Bursaphelenchus xylophilus, commonly known as pine wood nematode or pine wilt nematode (PWN), is a nematode

Nematode

The nematodes or roundworms are the most diverse phylum of pseudocoelomates, and one of the most diverse of all animals. Nematode species are very difficult to distinguish; over 28,000 have been described, of which over 16,000 are parasitic. It has been estimated that the total number of nematode...

 (worm) that infects pine

Pine

Pines are trees in the genus Pinus ,in the family Pinaceae. They make up the monotypic subfamily Pinoideae. There are about 115 species of pine, although different authorities accept between 105 and 125 species.-Etymology:...

 trees and causes pine wilt. It originates from North America

North America

North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...

, but has now spread to Europe

Europe

Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...

 and East Asia

East Asia

East Asia or Eastern Asia is a subregion of Asia that can be defined in either geographical or cultural terms...

 and has become a worldwide quarantine pest.

It is particularly damaging to matsutake

Matsutake

Matsutake is the common name for a highly sought after mycorrhizal mushroom that grows in Asia, Europe, and North America...

 mushroom cultivation, given as how the worms damage and kill the mushrooms' host trees.

Distribution

Originally thought to be native to North America, the PWN has now spread overseas to countries such as Japan, China, Vietnam, Australia, and Portugal, where it has caused severe damage in certain species of pine. Perhaps the most notable PWN epidemic has occurred in Japan, where pine wilt is credited with the destruction of some 26 million cubic meters of timber since WWII.

Pine wilt, as a disease, was first identified in Japan

Japan

Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...

 in 1905. Recent research has now indicated it may have originated from Japan and Asia.and been transferred via shipping. It was found in the United States in Missouri

Missouri

Missouri is a US state located in the Midwestern United States, bordered by Iowa, Illinois, Kentucky, Tennessee, Arkansas, Oklahoma, Kansas and Nebraska. With a 2010 population of 5,988,927, Missouri is the 18th most populous state in the nation and the fifth most populous in the Midwest. It...

 in 1931. PWN was identified as the pathogen responsible for pine wilt in 1979.

Morphology

Distinguishing PWN from other non-pathogenic species within the same genus (Bursaphelenchus) is challenging due to highly similar morphological features, but a positive ID can be achieved using molecular analyses such as RFLP.

Life Cycle/ reproduction

The basic propagative lifecycle of the PWN is typical of most nematode species, having four vermiform juvenile stages (J1-J4), followed by an amphimictic (male and female) adult stage.This basic life cycle takes place in dead or dying wood when the nematodes are said to be in there “mycophagous” phase, feeding on fungus within the wood and NOT on the wood itself. The PWN is unable to travel outside of the wood by itself, and in order to reach another host tree (living or dead) it must be carried by an insect vector.
B xylophilus has the shortest life cycle compared to any parasitic nematode. One generation is completed within four days after the nematode is cultured on fungi at it optimum. During the summer, the parasite reproduces very rapidly to very high numbers resulting in the spread throughout the resin canal system of susceptible pines, into the trunk and therefore all of its branches (even into the roots). If living tree cells are no longer available the parasite feeds and reproduces on the fungal hyphae that is growing through the resin canals. In the fall and winter the parasite becomes inactive but then resumes activity in the spring.

Vector

Though the PWN is known to be vectored by a number of bark beetles and wood borers, it seems to be most often associated with species in the genus Monochamus

Monochamus

Monochamus is a genus of longhorn beetles found throughout the world. They are commonly known as sawyer beetles or sawyers, as their larvae bore into dead or dying trees, especially conifers such as pines. A more ambiguous name is "bark beetles"...

, widely referred to as “pine sawyers”. Pine sawyers lay their eggs in the bark of dead timber. The growing larva feeds on the wood, and within the resulting cavity develops into an adult pupa. The J3 stage of the PWN congregate in the cavity around the pine sawyer pupa, molt into J4 (dauer stage) juveniles, and then migrate into the trachea of the adult beetle through its spiracles. During this “dispersive stage” the beetle may transport the PWN in one of two ways. In “secondary transmission” during vector oviposition, the nematode is transported to another dead tree and continues in the mycophagous phase.

Host parasite relationship

In “primary transmission” when the beetle feeds on susceptible host pines, the PWN will enter the tree and feed on the epithelial cells which line the resin ducts. This is referred to as the “phytophagous” phase of the PWN, and will result in pine wilt. When the host is infected, the transmission of water within the plant is restraining; under peak conditions, pine wilt can become apparent within a few weeks after infection, the needles turn brown, and the leaves turn yellow, and eventual death of the tree. From the time of infection to the death of the plant/tree/host, it only takes two to three months. Though the susceptibility of many species of pine is still a matter of contention, susceptibility has been proven (using Koch’s postulates) in a few species, which includes Scotch, Slash, Japanese red, and Japanese black pines. The Slash pine is the only susceptible species native to North America, and its susceptibility has been shown to be relatively weak compared to foreign pines. Because of this the direct economic impact due to pine wilt, in this part of the world, is limited to predominantly landscaping settings.

Management

Protective embargoes placed by the EU on untreated lumber transported from the U.S. and Canada have resulted in more indirect economic losses (2,3). Because there is no cure for pine wilt, management practices have concentrated on preventing the spread of the PWN. 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 PWN has recently made in appearance in Portugal, and threatens to spread to other countries in Europe.