At this time of year, it is easy to spot trees affected with pine wilt. The disease causes sudden appearance of off-color needles or a quick change from green to brown needles on pines. Usually older trees are affected, with 15- to 20-year-old trees being the norm. In most cases, there are healthy trees of similar species around an affected tree. The affected tree sticks out like a sore thumb. Pine wilt might be confused with Sphaeropsis blight, but pine wilt affects the entire tree rather quickly. Salt injury can appear similar, but that injury is one-sided (exposed side of tree) and injures tips of branches first. Drought stress, root injury, and construction damage might also resemble pine wilt.
The “pine wilt” name appropriately describes the sudden gray–green, wilted appearance of 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.
Pinewood nematodes are vectored (spread) by Sawyer beetles and a few related long-horned beetles. The nematode lives in the wood of the tree. The nematode is microscopic and causes blockage of the water-conducting tissues, resulting in a wilt symptom. The nematodes are not visible with 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 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. 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 clogs the water-transport system of the tree.
All pines grown in Illinois except white pine are susceptible to pine wilt. Because white pines have many problems of their own (see issue no. 6 of this newsletter), that species would not be encouraged as a replacement species unless the site has been carefully selected to avoid stress problems. Replace dead pines with Norway or blue spruce, Douglas-fir, 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/%7Evista/horticul.htm.