|male with spicule visible|
(Steiner & Buhrer) Nickle
Bursaphelenchus xylophilus, commonly known as pine wood nematode or pine wilt nematode (PWN), is a species of nematode that infects pine trees and causes the disease pine wilt. It is probably native to North America, where it was first described from a longleaf pine in Louisiana. It occurs in much of the United States, Canada, and Mexico. It also occurs in Japan, China, Taiwan, Korea, and Portugal.
This nematode causes pine wilt, a disease first identified in Japan in 1905. The nematode was described in 1934 and was identified as the cause of the disease in 1979. Pine wilt nematode epidemics have occurred in Japan, especially during warm, dry summers.
Species of the genus Bursaphelenchus are difficult to distinguish because they are similar in morphology. A positive identification can be made with molecular analyses such as restriction fragment length polymorphism (RFLP).
The pine wilt nematode has a typical nematode life cycle, with four juvenile stages and an adult stage with both male and female individuals that reproduce sexually. The mycophagous phase of the life cycle takes place in dead or dying wood, where the nematodes live and feed upon fungi, rather than the wood itself. The nematode cannot travel outside of the wood independently; it must be transported by an insect vector.
B. xylophilus has the shortest life cycle of any known parasitic nematode. In laboratory studies in which it is cultured on fungal media, its life cycle is completed in four days. In nature 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. If living tree cells are no longer available the parasite feeds and reproduces on the fungal hyphae growing in the resin canals. In the fall and winter the parasite becomes inactive.
The pine wilt nematode is vectored by a number of bark beetles and wood borers, and is most often associated with beetles in the genus Monochamus, the pine sawyers. Pine sawyers lay their eggs in the bark of dead timber. 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.
In primary transmission, when the beetle feeds on a susceptible host pine, the pine wilt nematode enters the tree and feeds on the epithelial cells which line the resin ducts. This is referred to as the phytophagous phase of the nematode, and it results in pine wilt disease. Water transport in the tissues of the infested tree is disrupted, and the disease can manifest within a few weeks. Signs include browning of the needles or yellowing of the leaves, and the tree may die within two to three months. Susceptible pine species include Scots, slash, Japanese red, and Japanese black pines. The slash pine, the only susceptible species native to North America, is weakly susceptible compared to other species. In North America, pine wilt occurs most often in landscaping settings where nonnative pines are cultivated.
Embargoes have been placed on untreated lumber from the United States and Canada. Management practices have concentrated on preventing the spread of the nematode. Infected trees are cut and either burned or chipped, soft wood timber is stripped of its bark to prevent oviposition by vectors, and all lumber shipped overseas is either fumigated or kiln-dried. Despite these preventative measures the nematode has been confirmed in Portugal, and may spread in Europe.
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- Bursaphelenchus xylophilus, Pine Wilt Nematode. Nematology. University of Nebraska, Lincoln.
- Bursaphelenchus xylophilus. Nemaplex. UC Davis.
- Cram, M. and J. Hanson. How to Identify and Manage Pine Wilt Disease and Treat Wood Products Infested by the Pinewood Nematodes. NA-FR-01-04. USDA, Forest Service
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