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Pine Wilt

USDA Forest Service

Photo 1: Scots pine killed by pine wilt

Dan Miller, MN Landscape Arboretum

Two mature Scots pines (Pinus sylvestris) at the Minnesota Landscape Arboretum started showing tan-colored needles at the end of the summer this year and by late September both trees were dead. When one of the trees was being removed, Assistant Gardener Mike Walters noticed a blue stain in the sapwood of the tree and from his previous experience with a tree care company in southeastern Iowa; he suspected the tree had been killed by nematodes. Cross-sections of the blue-stained wood were soaked in water and nematodes, microscopic roundworms, could be observed with a dissecting microscope. A sample was then sent to the University of Minnesota Plant Disease Clinic and they confirmed the presence of the pine wood nematode (Bursaphelenchus xylophillus). This nematode is the primary cause of pine wilt disease.

Dan Miller, MN Landscape Arboretum

Photo 2: Cross section of a Scots pine infected with blue stain fungus

Pine wilt disease is an interesting and complex disease. Two insects, the nematodes, and a fungus are all involved. The nematodes are transmitted by the pine sawyer beetle (Monochamus spp.). The adult pine sawyers feed on the young shoots of pine trees and even though they don't cause much damage to the tree, the feeding wounds create entry points for the hitch-hiking nematodes. When the nematodes enter the tree they feed on the cells surrounding the resin ducts causing resin to leak and plug the water transport system of the tree. As the tree is weakened and becomes stressed, bark beetles are attracted. When the bark beetles bore into dying pines, blue-stain fungi living in the beetles also enters the tree. This fungus provides another food source for the nematodes so their numbers multiply even faster.




Natasha Wright


Photo 3: White spotted pine sawyer; the beetle that transmits the pine wood nematode



Pine wilt disease was first reported in Minnesota by Dr. Robert Blanchette (University of Minnesota Professor of Plant Pathology) in the early 1980s but the nematode is believed to be native to North America. Pine wilt disease occurs most commonly in stressed nonnative trees. In the Midwest, 90 percent of the trees killed by pine wilt are Scots pine. The disease occasionally appears in Austrian (Pinus nigra), mugo (Pinus mugo), and Japanese red (Pinus densiflora) as well. Native pine species are usually not susceptible. In most cases, only trees greater than 10 years old are attacked. Once the tree is attacked, it dies within a few weeks.

Y. Mamiya

Photo 4: Pine wood nematode inside the resin canal of a pine tree

At this point management options are limited. Insecticides and nematicides have not proven to be practical or effective. The best strategy is sanitation. Dead trees should be removed in the fall or early spring before the adult pine sawyers emerge and should be burned, buried, or chipped. Scots pines are not recommended for new plantings.


Mary H. Meyer, extension horticulturist and professor, University of Minnesota

Mary Meyer

Table 1: BEST Crabapples for Minnesota

References and Further Reading:

Beckerman, J., J. Chatfield, and E. Draper. 2009. A 33-year Evaluation of Resistance and Pathogenicity in the Apple Scab-crabapples Pathosystem. HortSci. 44(3):599-608.

Chatfield, J. A. E. A. Draper, and B. Cubberley. 2010. Why Plant Evaluations Matter. American Nurseryman 210(9):10-15.

Draper, E. K., J. A. Chatfield, and K. D. Cochran. 2005. Marvelous Malus--Ten Crabapples Worthy to Know, Show, and Grow. Brooklyn Botanic Garden. Accessed October 6, 2014.

Green, T. L. 1995. Results of the national crabapple evaluation program. Accessed online October 3, 2014.

Green, T.L. 1996. Crabapples--When you're choosing one of those apple cousins, make flowers your last consideration. Amer. Horticult. 75:18-23.

Guthery, D.E. and E.R. Hasselkus. 1992. Jewels of the landscape. Amer. Nurseryman 175(1):28-41.
Iles, J. 2009. Crabapples..... With No Apologies. Arnoldia. Accessed online October 10, 2014.

Koetter, R. and M. Grabowski. 2014. Managing apple scab on ornamental trees and shrubs. Accessed online October 10, 2014.

