Itís about that time of year when this devastating disease of pines begins to become more apparent. We saw fewer cases of pine wilt this spring than in most years, but it will be interesting to see whether the near-continual rains over most of the state will delay the appearance of this perennial problem. Pine wilt is caused by the pinewood nematode. It is vectored (spread) by the sawyer beetle and a few related long-horned beetles. Many readers are familiar with nematodes as soil- or root-related pathogens, but in this case the nematode lives in the wood of the tree. The nematode is microscopic and causes blockage of the water-conducting tissues--resulting in a wilt symptom much like the fungal wilt diseases. The nematodes are not visible with the naked eye, but symptoms are quite apparent.
Pine wilt causes a sudden decline and death of an entire tree within a few weeks or a few months after the first sign of disease. Symptoms occur in four stages: Needles initially appear light grayish green, then yellowish green, then yellowish brown, and finally completely brown. An affected tree shows this color change either branch by branch or over the entire tree. The exception may be on Austrian pine--we have documented positive cases of pinewood nematodes in Austrian pine that initially showed symptoms on branch tips only. Pines with root problems, water-related stress, or cold injury decline from the top downward, or starting at the bottom and moving up the tree, or possibly from the tips inward. Needle color, however, does not progress from gray green to brown. Instead, necrosis is fairly quick with these noninfectious problems.
Samples to be tested for pine wilt should be sent to the Plant Clinic or another lab where a nematologist is available. Our fee is $15. Branch samples should be one to two inches in diameter and long enough to put into a vise so that wood discs can be cut from both ends of the branch. The pinewood nematode is not uniformly distributed within a tree. We find that the most reliable samples are from branches that have brown needles still attached.
There are no known effective chemical controls for pine wilt or its vector. Affected trees should be burned or buried to reduce reservoirs of infection. (Recent research shows that it is probably safe to chip the trees for mulch. Still, you might want to compost the mulch before use or spread it out to dry before placing it near pines.) Prune dead branches from live trees to minimize attractiveness to beetle feeding. Beetles that emerge from the dead wood may carry the nematode and fly to healthy pines several miles away. When the beetle feeds on a healthy pine, it may transmit the nematode to the tree through feeding wounds. The nematode enters the resin canal and eventually clogs the water transport system of the tree.
Replace dead pines with Norway or blue spruce, Douglas-fir, cedar, hemlock, or other nonsusceptible species. Consult Report on Plant Diseases No. 1104 for details about this disease.