Pine Wilt Disease
by Don Janssen, Extension Educator
Sudden death of Scotch pines (Scots pine) or some other evergreens may be Pine Wilt Disease caused by the pinewood nematode. Other pine species occasionally killed by pine wilt include Austrian (Pinus nigra), jack (P. banksiana), mugo (P. mugo), and red (P. resinosa) pines, and rarely in white pine (P. strobus). In the Midwest more than 90 percent of the trees killed by pine wilt have been Scotch pine. In Nebraska an estimated 5000 pines have been killed
Pine wilt is caused by a microscopic, worm-like organism called the pinewood nematode. These nematodes live in pines and are carried from tree to tree by insects called pine sawyer beetles. Once inside the tree, the nematode disrupts the flow of sap, causing the tree to turn brown and die.
The symptoms on the trees are minimal but dramatic. If a tree is infected in the fall, brown limb die-outs (flagging) in or near the crown, or an absence of candle growth will usually occur in the spring. The most noticeable symptom appears in mid to late summer when the greatest need for moisture is required. In a matter of four to six weeks the tree becomes a dull green, to yellow, and finally brown.
Another symptom is the reduction of resin production. When branches from a healthy tree are cut, a thick, sticky resin is produced at the site of the wound. On a diseased tree, resin may be absent, so branches and twigs will become dry and break easily.
The pine sawyer beetles are wood borers. They emerge in May or June from infested and non-infested pine trees, and fly to new host trees, where they feed on the bark of young pine shoots. As the nematode carrying beetles feed and deposit eggs within the tree, the nematodes are transferred to the healthy tree and move to the water-conducting tissue of the pine. Their populations increase rapidly, and block the resin canals. Within four to five weeks, the nematodes will spread throughout the tree. Infested trees usually die within three months.
A chemical treatment to protect healthy pines may become available in the future but its expense will limit its use. For now, to control the spread of the disease, you must remove dead trees and burn, chip or bury the wood before the beetles emerge. If you spot dead trees in the late fall, you can wait until April to remove them because the beetles will not emerge until the weather warms in the spring. The key here is to stop the reproduction of nematodes and beetles.
(This resource was updated May 2006 and appeared in the Lincoln Journal Star Newspaper Sunday edition. For information on reproducing this article or using any photographs or graphics, read the Terms of Use statement)
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