We’ve begun to see some pine mortalities from the pinewood nematode, the cause of a vascular disease called pine wilt. The 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. Watering does not help because the plant cannot absorb the water.
In Illinois, we see pine wilt on all pine species except white pine. The disease affects entire branches or trees, 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. If you are uncertain of the differences, consult issue no. 1 of this newsletter.
Pinewood nematodes are vectored (spread) by Sawyer beetles and a few related longhorned beetles. Readers may know nematodes as soil- or root-related pathogens, but this nematode lives in the tree’s wood, blocking 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 clinic or another lab with a nematologist. 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. The pinewood nematode is not uniformly distributed within a tree. We find that the most reliable samples are from branches with brown needles still attached. When sampling Austrian pine, 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 dry it before placing it near pines.) Prune dead branches from live trees to minimize attractiveness to beetles. Beetles emerging from the dead wood may carry the nematode and fly to healthy pines miles away. When a beetle feeds on a healthy pine, it may transmit the nematode. The nematode enters the resin canal and eventually clogs the water transport system of the tree.
All pines in Illinois, except white pine, are susceptible to pine wilt. Because white pines have problems of their own, that species is not encouraged as a replacement unless the site has been carefully selected to avoid stress (see issue no. 4). 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. Consult Report on Plant Disease (RPD) no. 1104, “Pine Wilt Disease,” for details. RPDs are available in Extension offices and on the Web at http://www.ag.uiuc.edu/~vista/horticul.htm.