Pine Wilt Disease: A Sad and Complex Story

Scots pine (Pinus sylvestris) is commonly grown in Missouri as a Christmas tree species and, until recently, has been widely planted for windbreaks or as an ornamental. A 30-year-old Scots pine, with its blue-green foliage, orange-colored bark and interesting canopy architecture makes a bold statement in the landscape. Unfortunately, because this species is highly susceptible to a problem called “Pine Wilt Disease” (PWD), it is rare to find a Scots pine old enough to create this effect. Because PWD rarely affects trees less than ten years old, it is not usually a problem for Christmas tree production. However, the wisdom of planting Scots pine in windbreaks or landscapes in Missouri is questionable.

Pine wilt has been known for about 100 years in Japan, where it has devastated forests of Japanese Black Pine (Pinus thunbergiana) and Japanese Red Pine (P. densiflora). Although the causal agent is likely native to North America, the disease was not diagnosed in Missouri until 1979, when a visiting Japanese scientist recognized the symptoms on a sample in the MU Plant Diagnostic Clinic. Since then, thousands of Scots pines in Iowa, Illinois, Missouri, Kentucky, eastern Kansas and southeastern Nebraska have died from PWD. Interestingly, there have been few reports of PWD from the neighboring states of Minnesota, Indiana and Ohio or in other regions of the country. A possible explanation for the severity of pine wilt in the lower Midwestern states is that high temperatures and frequent droughts in our region may stress trees, favoring infestation and rapid development of the disease. Scots pine is native to cooler climates, such as those of Norway, Scotland, western Asia and eastern Siberia. Our native shortleaf (P. echinata) and loblolly (P. taeda) pines are rarely affected by PWD. Other frequently planted pine species such as white (P. strobus) and Austrian (P. nigra) are also not commonly affected by the disease.

The disease cycle of PWD is complex. The main causal agent, the pinewood nematode (Bursephelenchus xylophylus), is a microscopic worm-like organism that feeds on the cells surrounding resin ducts in the xylem of pine wood. The nematode infests a healthy tree through wounds on young twigs created by the feeding of a beetle called the pine sawyer (Monochamus carolinensis). The pine sawyer beetle lays its eggs under the bark of dying pine trees. When the larvae hatch, they feed under the bark and then burrow into the wood to pupate. If the tree is infested with pinewood nematodes, the emerging adult pine sawyer beetle picks up nematodes in breathing tubes in its thorax. As it flies to feed on the foliage of a healthy tree, the beetle may carry thousands of nematodes that are ready to enter the new tree as soon as the feeding wounds have been created at the branch tips. Later, as the newly infested tree begins to decline, pine sawyer beetles will lay eggs under its bark to start the cycle again. To further complicate the story, two more organisms may affect the development of PWD. In addition to the pine sawyer, several species of secondary beetles are also attracted to dying pine trees. Some of these beetles carry with them a fungus, called the blue-stain fungus, that causes the wood to turn bluish as it colonizes. The blue-stain fungus is a preferred food of the pinewood nematode, and it accelerates the growth of the nematode when it is present. Once the nematode has infested a susceptible host like Scots pine, it multiplies rapidly, spreading throughout the tree and interfering with water movement through the xylem. An infested tree soon develops a gray green foliage color, and then the needles turn tan and persist on the dying tree. Another characteristic of infestation is the absence of pitch in the trunk and branches. The time between nematode infestation of a Scots pine and tree death ranges from a few weeks to 6 months, depending on weather conditions.

Unfortunately, the management options for PWD are limited. There are currently no insecticides or nematicides that have been proven effective and practical for controlling the disease. As always, the first step in management is definitive diagnosis. This involves soaking a sample of wood from the dying tree in water. The best sample is a 1-inch-thick slice of trunk 3-4 inches in diameter. Wood chips or branch pieces can also be used, but are not as reliable. If nematodes are present, they will exit the wood and will be visible in the water using at least 20 x magnification. Positive identification requires the services of a trained diagnostician or nematologist, since harmless species of nematodes may also inhabit pine wood. (See agebb.missouri.edu/pdc/ for information on the MU Extension Plant Diagnostic Clinic). If PWD is confirmed, it is important to remove and destroy the tree to reduce the potential that pine sawyer beetle may spread nematodes to healthy trees. Do not save logs from wilt-killed pines as firewood. Burn or bury the tree or chip it and allow the chip pile (at least 3 feet deep) to heat and then weather for at least 6 weeks before using the material as mulch. Do not pile chips around the trunks of trees, since there is a remote chance that nematodes could move from the chips into wounds in the bark.

For more information on Pine Wilt Disease, see muextension.missouri.edu/explore/miscpubs/mx0858.htm

Chris Starbuck, Woody Ornamental Horticulture, UMC (573) 882-9630