Pine Wilt (Wood) nematode
Overview
The pine wilt nematode (PWN), bursaphelenchus xylophilus , is a species of nematode that causes pine wilt disease. Nematodes are “roundworms” of which there are over 80,000 described species. Most plant-parasitic nematodes are associated with plant roots, but the pine wood nematode is found in the aboveground parts of the tree. It has the shortest life cycle of any known parasitic nematode. It reproduces most rapidly in the summer, producing large numbers of individuals that spread throughout the resin canal system of susceptible pines, into the trunk, the branches, and the roots. In the fall and winter the parasite becomes inactive. It was not known until 1972 that this new nematode species was the pathogen behind pine mortality.
The nematode cannot travel outside of the wood independently; it must be transported by an insect vector. The pine wilt nematode is spread by a number of bark beetles and wood borer but is most often associated with the pine sawyers beetle. Pine sawyers lay their eggs in the bark of the tree. The growing larva feeds on the wood and pupates in the resulting cavity. Nematodes of the third juvenile stage congregate in the cavity around the pupa, molt into the fourth juvenile stage, and invade the trachea of the adult beetle. During this dispersive stage the beetle transports the nematode to other trees.
The mechanism from initial nematode infection to the resultant death of the pine tree is not fully understood. In primary transmission, the pine wilt nematode is carried in the insect’s respiratory system, when the beetle feeds on a susceptible host pine, the nematodes emerge and enter the trees through feeding wounds created by the beetle and feeds on the epithelial cells which line the resin ducts. Nematodes kill the tree by feeding on the cells surrounding the resin ducts. This causes resin to leak into the tracheids, resulting in “tracheid cavitation” or air pockets in the water transport system. Just as a person cannot drink through a straw with holes in it, the tree cannot move water upward and consequently wilts and dies. Signs include browning of the needles or yellowing of the leaves, and the tree may die within two to three months.
Extent and Impat Of Infestation
The pine wilt nematode, is native to North America and is widely distributed in the United States with the highest incidence of the disease in the Midwest. The disease is most serious on pine species that are not native to North America. Scots, Austrian, and other non-native pines are often killed by this disease. Eastern white pine, a native pine, is also affected and may be killed by pine wilt disease. The nematode commonly infects other native pines and some native conifer species. However, most native species are resistant to the disease (e.g., native conifers may be infected and express little or no disease symptoms). Several months of hot, dry weather are necessary for pine wilt development and spread, thus climate change is expanding the range of the PWN. PWN spread in the early 20th century to Japan and in the latter half of the century to other areas of Asia, including China, Taiwan and Korea, as well as to Europe, including Portugal and Spain. Age of the tree also influences susceptibility, with an increased risk of developing pine wilt when trees are greater than 10 years of age. Worldwide, the problem is epidemic in the native pine forest of Japan and other parts of Asia, where their native pine forests that are at risk.
In the Midwestern United States, the disease has the greatest impact on homeowners and in planted natural areas due to the prevalence of susceptible, non-native, and poorly adapted pine species. Use of Scots pine in windbreaks and landscape plantings has been especially common in the Midwest, where this tree species was considered to be well adapted to the environmental conditions. Stands of single tree species (monocultures), in particular, provide an excellent breeding site for the insects and associated nematodes. Because Scots pines become more susceptible after they reach 10 years of age, large, well-established trees are disproportionately affected and their death has considerable impact on the landscape.
PWN disease has also had a big impact on U.S. exporters of wood products. It is listed as a quarantine pest by more than 40 countries. The European Union in 1993 imposed a ban on the import of untreated and unseasoned wood from areas where the pine wood nematode was known to occur. Softwood exports from the U.S. and Canada declined by approximately one-third and two-thirds, respectively, the following year.
The impact of pine wilt is greatest outside of the U.S. and Canada. The disease has had a devastating impact on native forests in Japan, China, Korea, and Taiwan. In Japan, where 28{c84f4263cff769fd6fda75e481f8c50ebc0ba520b41f3ff17aef52667440c57e} of the total pine forest area is estimated to be infected with PWN, tens of millions of dollars are spent annually to control the nematode. Additional tens of millions of Euros have been spent on control measures following the establishment of PWN in Portugal and Spain.
Status Of A Solution
The Foundation (ISF) has identified a candidate fungus and signed an MOU with the owner of this fungus to develop and commercialize it for forestry applications. Further testing, proof-of-concept, toxicology and product development is necessary to commercialize this fungus as a solution. Our Corporation will stay on top of the research and coordinate the development of manufacturing and marketing once an efficacious solution has been developed.
