If one were to rank the relative abundance of microorganisms in soil, from viruses and bacteria to fungi and mites, the group of non-segmented worms collectively known as nematodes would rank within the top seven groups. Nematodes were first described early in the 19th century, and then classified and reclassified a number of times until almost the 20th century when they were finally classified as a separate phylum, one of the most abundant phyla in the animal kingdom. Most nematodes are microscopic, but at least one of 8 meters in length was discovered in the reproductive tract of a whale.
There may be as many as 1.7 million different species. Most sources, however, figure the number may be closer to 100,000, nearly 20 percent of which are considered parasitic. Within any given gram of topsoil there are at least 10 and up to 100 or more individual nematodes with a combined mass of from 1 to 100 kilograms per hectare (kilograms per hectare is roughly the equivalent of pounds per acre). Bacteria and fungi are far more prevalent in soil ecosystems, but they share the same communities as nematodes, and many nematode species also rely on fungi as a source of food.
As a group, nematodes epitomize the metaphor that one bad apple will spoil the bunch. Even though most nematodes species are innocuous or beneficial, they get a bad rap for the relative few that are perniciously parasitic. Although relatively little is known about this abundant group of organisms, most nematodes are soil dwelling. It is difficult to generalize the circumstances that precipitate nematode injury to tree roots, but most serious infestations occur on sandy, nutrient-poor soils supporting a monoculture of an economically important species like citrus and other long-established fruit crops.
There are many instances where nematodes survive outside of a soil environment, even groups that live in the digestive tracts of humans. By some estimates, as much as 75 percent of the world’s population support nematodes in their intestines. Common nematode species that are known human parasites include: roundworm (Ascaris lumbricoides), hookworm (Necator Americanus) and pinworm (Enterobius vermicularis). Other species infest cats, dogs, horses and other livestock. A few nematode species are also known to live in trees.
In 1934, a species now known as the pinewood nematodes (Bursaphelenchus xylophilus) was first discovered in a pile of longleaf pine logs at a mill in Louisiana. The nematodes were associated with blue stain, a fairly common superficial fungus of the Ceratocystis genus that often develops on freshly cut pine surfaces during the summer months. The blue stain does not affect the integrity of wood, and the nematodes appeared to be feeding on the fungus, so it was forgotten. Forty-five years later, in 1979, B. xylophilus was rediscovered, this time on recently dead Scots pine near Columbia, Mo. The same species has been isolated on pines in Japan and China, where it is a serious, exotic and invasive pest akin to Dutch elm disease or gypsy moth here in the United States. Today, B. xylophilus is listed in the top 10 of quarantined species with at least 46 countries that restrict the import of green pine products and nursery stock of affected species.
What is most unusual about the pinewood nematode is its lifestyle, feeding and reproducing in the active xylem tissue of susceptible pines. No one is sure where the organism originated, but most scientists suspect that it is native to North America. It has been located in 36 states, but causes little or no damage to endemic coniferous species. This fact makes researchers all the more suspicious that the pinewood nematode coevolved with trees in North America, since most parasites eventually “learn” how to use a host without killing it. Another theory is that the pinewood nematode has yet to express their pathogenic ways in most of North America, especially in northern coniferous populations. Climate change, especially warming, will favor the pinewood nematodes, so researchers are keeping a close watch on this species in the North.
So far, the nematode has been discovered in 20 pine species, both native and naturalized, in the United States, including popular landscape trees such as Japanese red and black pines, and Scots and Austrian pine. Other conifers that host pinewood nematodes include balsam fir, larch, spruces, a couple of true cedars in the West and Douglas fir.
Another interesting habit of the pinewood nematode is the well-established commensal relationship it has with Ceratocystis fungi and a group of long-horned, wood-boring beetles collectively known as cerambycids, including the likes of the pine sawyer and the Asian longhorned beetle. If not for these relationships that provide the nematodes with a vector that allows it to move from host to host, it is doubtful the pinewood nematode could exist outside of a soil environment. The pine sawyer and other wood-boring beetles are attracted to monoterpenes and ethanol produced by stressed trees and by stems that have been recently felled. Pinewood nematodes feed on the products of photosynthesis in live trees but then switch to fungi as a food source in dead wood.
Nematodes are delivered to trees by the feeding and ovipositioning habits of bark beetles, which also serve as a vector for fungi. Once nematodes are deposited in active xylem tissue, they spread into the tree along resin canals, feeding on photosynthates by puncturing the thin-walled epithelial cells. Trees succumb due to a disruption of transpiration caused by the nematode’s behavior or due to a defensive response on the tree’s part. The foliage shows symptoms of drought then browning, sometimes within days of the nematode’s arrival. Such rapid progression is due to the nematode’s fecundity.
When a developing, long-horned, wood-boring beetle emerges from the bark it carries thousands of nematodes—sometimes hundreds of thousands—that have attached directly to the surface of the beetle. Both nematodes and blue stain spores are deposited in a new host tree usually a few hundred yards away, but as far away as a couple of miles, when the adult beetle flies off to locate a new host. Without the wood-boring beetle and the blue stain fungus, nematodes are severely limited in their ability to find a new host.
After the tree is dead, and there is no longer any living xylem cells as a food source, nematodes will feed on blue stain and other fungi that begin to colonize the wood. Thus the blue stain serves a dual role in the life of a pinewood nematode, first by helping them attach to adult wood-boring insects, then as a source of sustenance for nematodes when the tree is no longer capable of supplying carbohydrates. It is quite possible that this dual role played by Ceratocystis fungi is the bridge that allowed the pinewood nematodes to leave the soil.
When a susceptible tree is infested with nematodes, there are no cures. Although there is promise of an effective systemic nematicide, so far the only available compounds require treatment to commence within days after the nematodes arrive at the host. The standing recommendation for treatment of susceptible trees is to keep them as vigorous as possible, and if they show any signs of wilting, with no other obvious symptoms, the tree should be removed from the landscape as soon as possible. All exposed stem surfaces should be buried, burned or, at a minimum, debarked so bark-boring beetles will not use the material as a nursery.
In regions that are known to host wood-boring beetles and pinewood nematodes, avoid planting susceptible conifer species. Since any pine is a potential host, keep a close eye on even somewhat resistant species.
Finally, if the landscape plan calls for pine and nothing else will do, select species known for resistance and make sure that plantings are kept vigorous. At the first indications of stress, look for signs of wood-boring insects. If trees have been visited by cerambycids, you must assume the specimens have also been infested with pinewood nematodes. At this point, your choices are few. Either begin treatment with a registered nematicide or cut the trees down and bury the wood.
The author is a professor and extension forester with the Rubenstein School of Environment and Natural Resources at the University of Vermont.