the tree healthy?

· Will the tree remain stable if it stays in the landscape?

· What is the relative value of the tree to you?


Once a crack has started on a mature, large tree, cabling should not be considered a reliable option. Remove the tree. Beth Jarvis.

· What can your budget reasonably handle?

By the time these four questions are answered, the fate of the tree's future should be obvious.

Is the tree healthy?

Health is a measurement of a tree's ability to photosynthesize normally, store energy for growth and tolerance to stresses, and the eventual release of that energy. If a tree has a species-characteristic crown density, leaf color and size as well as a sufficient live crown ratio, it's generally considered to be healthy. Stand under the tree and look up through the foliage. If it's a Norway maple, you shouldn't be able to see a lot of sky. If it's a honeylocust, you should see blue clearly through the crown. That's an example of characteristic crown density. The same goes for leaf color and size. Don't compare oaks with service berry.

Live crown ratio refers to the vertical mass of foliage. A healthy tree should have at least a 60% live crown ratio, that is, 60% of its height is photosynthetic foliage. If a tree has abnormally small leaves, a thin crown, a deficient live crown ratio, lots of dieback (therefore, less foliage) and defoliates early due to insects or's unhealthy.


Would you consider this a reliable treatment to prevent this large ash from tipping any further? No? Good! Remove the tree.Unstaged photo courtesy of Mike Meisch.

Trees in Minnesota have a relatively short amount of time to create chemical energy (e.g. sugars) and store them up for normal functions (e.g. starches provide energy to grow, leaf out, tolerate defoliations). Full leaf expansion doesn't occur until mid-June (sometimes later) and the photosynthetic season is essentially over by the end of August. Weakened trees have difficulty tolerating or recovering from common stresses. If your tree mirrors the image of an unhealthy tree and it's been that way for awhile, it's not likely that investment s of chemicals and money will turn it around.

Declines in health are most often associated with repeated defoliations (e.g., Japanese beetle or gypsy moth, hail storms, or anthracnose) or chronic drought (several years of seasonal drought). If nothing has been done in the past to intervene and lessen these stresses, trees progress into a decline spiral from which they rarely recover. Healthy trees on the other hand can recover from problems that are shorter termed (termed inciting events) especially with a little care such as watering, mulching, and controlling defoliating insect pests.

Will the tree remain stable?

One of the more difficult decisions to let go happens when a tree is obviously "healthy" but is unstable, too risky for the landscape. What? How can that happen? How can something be that healthy looking be bad for us?


Many trees can live for years after root loss on one side, but this drastically reduces a tree's ability to stay vertical during wind storms.Jacob Ryg.

Decay. Decay is the most common reason trees fail in wind storms (well, other than the wind). A healthy tree can have extensive decay in the trunk or buttress roots, which makes it an unacceptable risk in the landscape...too likely to prematurely or suddenly fail and cause property or personal damage. If the decay is extensive and especially if there's an opening to the cavity, don't think twice, find someone with a chainsaw. A good arborist can calculate the strength loss due to decay for you. If it's greater than 33%, the risk isn't worth keeping the tree in the landscape.

Dysfunctional roots. Stem girdling roots, roots of big trees squeezed into tiny spaces (like narrow boulevards or small planting spaces) are often the causes for complete tree failures during wind storms. The bigger the tree, the more severe the root problem, the more likely massive damage will result...not may result, but will result.

Severed root systems. Street widening projects, new or repaired buried utilities, roots cut during house construction activities cause instability issues. Most trees (if they're healthy...ironically) recover from construction activities or anything that cuts the normal root spread...only to topple during the next wind storm or the next one or the next one. It's not worth it. If roots are cut within a few feet of a large tree on two or more sides, it's unstable. If it's near a house, utility wires, roads, etc, it's an unacceptable risk.


When large trees are left on narrow boulevards after street widening projects remove much of their roots, this is a pretty common scene for several years. Gary Johnson.

What's its relative value?

You could hire a certified arborist to calculate the monetary value of the tree, but that's not what I'm referring to. Is there an emotional or sentimental value to the tree? Did Grandpa plant it with you on Arbor Day when you were a kid? Did Mom love the apples it grew and made the best pie from them? Oooh, tough call if it's removal time.

Relative values for everything are as unique as the individuals that own them. Some people collect objects, take pride in weed-free lawns or drive pick-up trucks with carpeting in them. Others purge, are willing to clean bathrooms every day if it means avoiding lawn work or buy trucks for work, not show and wouldn't pay an extra nickel for carpeting.

Trees that are special (unusual species, extra large size), well -placed for shade or blocking a nasty view, showy in the spring or autumn or host a tire swing or tree house are trees that are hard to let go. You may never see another tree like that again, so the cost of care may not be an issue. Whatever it takes to save it will be long as it's relatively healthy and stable.