Romer, J., J. Iles, and C. Haynes. 2003. Selection Preferences for Crabapple Cultivars and Species. HortTechnology 13:522-526.

Schmidt, J. Frank and Sons. 2014. Crabapple Information Chart. Accessed online October 8, 2014.


Mary H. Meyer, extension horticulturist and professor, University of Minnesota

Mary Meyer

Photo 1: Adirondack close-up

Mary Meyer

Photo 2: Adirondack - whole tree

Mary Meyer

Photo 3: Beverly close-up

Mary Meyerd

Photo 4: Beverly - Whole tree

Mary Meyer

Photo 5: Bob White close-up

Mary Meyer

Photo 6: Bob White - whole tree

Mary Meyer

Photo 7: Donald Wyman - whole tree

Mary Meyer

Photo 8: Firebird close-up

Mary Meyer

Photo 9: Firebird - whole tree

Mary Meyer

Photo 10: Louisa close-up

Mary Meyer

Photo 11: Louisa - whole tree

Mary Meyer

Photo 12: Pink Spires close-up

Mary Meyer

Photo 13: Pink Spires - whole tree

Which one would I plant in my yard? Anyone from this list, but something in the name 'Professor Sprenger' does resonate with me! It is a lovely tree that greets visitors on the Snyder Terrace at the Arboretum. Others you can see easily at the Arboretum are: 'Donald Wyman' planted in mass in the first parking lot bay across from the Oswald Visitor Center; 'Adirondack' marks the entrance to the espalier in the Cloister Herb Garden; 'PrairiFire' makes the double allée at the Sensory Garden, 'Pink Spires' flanks the entry to the new Green Play Yard at the Andrus Learning Center, and two 'Prairie Maid' trees fill an island in the staff parking lot.

Mary H. Meyer, extension horticulturist and professor, University of Minnesota

Mary Meyer

Photo 1: Prairie Maid close-up

Mary Meyer

Photo 2: Prairie Maid - whole tree

Mary Meyer

Photo 3: PrairiFire close-up

Mary Meyerd

Photo 4: PrairiFire - Whole tree (left) Sargentii espalier (right)

Mary Meyer

Photo 5: Professor Sprenger close-up

Mary Meyer

Photo 6: Professor Sprenger - whole tree

Mary Meyer

Photo 7: Red Jewel close-up

Mary Meyer

Photo 8: Red Jewel - whole tree

Mary Meyer

Photo 9: Royal Raindrops close-up

Mary Meyer

Photo 10: Royal Raindrops - whole tree

Mary Meyer

Photo 11: Sargentii close-up

Mary Meyer

Photo 12: Sugar Tyme close-up

Mary Meyer

Photo 13: Sugar Tyme - whole tree

This summer I was asked so many times "What is wrong with my crabapple?" that I started LOOKING anew at crabapples. 2014 was a banner year for apple scab, discoloring the foliage and causing premature leaf and even fruit drop. Affected plants looked dormant, or as many homeowners feared, dead. Apple scab can weaken trees, but rarely is fatal. Scab may allow secondary organisms to attack the tree and can decrease its winter hardiness, so it is best to purchase a scab-resistant crabapple. WHICH crabapples are resistant to scab, is complicated as the newest study (Beckerman et al, 2010) shows a new strain of this disease may now infect previously resistant cultivars. Additionally, we tend to think only about the FLOWERS on crabapples, and especially LOVE the showy pink or red flowers that are unfortunately often more susceptible to scab.

I recommend the first criteria for selecting a crabapple should be the ultimate SIZE, height and shape of the plant, followed by scab resistance, fruit, and finally the flowers. It is a misconception that crabapple fruit is messy: the small colorful fruit (5/8 inch or less) is a valuable food source sought by birds throughout the winter, and adds color and interest for many months.

Late fall is an ideal time to walk the crabapple collection at the Minnesota Landscape Arboretum and pick your favorite, because you can easily select scab resistant plants with clean, healthy leaves. Additionally, you can evaluate the fruit which varies widely in color and can last for MONTHS, easily six or even eight months. Remember that crabapple flowers last only for DAYS, perhaps a week. Why not select a crabapple for healthy leaves, and attractive fruit and just let the flowers be an added bonus?

