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Symptoms and signs

Pine wilt is a dramatic disease because it usually kills affected trees within a few weeks to a few months (Figure 1). The pine wood nematode (Bursaphelenchus xylophilus) that causes death of the trees is microscopic. Most plant-parasitic nematodes are associated with plant roots, but the pine wood nematode is found in aboveground parts of the tree. 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. Just as a person cannot drink through a straw with holes in it, the tree cannot move water upward and consequently wilts and dies. Pine sawyer beetles (Monochamus spp.) vector the nematodes. The nematodes hitch a ride with the beetles and thus move from tree to tree as the beetles feed on the young shoots of pine trees.

Figure 1.

Scots pine is the most susceptible tree species, although other conifers can be affected, such as Austrian, jack, mugo, and red pines and, less commonly, white pines. Tree death usually progresses from the top of the tree downward, distinguishing this disease from needle diseases. Needle discoloration is usually the first symptom. Needles change from their normal color to a grayish green color and finally a tan to brown color. Needles do not fall from the branch, which is another diagnostic feature. Sometimes, a branch will die, and no disease progression will be seen until the next growing season.

The pine wood nematode was found widely distributed in the United States after its rediscovery in a dying tree in 1979. In the United States, the highest incidence of the disease is currently in the Midwest. Worldwide, the problem is epidemic in Japan and other parts of Asia. In all areas, the disease is most serious on stressed coniferous trees not native to North America. Age of the tree also influences susceptibility. There is an increased risk of developing pine wilt when trees are greater than 10 years of age.

Diagnosis requires observation and identification of the nematode, which can be obtained from cross-sections of symptomatic limbs greater than 2.5 cm (1 in.) in diameter soaked in water. The greatest success in finding the nematode involves soaking a trunk section in water for 24 to 48 hours. One can also sample wood by drilling into branches or the trunk with a large diameter (1-2.5 cm/ 0.5-1 in.) auger drill bit and soaking the resulting wood chips in water. Nematodes may not be well distributed throughout the tree, so it may be necessary to test several samples from different areas of the tree to find the nematodes. Microscopic examination of the liquid is needed to identify the nematode.

Several different bacterial-and fungal-feeding nematodes can be found in the dead or dying wood. Removal of bark before the samples are soaked can reduce the number of other nematodes. There can be several species of Bursaphelenchus present, but not all species are causal agents of pine wilt.

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