Introduction
Pine wilt disease is an affliction of pine caused by the pine wood nematode, Bursaphelenchus xylophilus. The pine wood nematode is a microscopic parasite. It is transmitted to susceptible pine trees by pine sawyer beetles, which feed on the young shoots of healthy pines, while laying their eggs on dead or weakened pines. The pine wood nematode is native to North America. It was first documented in 1934, when it was observed infesting diseased timber. The nematode was initially associated with the fungi residing within the timber. The nematode was discovered once again in 1979. This time the nematode was reported in Missouri, where it was inducing pine wilt disease in numerous pines. Researchers have since determined that the pine wood nematode poses a significant problem for some exotic pines. It is not a concern for native pines. Pine wilt disease progresses quickly, with most trees succumbing to infection within a few months.
Distribution & Habitat
The pine wood nematode is widely distributed throughout North America. It has been reported in the United States, Canada, and Mexico. In the United States, the nematode may be found wherever there are susceptible pine trees, though infections are most common in the northeastern and midwestern states. The pine wood nematode was inadvertently introduced to Asia, where it has since decimated pine tree populations across Japan, China, Korea, and Taiwan.
Hosts
Pine wilt disease occurs on Scots pine, Austrian pine, jack pine, mugo pine, black pine, and Japanese red pine. Scots pine is the most vulnerable to infection. Infections have occasionally been reported on white pine. Pines that are undergoing environmental stress, or are infested by insects are more prone to infection.
Disease Cycle
Pine wilt disease results from interactions that take place between the pine wood nematode, and the pine sawyer beetle. Spread of the pine wood nematode occurs via the pine sawyer beetle. The nematodes are carried in the tracheae of the beetle’s respiratory system. Some of the beetles navigate to healthy pines to feed. As the beetles commence feeding on the branches of healthy pines, they create feeding wounds that the nematodes use to infiltrate the host. Other beetles migrate to dead, and dying trees for oviposition, or egg laying. Trees with bark are required for oviposition, and larvae development. Once the larvae hatch, they feed for several weeks in the cambial wood. As the larvae develop, they bore into the sapwood, where they continue to feed. The nematodes are deposited on the tree, along with the eggs.
In both scenarios, the nematodes migrate to the tree bole through cortical resin canals, or xylem resin canals in the branches. In young trees, the nematodes utilize the cortical resin canals. Trees that exceed four years of age lack cornical tissue. As such, in mature trees, the nematodes seek out the xylem resin tissue instead. As the nematodes advance downwards through the tree, they feed on the surrounding tissue, effectively destroying it. When not feasting on the plant cells, the nematodes consume blue-stain fungi, and other fungi that have colonized the wood. These wood invading fungi are often introduced to trees via bark beetles once the host has been weakened or killed. The nematodes tend to feed more vigorously when a blue-stain fungus is present within the wood. This phase of the disease generally culminates in tree mortality.
The pine wood nematode is a microscopic unsegmented worm that measures just under ½ an inch in length. The nematode engages in a propagative cycle, and a dispersal cycle. Under favorable conditions, the nematodes reproduce. They develop quickly, progressing through six stages: an egg stage, four instars, and an adult stage. If there is adequate soil moisture, temperature, and nourishment, the nematodes may pass into the adult stage within four to five days. As the eggs hatch, they reveal masses of tiny worms, which soon molt into third stage larvae. There are two forms of third stage larvae. Some morph into fourth stage larvae, eventually maturing into adults that remain within the infected trees. Others enter a non-feeding dispersal cycle. The dispersal cycle is triggered when the host tree expires, or nutritional resources have become scarce. These larvae, also referred to as dispersal larvae, crawl to the pupal chambers of the pine sawyer beetles, and migrate toward the insects’ tracheae, which is located in the spiracles. Once they have settled in the insects’ tracheae, the larvae molt into dauerlarvae. The dauerlarvae is a non-feeding larval stage that is specialized for survival during the transport phase of the dispersal cycle. Upon completing pupation, the beetles chew their way through the host’s sapwood, and vacate the tree. Some of the beetles carry the nematodes to susceptible trees, where they feed on healthy foliage. Others travel to dead, or dying trees to lay their eggs. Once the nematodes have been vectored to new hosts, they molt into adults, and the cycle begins anew.
Symptoms of Infection
Pine wood nematodes kill host trees by feeding on the cells surrounding the resin ducts. The vigorous feeding of the nematodes causes resin to leak into the tracheids. The tracheids are water conducting cells in the xylem. This results in the formation of air pockets within the tree’s vascular system. The air pockets prevent the tree from distributing water throughout the crown, causing it to wilt, and die. Wilting begins in the top of the tree, and progresses downwards. This is a distinguishing characteristic of pine wilt disease. Many other needle diseases affect the lower branches first. Needle discoloration is the first visible symptom of infection. Affected needles turn greyish-green, before transitioning to tan or brown. Wilted needles are not shed from the branch, another characteristic trait of the disease. Occasionally, a branch may die back, with disease progression not becoming apparent until the following year.
Diagnosis of pine wilt disease requires identification of the nematode. This can obtained from cross-sections of symptomatic limbs soaked in water. Limbs should measure at least one inch in diameter. Wood chips may also be collected for analysis, and soaked in water. To increase the probability of detection, several test samples should be acquired from different areas of the tree. Once the wood samples have been collected and submerged in water, a microscopic examination can be conducted on the liquid. Several bacterial and fungal feeding nematodes may be discovered in the water. Not all of these species are causal agents of pine wilt disease.
Management
- Avoid planting susceptible tree species in landscape settings.
- When planting, select resistant tree species. Some of the tree species that exhibit resistance to pine wilt disease are balsam fir, concolor fir, nikko fir, Korean fir, Chinese juniper, Rocky Mountain juniper, eastern red cedar, Norway spruce, white spruce, Serbian spruce, Colorado spruce, Chinese white pine, jack pine, lacebark pine, Swiss stone pine, pinyon pine, lodgepole pine, limber pine, Bosnian pine, Korean pine, mugo pine, Japanese white pine, ponderosa pine, Balkan pine, Virginia pine, Douglas fir, Japanese yew, anglojap yew, eastern arbovitae, western arbovitae, and eastern hemlock.
- Cull symptomatic trees to reduce the spread of the nematodes, and beetles. Remove and dispose of dead, dying, or freshly cut wood to deter the pine sawyer beetles.
- Maintain tree vigor through sound cultural practices. Ensure that trees are sufficiently watered, especially during extended periods of drought. Apply a layer of organic mulch around the base of susceptible trees to improve soil quality, moderate soil temperature, and retain soil moisture.
- Due to the rapidity at which the pine wood nematode spreads, nematicide and insecticide treatments are often costly, and ineffective.
- Heat treatments have proven effective at eliminating the pine wood nematode from wood products. To ensure success, infected wood must be heated to a core temperature of 132.8°F.
Photo courtesy of Kansas Forest Service.
