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Disease Cycle and Epidemiology

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Figure 8. Life cycle of pine wilt disease illustrating propagative and dispersal stages of disease (Redrawn by permission from Wingfield, ed. (1987) Pathogenicity of the Pine Wood Nematode, APS Press, St. Paul, MN. (Courtesy N. Upchurch)
Figure 8.

Epidemiology

Several months of hot, dry weather are necessary for pine wilt development and spread. Nematode dispersion and multiplication are key factors in disease symptoms and tree death. The pine wood nematode can be a primary or secondary invader of plant tissue (Figure 8); therefore, its presence in dead or dying wood is not proof that the nematode killed the tree.

Adult beetles feed on bark of young branches (Figure 9) and the nematodes present in the insect spiracles (Figure 7) enter the tree through feeding wounds caused by the beetle. Inside a susceptible tree, the nematodes develop into adults and migrate throughout the tree, feeding on parenchyma cells of the ray canals, and reproduce. When conditions favor the nematode (susceptible host, optimal temperature, virulent nematode isolate), internal host responses can be seen within one to three days after infection. Cell death, increased host respiration, decreased water conductivity, increased ethylene production, and increased phytotoxic compounds are found. Trees infected in this manner usually wilt and die rapidly. The nematode is the primary pathogen in this case, and the result is pine wilt. It has been theorized that in susceptible trees the rapid spread of the nematode throughout the tree triggers hypersensitive reactions from the tree in cells behind the nematode. The combination of the nematode movement, feeding, and exudation stimulate the tree to essentially kill itself.

Figure 9. Monochamus (pine sawyer) beetle feeding damage. (Courtesy L.D. Dwinnell, copyright-free) Figure 9. Monochamus (pine sawyer) beetle feeding damage. (Courtesy L.D. Dwinnell, copyright-free)
Figure 7. Figure 9.

The nematodes can also be a secondary invader of dead or dying trees when they enter the wood when the sawyer beetle is ovipositing (Figure 10). In these cases, the cause of the tree death may not be the pine wood nematode or pine wilt. There is evidence that the nematode exists naturally independent of the disease. These populations of the nematode serve as reservoirs of the nematode for later dispersal by sawyer beetles.

Figure 10. Beetle oviposition pit for dispersal transmission. (Courtesy L.D. Dwinnell, copyright-free)
Figure 10

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by The American Phytopathological Society