Not everyone has heard of nematodes, and certainly many have never seen one. They are common in soil and may help decompose organic matter. Although most nematode species are not harmful, some nematodes are parasitic on animals (for example, pinworms, heartworms), and some are plant parasites.
Pine wilt is a disease of pine that is caused by a nematode called the pinewood nematode. This nematode is the pathogen causing the pine wilt disease, just as a fungus or bacterium can be a pathogen causing other diseases. 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 pinewood nematode is microscopic and causes blockage of the water-conducting tissues, resulting in a wilt symptom, much like the fungal wilt diseases. You are not able to see the nematodes without a microscope, but you can see symptoms of infection.
Pine wilt appears as a sudden decline and death of the entire tree within a few weeks or months after the first symptoms of disease. Needle color progresses from off-green to gray-green to completely brown in from a few weeks up to a few months. The affected trees show this 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 blight (see issue No. 3) but without the diagnostic fruiting bodies of that fungal disease. 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 will be able to distinguish nematode species to know whether the 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 both ends of 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.
There are no chemical controls that have been shown to be effective for pine wilt. Infected trees should be burned or buried to reduce reservoirs of infection. Prune dead branches from live, uninfected 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 causes blockage of the water-transport system of the tree.
Often the question arises as to whether pinewood nematode infested wood can be chipped and safely used for mulch. Research to this point would lead us to believe that such a practice should be fairly safe. It has been shown that the insects do not survive the chipping process, that the nematode does not infect through the roots, and that the insect 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 Diseases (RPD) No. 1104 for details about this disease. The RPD is also available on the Web at http://www.ag.uiuc.edu/~vista/horticul.htm