Most of the early horticultural samples at the Plant Clinic have involved evergreens. A good number of the pines have been infested with pinewood nematodes, the cause of pine wilt disease. We have already seen many more positive cases of this disease than occurred last spring, and I have no explanation. The spring of 1998 was a particularly light year for pine wilt at the Plant Clinic.
Pine wilt causes a sudden decline and death of an entire tree within a few weeks to several months after the first sign of disease. Infected trees will show symptoms much like the fungal vascular wilts on deciduous trees. There may be a branch-by-branch decline in one section of the tree, or the tree may appear to die all at once. If the disease is seen early in its development, needles appear light grayish green, but the needles quickly become brown. The disease affects entire branches on all pines except Austrian pine. We have documented cases of pinewood nematodes in Austrian pine that initially showed symptoms on branch tips only. This symptom can be confused with Sphaeropsis blight.
Pinewood nematodes are 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. The nematodes are not visible with the naked eye, but symptoms are quite apparent.
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 $18.50. Branch samples should be 1 to 2 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. The most reliable samples are from branches with brown needles still attached. When sampling Austrian pine, also include the terminal 12 inches of a stem with brown needles 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 for 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.
All pines that we grow in Illinois, with the exception of white pine, are susceptible to pine wilt. White pines have many problems with Illinois soils and temperatures and are not encouraged as replacement tree species. Replace dead pines with Norway or blue spruce, Douglas fir, cedar, hemlock, or other nonsusceptible species. Consider the site, soil, and space when selecting a replacement tree. Consult Report on Plant Disease No. 1104 for details about this disease.