Exotic pathogen of concern – Pinewood Nematode (PWN)
Bursaphelenchus xylophilus
What is it?
The pine wood nematode (Bursaphelenchus xylophilus) is a microscopic unsegmented worm about 1 mm in length and is native to North America but is found in Portugal and parts of Asia (including Japan). The nematode has caused widespread losses to pines and other conifers overseas. The pine wood nematode is transmitted by a range of wood boring beetles, but it is primarily spread by large (up to three cm long and approximately one cm wide) mottled, brown–grey coloured beetles with long antennae which belong to the Monochamus genus. All of the known vectors of this nematode are exotic species for Fiji, including the Japanese Pine sawyer beetle (Monochamus alternatus) and the Pine sawyer beetles, M. galloprovinicialis and M. carolinensis. The pine wood nematode has been killing native pine trees (Pinus densiflora, P. thunbergii, P. luchuensis) in Japan since the early 20th century. It is the number one forest pest in Japan and has been spread to China, Korea and Taiwan. Although the nematode is native to North America (Canada, USA, Mexico), it is thought to have been carried to Japan at the beginning of the 20th century on timber exports.
Host range
The pine wood nematode almost has a worldwide distribution; the problem is epidemic in Japan and other parts of Asia, where it is the native pine forests that are at risk. Scots pine is considered the most susceptible species particularly in the United States, although many of the other species in the Pinus (Sylvestres) subsection (e.g., Austrian, mugo and Japanese black pines) are also highly susceptible. In contrast, species in the Australes subsection (e.g., loblolly, longleaf, pitch and slash pines) are generally highly resistant. Pine sawyer beetles (Monochamus spp.) attack a range of conifers including Pine (Pinus), Spruce (Picea), Larch (Larix) and Fir (Abies). Pines are the preferred hosts.
Symptoms
Pine wilt is a dramatic disease that typically kills affected trees within a few weeks to a few months. 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. , the tree cannot move water upward and consequently wilts and dies. The first symptom is wilting foliage. Wilted trees will turn yellow to brown within three months after becoming infested. The dispersal stage of the nematode is carried in the vector insect’s respiratory system and thus spread from tree to tree as the beetles feed on the young shoots of pine trees. The pine wood nematode can only be identified under microscope magnification.
Life cycle
Long-horned beetles in the genus Monochamus have been shown to transmit the pine wood nematodes. These beetles are known as sawyers. The pine wood nematode is transmitted to new pine hosts during beetle feeding. Adult sawyers emerge from nematode infested trees, carrying hundreds of nematodes in their bodies. As the adult beetles feed on healthy pine shoots, the nematodes leave the sawyers and enter the new pine host through beetle feeding wounds. The pine wood nematode first colonizes the resin ducts of the tree and then moves into the water conducting tissue, xylem. The nematode reproduces rapidly in the xylem, which leads to wilting and tree death. Sawyers are attracted to dying or dead trees in which they can lay their eggs. The nematodes, by killing trees, provide breeding areas for the sawyer beetles. Pupal chambers are created in the dead wood, where the sawyer beetles complete their lifecycle. In the pupal chamber, juvenile nematodes enter the bodies of young adult beetles just before they emerge from their chambers. These young adults fly away from the dead host carrying the nematodes in their bodies in search of healthy pine trees to feed on, repeating the life-cycle.
What it can be confused with?
The long antennae of Monochamus beetles makes them fairly distinctive from other beetles likely to be found in pine plantations. When the nematodes infect a susceptible tree they cause yellowing and wilting symptoms that could also be caused by other disorders (e.g. Sirex wood wasp, Diplodia canker or Five-spined bark beetle (Ips grandicollis)), but the rapid onset of symptoms (as short as four weeks) and tree death is unlike other endemic pests or diseases.
Spread
The pine wood nematode itself in infested material is the principal route for dispersal over greater distances while short distance dispersal is usually through beetles of the Monochamus species. Human activity through importation of timber and wooden packaging material from countries known to have pine wood nematode could have the potential to spread the pest to new areas or countries.
Management
Successful management strategy includes targeting both the beetle vector and the nematode pathogen. Timely removal and destruction of trees killed by pine wilt disease eliminates the breeding habitat of the vector and prevents nematode transmission to healthy trees. Infected trees need to be disposed of by cutting the trees off at the ground and then burning, chipping or burying the wood. The primary goal is to destroy the larvae and pupa of the pine sawyer.
Reporting
If you notice anything unusual on pines in Fiji especially wilting symptoms described above, please do not hesitate to contact the Biosecurity Authority of Fiji on 3312512 to speak with a Nematologist or one of the Plant Pathologists from the Plant Scientific Team. You may also send an email to info@baf.com.fj