Pine wilt disease caused by the pine wood nematode: The induced resistanceof pine trees by the avirulent isolates of nematode

Since the beginning of the 20th century, pine trees in Japan have been seri ously damaged by the pine wilt disease. This disease is caused by the pine wood nematode, Bursaphelenchus xylophilus, which is transmitted by the Japa nese pine sawyer, Monochamus alternatus. The control of disease depends to a large extent on chemicals, but the public is now demanding environmentall y friendly control methods. The virulence of B. xylophilus varies very wide ly. Pre-inoculation of young pine trees in a nursery with avirulent B. xylo philus has induced systemic resistance of trees against a subsequent inocul ation with virulent B. xylophilus. This induced resistance was considered a hopeful means for developing a biological control for the disease. The ind uced resistance by the avirulent nematodes was also expressed in mature pin e trees in a forest where the disease was naturally epidemic. However, the effects of induced resistance were not satisfactory for practical biologica l control. Since the inoculation with higher concentrations of the avirulen t B. xylophilus induced the resistance more effectively, the pre-inoculatio n method will need to be improved to develop the biological control. The in duced resistance of pine trees by avirulent B. xylophilus should be one of the candidate biological control methods against pine wilt disease. This in duced resistance also provides an experimental system to clarify physiologi cal interactions between the nematodes and pine trees.