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Extension > Yard and Garden News > Pine Wilt

Monday, November 10, 2014

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 xylophilus). 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.

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