This tree has a live crown ratio of more than 90%. A healthy tree should have 60% or more as a measure of its photosynthetic potential.Gary Johnson.

Individual windbreak trees, individual trees that are part of a woodland or trees that are so far from your house or normal activities area have less value. Their absence will not be as noticeable as the sentimental trees or the tree that your always rely on for shading the patio in the afternoon. Younger or smaller trees tend to tug at the heartstrings less, too. If the tree has only been in the landscape for a few years, there's usually less of an emotional attachment and it probably isn't shading the landscape that much.

What's your budget?

It often comes down to this: groceries or the ginkgo. Tree care doesn't come cheap, especially if it's a large tree and the care is long term. Trees are an investment, part of the infrastructure of your landscape, just like fences, garages and patios. Deferred maintenance has never worked for building longevity and quality and it doesn't work for trees either.

If a tree is regularly straining your budget by demanding life support maintenance, you'll probably be in favor of removing it. Why on earth would someone on a limited budget invest gj7.jpg

Yes, this tree is alive but along with having less than 30% live crown ratio, it supports half of a crown of leaves. Gary Johnson.

money year after year controlling apple scab on a crabapple when there are crabapples that are disease resistant? Get rid of the money pit and plant a new, lower maintenance crabapple...they're just as pretty.

If, however, the tree is well-placed, provides a valuable service, is healthy and stable, the investment to keep it healthy and stable is probably money well-spent. Inject that American elm with fungicides that will prevent Dutch elm disease. Treat those bur oaks with oak wilt...they can be saved. Don't let gypsy moth or Japanese beetle repeatedly defoliate that linden...there are both chemical and biological controls for those problems.

A reality check is needed for comparing control options to giving up and removing the trees. The expenses related to keeping trees free of oak wilt or emerald ash borer may seem onerous, sometimes $100-300 every 2-3 years. However, removing large trees near homes doesn't come free, either. A large tree within dropping distance of a home can cost $1,000 to $6,000 or more to remove...and then you're left with nothing but fire wood. No shade, no fragrance, no privacy. All of a sudden, maintenance money seems a bit cheaper.


This stressed maple has one of the perennial, fungal "target" cankers that are so common on maples in Minnesota. This will not get better, so remove and replant with something other than a maple.
Gary Johnson.

The final decision.

That well-placed, healthy, mature tree needs some significant pruning and cabling work on it, as well as some other health management it worth it? Most likely. Keep in mind that it could be an ash. There are very effective treatments for preventing or treating ash trees for emerald ash borer. Don't give up if the tree is worthy of saving just because it's an ash.

That tree has been repeatedly topped under the power lines for years and looks like a mop on a tree trunk. Get rid of it! It's most likely filled with decay and there are many better alternatives such as smaller trees or trees not planted under the lines.

Construction activities have cut the roots within 4-5 feet of your mature silver maple on three's too risky for it's own good and it's time to replace it. Don't take a chance.

The apples on that Yellow Transparent are unbelievably delicious and you can't buy them in stores anymore! Control the apple maggots and apple scab.

Your male ginkgo has a little surprise for's a female and the odor gets a bit stronger each year if you don't clean up the mess in the autumn! What to do? It's your call on this one.

September Home Lawn Care To-do List

Bob Mugaas, UMN Extension Educator

Thumbnail image for bob2.JPG1. One of the very best times of the year to be fertilizing your lawn is from about Labor Day through the middle of September. Applications that put down about one pound of nitrogen per 1000 square feet will help provide the necessary nutrition through the late summer / fall period - a time of active grass shoot and root growth.

2. It is important to avoid serious water stress on lawns this time of year. As noted above, the late summer /early fall period is a time of active growth. Hence, not only is sufficient nutrition important but ample soil water is just as important to sustain and encourage growth. Early in September, the average rule of thumb of one inch of water per week including rainfall should be sufficient. As we get later into September and early October, that same one inch of water may be sufficient for two or even three weeks depending on weather conditions. That is, the warmer and drier the weather the more frequently plants will need watering. The cooler, and more moist the weather, the less frequently plants will need to be watered. Remember, soils should be just damp to the touch not soaking wet to provide a healthy place for roots to grow and beneficial soil microbes to flourish.