From endless lists of hundreds of crabapples, the short list below was developed based on fall appearance with clean foliage at the Arboretum. Additionally, these plants are also top recommendations from long-term research trials conducted at the Ohio State University (their Crablandia field plots); Morton Arboretum, Illinois; Purdue University, Indiana; and the multi-state National Crabapple Trials.

I propose these 13 crabapples as "the best" for Minnesota. If your favorite is not here, let me know! All of the plants listed show good to excellent resistance to apple scab with an asterisk * for those showing some susceptibility to the new apple scab strain now in the Midwest. 'Red Splendor' is a showy red-pink prolific flowering crabapple that originated in Minnesota, however, it is susceptible to scab, needs plenty of space due to its large size and can be defoliated and defruited in mid-summer due to scab.

Which one would I plant in my yard? Anyone from this list, but something in the name 'Professor Sprenger' does resonate with me! It is a lovely tree that greets visitors on the Snyder Terrace at the Arboretum. Others you can see easily at the Arboretum are: 'Donald Wyman' planted in mass in the first parking lot bay across from the Oswald Visitor Center; 'Adirondack' marks the entrance to the espalier in the Cloister Herb Garden; 'PrairiFire' makes the double allée at the Sensory Garden, 'Pink Spires' flanks the entry to the new Green Play Yard at the Andrus Learning Center, and two 'Prairie Maid' trees fill an island in the staff parking lot.

There is an amazing variation that exists in these tough plants. A crabapple that grows well in Ohio, may not show the same disease resistance to apple scab here in Minnesota. The weather and climate makes a difference. Touring the famous crabapple collection at the Arboretum and the newer plantings in the display gardens can give you a first-hand look at how the plants grow in our climate. Ideally, we would annually rate crabapples three times: for foliage and fruit in September and October; for winter interest and fruit (bird food) in January; and flowering in May. Look for yourself at a garden center or the Arboretum, so you can decide which form and fruit is best for your garden and landscape.

Millipedes in vegetables

Jeffrey Hahn, Extension Entomologist

Jerry Wenzel

Photo 1: Despite the circumstantial evidence, the millipedes did not damage this carrot; they are taking advantage of previous damage.

A couple of home gardeners encountered millipedes in some of their vegetables during October. In one case they were in a few potatoes, in another instance they were infesting a carrot. There was concern whether the millipedes were attacking healthy vegetables. Fortunately, the millipedes were not causing damage in the garden. They have weak mouthparts and are only capable of feeding on decaying organic matter. It is possible for them to feed on plants that have already been damaged but they are not attacking healthy plants.

A 2012 research article in the Journal of Applied Entomology looked at the potential of millipedes and wireworms to attack carrots (also sweet potatoes). They found the presence of the millipedes was associated with wireworm damage to carrots. The millipedes themselves were not causing damage but were there as a result of preexisting wireworm injury. That is also what is probably happening with the presence of millipedes in the potatoes. The millipedes were not damaging the tubers but were there because of other damage (probably wireworms).

Fortunately wireworm damage is not common in home gardens and this kind of injury (as well invasion by millipedes) should not be a problem very often.

Don't worry about snowfleas

Jeffrey Hahn, Extension Entomologist

Diane Peterson

Photo 1: These strange black lines are composed of large numbers of snowfleas

A couple of homeowners discovered an odd situation in their lawns during mid to late October. From a distance, they could see long, black lines in the grass. Upon closer inspection, they discovered that the black lines were actually due to many tiny insects. Examining the insects under magnification revealed that they were snowfleas, a type of springtail.

Springtails get their name because of their ability to jump. They feed on decaying organic matter as well as fungi, pollen, and algae. They are very abundant insects but because of their small size and that they are usually found in leaf litter, soil, and other generally hidden places, people do not usually notice them. Until, that is, they occur in large numbers.

Jeff Hahn, Univ. of Minnesota Extension

Photo 2: Snowfleas are most commonly seen on top of snow.

Snowfleas are particularly interesting because they are cold tolerant. They are typically seen during late winter and early spring as the snow starts to melt and they congregate, often in large numbers.  Fortunately, whether you see snowfleas now or on top of snow later, they are harmless to turf and should be ignored. They will eventually go away on their own.

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