Pine mortalities from the pinewood nematode, the cause of a vascular disease called pine wilt, are submitted at all times of the year. The greatest number seems to coincide with stress. The pine wilt name appropriately describes the sudden gray–green, wilted appearance of limbs or entire mature pine trees. There is no recovery, and trees quickly turn brown in heat. Trees do not recover with watering because they cannot absorb the water.
Pine wilt is most common on Scotch and Austrian pines in Illinois. Still, it may infect all pine species except white pine. The disease affects entire branches or entire trees and not just branch tips. Of course, there is always an exception; and in this case it is Austrian pine. We have documented cases of pine-wood nematodes in Austrian pine that initially showed symptoms on branch tips only. This symptom can be confused with Sphaeropsis blight. Sphaeropsis was discussed in issue no. 4 of this newsletter.
Although many nematodes infest the soil and roots, the pinewood nematode is present in the wood. It does not move into the root system of pines. Pinewood nematodes are vectored (spread) by Sawyer beetles and a few related long-horned beetles that feed at the top of trees moving the nematode from tree to tree as the beetle feeds. The nematode is microscopic and causes blockage of the water-conducting tissues—resulting in a wilt symptom. Nematodes are not visible to the naked eye, but symptoms are 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 $18.75. 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. 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. 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 to beetle feeding. Beetles that emerge from the dead wood may carry the nematode and fly to healthy pines several miles away.
Replace dead pines with Norway or blue spruce, Douglas-fir, cedar, hemlock, or other nonhost species. Consider the site, soil, and space when selecting a replacement tree. Consult Report on Plant Disease (RPD), no. 1104, “Pine Wilt Disease,” for details about this disease. RPDs are available in Extension offices, as well as on the Web at the Extension VISTA Web site.