3. Early September is an excellent time to be doing some overseeding of damaged or thin areas of the lawn. Seed germination is usually much quicker due to the warmer soil temperatures. Hence, seedlings are able to get out of the ground and more quickly establish the area prior to the onset of colder conditions of late fall and early winter. If you're struggling with trying to get some grass growing under the shade of some maturing shade trees, try growing some of the fine fescues. They are well adapted to dry shade conditions and are tolerate much lower inputs of fertilizer and water while still remaining healthy. If seeding areas shaded by trees, be sure to keep lightly removing tree leaves as they fall. That will help ensure that seedlings receive sufficient sunlight throughout the fall resulting in better establishment. Keep newly seeded areas damp during the germination process and gradually back-off the water as they begin to get established - usually about three to four weeks.

4. One of the best times to be aerating the lawn is right around Labor Day. This minimizes the amount of germination from unwanted weed seeds making for less competition to the new grass seedlings. It also provides increased soil oxygen levels that encourage better root growth and a healthier soil microbial community. Lawn aerifiers that pull a core of soil and deposit that core on the lawn surface are the most effective units that are still relatively easy to use for homeowners. These soil cores can be left to decompose naturally over the next few weeks. There is usually not a need to remove these from a home lawn. If you are also planning some fertilizing, and/or want to do some overseeding, an excellent time to do that is right after you have aerified. Aerification can also be used to control the rate of thatch build-up as the decomposing soil cores help to reinoculate the underlying thatch with soil. In turn, that helps break down the thatch and keep the amount of thatch build-up to below damaging levels (i.e., less than ½ inch) Thumbnail image for bob3.JPG

When you think of fall as an active period of growth for your lawn grasses, the extra effort to ensure good growing conditions during that time helps ensure a healthy lawn going into the winter and a lawn quicker to recover and resume growth in the spring. Besides, late summer and early fall are some of the nicest conditions of the year to be tinkering with your lawn. Enjoy!

Dogwood Sawflies

Jeffrey Hahn, Asst. Extension Entomologist

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Photo 1: Dogwood sawfly defoliation. Chris Bauer.

Dogwood sawflies been found defoliating dogwoods during late August. In all of the reported cases, defoliation was severe and apparently appeared suddenly. However, despite the apparent sudden appearance, these sawflies have a actually been present for weeks but just went unobserved when they first started feeding.

Adults are active any time from late May through July. After the larvae first hatch, they are covered in a whitish material which some entomologists believe helps the sawflies resemble bird droppings, protecting them from predation. The young larvae skeletonize leaves, i.e. feed between the veins. Older larvae consume the entire leaf except for the midrib. As mature larvae, dogwood sawflies are about one inch long. Also, the whitish material comes off, revealing their greenish - yellowish, conspicuously spotted bodies.

When they are done feeding, they wander off looking for places to pupate, preferring rotted Thumbnail image for dogwood 3.JPG

Photo 2: Dogwood sawfly (note white material on body). Chris Bauer.

wood. Dogwood sawflies have also been known to bore into homes to pupate. They remain as pupae through the winter and in the following spring There is one generation of dogwood sawflies each year.

If you find your shrubs are being defoliated now, there is not a lot that you can do as dogwood sawfly feeding is either done or is nearly finished. However, if you did have a problem with them this year, watch for them next year starting in June and treat if they are abundant. There are a variety insecticides than can be used if you catch them while they are small, including insecticidal soap, spinosad, horticultural oil, permethrin, bifenthrin, and other pyrethroids. Safari (dinotefuran) and imidacloprid, both systemics, are also effective but need some time to be taken up by the shrubs.

Woolly Oak Gall

Jeffrey Hahn, Asst.Extension Entomologist

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Photo 1: Woolly oak gall. Brittany Koch.

People have been commonly finding about one inch long fuzzy or woolly looking growths on the leaves of oak trees, especially species in the white oak group. These galls are either reddish or whitish or sometimes both. Despite their appearance, these growths are not disease, but is abnormal plant tissue due to the feeding of the larvae of tiny non-stinging insects called cynipid (sin-IP-id) wasps. These galls are common and like other leaf galls, have very little, if any, impact on tree health, especially if they are vigorous, mature oaks. These galls generally do not become very abundant and their numbers vary from year to year on individual trees. By the time you see leaf galls, it is already too late treat them, just ignore them.

Mildew Aplenty on Minnesota's Pumpkins, Squash and Cucumbers

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
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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
Downy Mildew Y&G.jpg

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.

Winter Damage in Apples

Karl Foord, UMN Extension Educator

Preparation for winter

Plants acclimate or develop hardiness to freezing temperatures in response to changes in light duration and temperature. Acclimation is a two-stage process. The first stage is initiated by decreasing day length and results in partial hardiness. The second stage is initiated by subfreezing temperatures and results in full hardiness and acclimation. The hardening response in a plant may vary from year to year because of variation in temperatures over that year. The degree of cold hardiness of a plant is determined by the genetic capacity of a plant to acclimate to freezing temperatures and transform its tissues from a non-hardy to a winter hardy state. Plants are often given a cold hardiness rating based on the lowest midwinter temperature that plant tissues can endure relative to the USDA winter hardiness zone temperature bands. For example, Honeycrisp has a winter hardiness rating of USDA hardiness zone 3 (-40 to -30 F).

