A lethal disease of pines has shown it ugly head again this year. The disease is pine wilt, caused by a nematode pathogen. Affected trees become gray–green (dull green) and then rapidly turn brown. This process may take from a month to a year. Once a tree is infected, it dies. Pine wilt does not allow any recovery periods.
Not everyone has heard of nematodes, and certainly many have never seen them. Those causing plant disease, including the pinewood nematode, are generally microscopic. As with fungi and bacteria, many nematode species commonly occur in the soil, where they may help decompose organic matter. Also similar to fungi and bacteria, most nematode species are not harmful to plants. There are still hundreds of nematode species that can be serious problems on plants. One of these is the pinewood nematode, capable of causing the death of a mature pine.
Many readers will be familiar with nematodes as soil or root-related pathogens; but in the case of pinewood nematodes, the nematode lives in the wood of the tree. The pinewood nematode is microscopic and causes blockage of the water-conducting tissues, resulting in a wilt symptom, much like the fungal wilt diseases. You will not be able to see the nematodes without a microscope, but you can see symptoms of infection.
Most pine species grown in Illinois are susceptible to pine wilt. White pine is not affected. Affected trees show foliar color change, either branch by branch or over the entire tree. The exception may be Austrian pine. We have seen cases where infected Austrian pines initially showed symptoms on branch tips only. This may resemble injury from Sphaeropsis (Diplodia) blight, but without the diagnostic fruiting bodies of that fungal disease. Some other conditions can be confused with pine wilt. 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.
Samples to be tested for pine wilt should be sent to the Plant Clinic or another lab where a nematologist is available. The nematologist can distinguish nematode species to know whether harmless or parasitic nematodes are present. The Plant Clinic fee for pinewood nematode assays is $18.75. 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 the branch. This nematode is not uniformly distributed within a tree. We find that the most reliable samples are from branches that have brown needles still attached.
No chemical controls have been effective for pine wilt. Infected trees should be removed quickly and burned or buried to reduce reservoirs of infection. Pine wilt is a disease of pine that is vectored (spread) by the Sawyer beetle and a few related long-horned beetles. 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 causes blockage of the water-transport system of the tree. Prune dead branches from live, uninfected trees to minimize attractiveness to beetle feeding.
Often the question arises as to whether wood infested with pinewood nematodes can be chipped and safely used for mulch. Research would lead us to believe that such a practice should be fairly safe. The vector does not survive the chipping process, the nematode does not infect through the roots, and the insect vector does not frequent wood chips. Logically, there is no way for the nematode to move out of the chips and into the tree. To be safe, spread the mulch on a concrete or blacktop surface and let it dry before use, or put it through a true compost cycle.
Replace dead pines with Norway or blue spruce, Douglas-fir, cedar, hemlock, or other nonsusceptible species adapted to the site. Consult Report on Plant Disease (RPD), no. 1104, for details about this disease. The RPD is available at http://www.ag.uiuc.edu/~vista/horticul.htm.