Bursaphelenchus xylophilus

Bursaphelenchus xylophilus
male with spicule visible
Scientific classification
Kingdom:	Animalia
Phylum:	Nematoda
Class:	Secernentea
Subclass:	Tylenchia
Order:	Aphelenchida
Superfamily:	Aphelenchoidoidea
Family:	Parasitaphelenchidae
Subfamily:	Bursaphelenchinae
Genus:	Bursaphelenchus
Species:	B. xylophilus
Binomial name
Bursaphelenchus xylophilus (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.^[1][2] 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.^[2]

Pine wilt

This nematode causes pine wilt, a disease first identified in Japan in 1905. The nematode was described in 1934^[2] in the United States. In 1969, Japanese plant pathologists Tomoya Kiyohara (清原友也) and Yōzan Tokushige (徳重陽山) discovered the unfamiliar nematode in the dead pine trees at Kyushu.^[3] They tried inoculating the nematode to healthy pine trees. The pine trees are wilt and dead,^[4] so they conclude that the nematode is the pathogen of the pine wilt disease. Pine wilt nematode epidemics have occurred in Japan, especially during warm, dry summers.^[5][6][7][8]

Morphology

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).^[5][9]

B. xylophilus is distinguished by three characteristics: the spicule is flattened into a disc-shaped cucullus at the tip, the front vulval lip is flap-like, and the tail of the female is rounded.^[2]

Life cycle

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.^[5][9][10]

Vectors

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.^[5][9]

Host-parasite relationship

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.

Grade of resistance

Furuno(1982)^[11] observed standing pine trees in Japanese forest, and ranked their resistance to pine wood nematode.The "Highly resistance" pines are rarely killed by the nematode , but they killed weakn condition or young sapling.^[5][11]

Highly resistance

Pinus taeda, P. elliotti, P. palustris, P. rigida, P. taiwanensis

Low resistance

P. strobus, P. massoniana, P. resinosa, P. tabulaeformis, P. banksiana, P. contorta, P. thumb×P. masso

Low susceptible

P. bungeana, P. monticola, P. parviflora, P. strobiformis, P. densiflora

, P. pinaster, P. sylvestris, P. ponderosa, P. rudis, P. pseudostrobus, P. oocarpa, P. radiata, P. greggii

Highly susceptible

P. koraiensis, P. leiophylla, P. luchuensis, P. thunbergii, P. nigra, P. mugo, P. khasya, P. muricata

Management

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.^{[5][6][7][8][9]}

Gallery

• 

  Pointed tail end of the male

• 

  Austrian pine with pine wilt

• 

  A cross-section of an affected Austrian pine

• 

  Affected Scots pines on a Christmas tree farm

References

Wikispecies has information related to: Bursaphelenchus xylophilus

External links

• Pine wilt disease. American Phytopathological Society.