Pine wilt is a scary disease that can kill a mature pine in a season. We have been seeing this disease in Illinois for at least 25 years. It appears in Illinois each year; but there have already been two positive cases of pine wilt at the Plant Clinic this season, so it is time to once again review the symptoms.
Pine wilt is caused by a nematode called the pinewood nematode. 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 water.
In Illinois, pine wilt is most common on Scotch and Austrian pines. 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 pinewood 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. 3 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. The nematodes are not visible to 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 $18.75. 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. 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 is no cure other than to rapidly remove and burn or bury infected trees, which helps prevent the beetles from spreading the pathogen to healthy pines. Affected trees should be burned or buried to reduce reservoirs of infection. 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 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 found at http://www.ag.uiuc.edu/%7Evista/horticul.htm.