However, extreme low temperature is only one of a number of factors affecting winter survival. Winter damage frequently occurs during late fall or early spring due to extreme changes in temperature when the plant is not at its maximum hardiness. Injury is a function of the acclimation status of the plant at the time of the radical temperature change, and is often difficult to determine, and may not show for several years.

No two winters are the same

Every winter may have an episode that could cause some damage to some trees. For example the winter of 2008-9 was considered to be a more stressful year than the winter of 2009-10. This is borne out by the temperature graphs of the two years. Note that the winter of 2008-9 had a great deal of significant temperature fluctuation while 2009-10 was much smoother (Figures 1 & 2). Dr. Jim Luby noted three potentially damaging events that stand out in the winter of 2008-9 as follows: 1. The 43 degree temperature drop on December 15, 2008, 2. The 37 degree temperature drop on March 11, 2009, and 3. The five nights of low temperatures below - 20 from January 13 - 17, 2009. Many areas experienced a freeze event on Mother's Day (May 9, 2010) with varying amounts of damage. Some Minnesota apple orchards lost almost all their apple flowers and reports from Iowa noted lots of damage with bark splitting off the trees.



Carryover environmental effects

A dry season or a particularly heavy crop can reduce the vigor of the tree and make it more susceptible to winter injury. As Kathy Zuzek would tell you a rose variety that is defoliated by black spot in July is more susceptible to winter injury then one that has not been defoliated.

Other influencers of hardiness

Lack of snow cover during the coldest period can lead to root damage.

Pruning apples before they have accumulated their full hardiness can set trees up for winterkill, especially so with cultivars that are less winter hardy. In general, pruning cuts are dehardening in the early winter and the larger the cut the more dehardening occurs. Prune fruit trees in late spring before the buds become active.

Orchard topography is important because it affects cold air drainage. Lower areas where cold air accumulates can cause frost to settle and damage trees.


High intensity sunlight on a sunny winter day heats up the south and southwest side of thin-barked young apple tree trunks causing the cells to come out of dormancy and become active. After sunset temperatures can drop precipitously to levels well below freezing which kills active cells and conductive tissue. This often appears as a longitudinal crack running up and down the trunk.

Commercial tree wraps made of crepe paper, plastic spiral wraps or longitudinally cut drain pipe will intercept the sun and insulate the bark preventing sunscald. Wrap the trees from base to the lowest branch in the fall after leaf drop. I have used all methods. I have left the plastic wraps on for at least three years until the bark thickens and is less prone to sunscald. I removed and rewrapped the crepe paper each year.

The Taste of Minnesota Apples - An Adventure

Karl Foord, UMN Extension Educator

I did an apple tasting exercise with 24 young people last summer and we tasted the following apples traditionally available at grocery stores, Granny Smith, Golden Delicious, Jonagold, Tentation, Fuji, American Cameo, Royal Gala, and Red Delicious. More then half of the young people chose Fuji as their number one choice. Given this array of apples my taste buds agreed with theirs.

Then I started tasting local Minnesota Apples. Quite frankly I was astonished. These were excellent apples and in my opinion outclassed all of the commercially available apples previously tested. Granted the store apples had either been grown in Washington State and stored for 10 months, or had been shipped from either Chile or New Zealand. The Minnesota apples were fresh. Certainly tastes differ significantly among people and a recommendation is simply my opinion. You must discover for yourself as it is your taste and opinion that will render the final decision.

We are still early in the apple season. Some of the earliest of varieties have come and gone like State Fair. This is mostly due to the warm spring we had. However, many of the best are yet to come. My favorite early apples are Chestnut Crab and Zestar. I thought Paula Red was a good apple when captured within its optimum taste window. It doesn't keep that well and that is why my memory of Paula Red is one of a mushy apple. Check out our Early Apple Tasting video with Mike Dekarski (linked to this Y&G issue) to see some early season apples that we tested last week.

I look forward to SweeTango but we will have to see how many apples make it to the market. I also look forward to Honeycrisp, Sweet Sixteen, and SnowSweet.

Don't miss out on the Minnesota fresh apple season. Most orchards will let you taste before you buy so you can get the apple that tastes best to you.

I highly recommend that you give yourself a treat and experience the taste adventures available in local Minnesota Apples.

You can find an orchard near you at the Minnesota Grown .

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