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Pathogen Biology

Pine wilt results from interactions among the pine wood nematode (Bursaphelenchus xylophilus) (Figures 2 and 3), the pine sawyer beetle (Monochamus spp.) (Figure 4), a host coniferous tree, and sometimes blue-stain fungi (Ceratocystis spp., Ophiostoma piceae) (Figures 5 and 6).


Figure 2.	Figure 3.

Figure 4.


Figure 5.	Figure 6.

The pinewood nematode is a microscopic unsegmented worm about 1 mm in length. It is one of at least 49 described species of Bursaphelenchus, most of which have a symbiotic relationship with insects. Bursaphelenchus xylophilus is believed to be native to the United States. It was documented causing damage to trees in 1934 and was rediscovered in 1979. The nematode feeds on fungi within the wood and on living plant cells surrounding the resin canals of pines.

Spread of the nematode from tree to tree occurs via the pine sawyer beetle (Monochamus spp.). The nematodes are carried in the tracheae of the beetle's respiratory system (Figure 7). When the beetles feed on branches of healthy trees, the nematodes emerge and enter the trees through feeding wounds created by the beetles. The adult sawyers are attracted to recently dead or dying trees for oviposition (egg laying). Trees with bark are necessary for oviposition and larval development. The beetle larvae feed several weeks in the cambial wood and then bore into the sapwood.

Figure 7.

Once introduced into a tree seedling, the nematodes migrate mainly via the cortical resin canals in branches. Cortical tissue is not present in stem tissues of trees over four years of age, and therefore it is not likely that the nematodes spread within mature trees via cortical tissue. It has been suggested that spread within mature trees occurs through xylem resin canals. The nematodes migrate downward to the tree bole without interruption by the periderm (cork or bark). The nematodes can destroy surrounding tissue and invade cortical tissue until they invade the xylem resin canals and the trunk. When not feeding on the plant cells, the nematodes feed on fungi present within the wood. Bark beetles carry blue-stain fungi to trees, and these fungi, along with other wood-inhabiting fungi, colonize the tree after it is weakened or killed. Research indicates that there is an increase in nematode reproduction when a blue-stain fungus is the food source for the nematode.

The plant-feeding phase of the nematode, B. xylophilus, is usually characterized by rapid reproduction of the nematode and tree death. Laboratory studies show that reproduction can progress from egg to adult in four to five days in the laboratory, feeding on fungi at 25°C (77°F); however, generation time is temperature-dependent. Measurements on pine resin canal tissue are much harder and thus direct comparisons cannot be made here. The nematode goes through four juvenile stages punctuated by molts before becoming an adult. This pathway for the nematode life cycle can continue as long as conditions are favorable for growth and reproduction.

Tree death or scarce food supply results in the nematode switching to a dispersal mode. There is much closer physiological relationship in this mode between the nematode and the insect. The nematode will only molt into fourth-stage dispersal juveniles in the presence of pine sawyer beetles, and the pine sawyer beetle only vectors fourth stage larvae. The dispersal juveniles aggregate around the pupal chambers of the beetles, and migrate toward the insect tracheae located in the spiracles. The fourth stage dispersal juveniles can survive for a year. Laboratory research has shown that removal of the insect pupa from its chamber will trigger increased nematode reproduction. The adult beetles chew their way through the sapwood and exit the tree. Carrying the nematodes, adult beetles fly to healthy trees to feed or to dead or dying trees to lay eggs.