By Dr. Thomas T. Yamashita
Pine wilt is a disease caused by infestations of the pinewood nematode, Bursaphelenchus xylophilus, and is often fatal within just a few months of infection. Just about every pine species is susceptible to pine wilt, though Austrian, jack, mugo, red, and Scots pine are particularly vulnerable. The disease can also be found in many species of cedar, fir, spruce, larch, and other conifers.
The symptoms of pine wilt are obvious, with wilting of tree needles that swiftly gives way to comprehensive yellowing of foliage, necrosis, and death of the tree.
Typically, the first noticeable symptom of pine wilt is, suitably enough, the wilting or fading of needles. This may be isolated to a few limbs high up in the tree, one side of the tree, or may impact the entire tree.
Impacted limbs will dry out very quickly—within a couple of days, cutting a twig from an affected limb will yield no sap from the site of the cut. Rather than being sticky, as is typically with conifers, the inner wood will be completely dry. Limbs often die so quickly that needles will continue to cling to dead branches for months.
The progress of the disease can be shocking in its rapidity. The tree will appear to fade, transitioning from a healthy gray-green, to a faded tan, and then brown. Death of the tree occurs anywhere from a few weeks to a few months from the time of infection, depending on the tree species. Trees are usually infected in spring, and the time of death generally ranges from summer through spring of the following year.
Pine wilt disease is caused by the feeding of the pinewood nematode, which is introduced by the pine sawyer beetle.
The main propagator of pine wilt is the pine sawyer (Monochamus spp.), a genus of longhorned wood-boring beetles which feed on pine trees. The common vector species for pine wilt in the United States are M. scutellatus (commonly known as the white-spotted or spruce sawyer) as well as M. carolinensis. Because pine sawyers are particularly attracted to distressed and dying trees—whose cracked and peeling bark make it easy for sawyer larvae to access a meal of inner bark, cambium, and sapwood—they often cross paths with the pinewood nematode (B. xylophilus).
When the larval sawyers mature and emerge from the tree that has served as their cradle, their respiratory systems have been colonized by thousands of pinewood nematodes—each measuring no more than 1 mm in length and 1/40th of a millimeter in diameter. The small size of the pinewood nematode allows for easy transport of even very large colonies. As many as 175,000 larval nematodes have been found on a single sawyer beetle.
As the newly fledged sawyers migrate to new trees, the accompanying nematodes enter through the feeding holes conveniently made by their former hosts. Once in the tree, the nematodes begin to reproduce and feed in the resin canals of the tree, before spreading to adjacent cortical tissues, xylem resin canals, and vascular tissue.
In addition to the damage inflicted by their feeding, pinewood nematodes often carry with them Pseudomonas syringae and related subspecies of bacteria, which cause bacterial canker. Pine wilt is the consequence of the damage inflicted by innumerable pinewood nematodes and Pseudomonas.
After the tree has succumbed and food runs low, nematode juveniles will transition into a non-feeding dauer larvae stage, characterized by some researchers as a ‘dispersal mode.’ These individuals are particularly hardy and can withstand conditions that would otherwise kill them.
Once in this stage, the larvae will congregate around the pupae of larval sawyer beetles and colonize the beetles as they emerge. Together, the next generation of pine sawyers, nematodes, and Pseudomonas alight to the next host tree.
Pine wilt is widespread, and positive diagnosis requires laboratory testing of tissues from affected trees.
Pinewood nematodes and the pine wilt disease they inflict can be found throughout the United States, but are most commonly found throughout the Midwest. As the disease originated in the Americas, native pine species are not as susceptible. But non-native species farmed for Christmas trees or used for landscaping are extremely vulnerable.
To diagnose the disease, it’s necessary to secure cross-sections from limbs measuring at least 1 inch in diameter, preferably from recently infected trees. The density of nematode populations can vary throughout a single tree—but may number more than 1,000 individuals per gram of wood—so multiple samples must be taken. Samples are soaked in water for a period of 24 to 48 hours, and then the water is examined under a microscope for the presence of B. xylophilus.
It should be noted that pine trees can be host to many species of nematodes simultaneously. Thus, an expert eye is required to confirm the presence of the pinewood nematode and avoid making an errant diagnosis.
Controlling pine wilt requires a sanitation-based approach to mitigating the spread of disease.
There is no cure for pine wilt disease. Even if a tree has only recently been infected, there is no intervention which will prevent it from succumbing to the disease. Thus, it’s necessary to carefully monitor trees for signs of infection, such as paling of needle or an unusual number of dropped limbs. Trees older than 10 years of age are particularly prone to infection.
If you suspect the presence of the disease, gather samples as described above and send them to a diagnostics lab. In the case that you have confirmed the presence of the disease, any infected trees must be cut down and disposed of by burning, burying, or chipping the tree. Stumps should also be removed, and any wood from infected trees should not be retained for use as firewood or other purposes. Intact lumber can serve as a vector for the disease.
In addition, avoid the planting of non-native species of pine, if possible. Native tree species can tolerate pinewood nematodes quite well.
A few years ago, a preventative insecticide/nematicide, known by the generic name abamectin, was brought to market. Abamectin cannot be used to treat infected trees; it is only for use in preventing infection. However, it is extremely expensive, as large quantities must be administered via injection directly into the tree trunks of uninfected trees. Thus, this treatment is only suitable for use with very high value trees.