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Pine Wilt Continues to Kill Nebraska Trees
submitted by Sarah Browning, UNL Extension Educator
Evergreen trees can develop brown needles and branches in spring due to winter burn or from a Sphaeropsis tip blight infection. But the most serious cause of browning and death in evergreens is pine wilt, caused by the pine wood nematode. Many trees can be seen now in landscape plantings or windbreaks that are completely dead and brown, and a majority of these were probably killed by pine wilt.
Scotch pine is the main host of pine wilt, but the disease also occurs in Austrian, Jack, Mugo and Red pine. In the Midwest more than 90 percent of the trees killed by this condition have been Scots pine. The disease does not affect other conifers, such as spruces, firs, and red cedars or junipers.The pine wood nematode is a microscopic, worm-like animal that feeds on the living plant cells surrounding the water-conducting tissues of pine trees. This nematode is unusual, compared to other plant-parasitic nematodes, because it lives entirely in the above ground parts of the tree and never enters the soil. Once inside a susceptible pine tree the nematodes reproduce rapidly, and move throughout the tree.
As they destroy the water-conducting tissues the tree's water-moving system becomes clogged and resin flow slows, then stops. The tree then begins to display wilt symptoms and soon dies. This wilt typically kills Scots pine within a few weeks to a few months.Pinewood nematodes are transmitted from tree to tree by another native insect, the pine sawyer beetle, Monochamus spp. Also known as longhorn beetles, pine sawyers are attracted to the dying trees and the females lay their eggs under the bark. The grub-like larva (photo right) hatch out and tunnel through the wood of the dying tree. When the beetle larva pupates into an adult and begins to emerge from a pine wilt tree in spring, it is infected with thousands of the nematodes that enter its breathing holes. As the adult beetle flies to a new tree it carries the nematodes with it. Nematodes enter the new tree through feeding site wounds made by the adult beetle.
Once the pine sawyer beetle introduces the nematode, an infected tree typically dies within a few weeks or months. Some trees will fade during the summer, and more will begin to fade in August and September, continuing through the fall and into spring.Sanitation can prevent or slow the spread of pine wilt. Cut down infected trees before May 1st to prevent pine sawyer beetles from emerging and carrying the disease to other healthy trees. Do not hold the wood for firewood, instead burn, bury or chip the tree. The stump should be removed down to the ground, if possible.
A protective treatment for pine wilt is available. Greyhound (abamectin) an insecticide/nematicide product can be trunk-injected into uninfected trees. Treatment provides healthy trees about 95% protection from the disease for approximately three years. However, the treatment is expensive and will feasible only for high value trees. As an example, a tree with a trunk diameter of 10 inches would cost the owner about $200 to treat. Contact a local arborist for treatment prices.
University of Nebraska-Lincoln Extension in Lancaster County is your on-line yard and garden educational resource. The information on this Web site is valid for residents of southeastern Nebraska. It may or may not apply in your area. If you live outside southeastern Nebraska, visit your local Extension office
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