Graduate School of Agricultural and Life Sciences, The University of Tokyo, Tokyo 113, Japan

Background and objectives
Pine wilt caused by pine wood nematodes, Bursaphelenchus xylophilus, is a devasting epidemic disease of pine forests in Japan and which has already spread to East Asia, including China, Taiwan, and Korea, and which is endemic to North America. If it were to become established in the pine forests in Europe and/or Siberia, it could become one of the most serious threats to coniferous forests worldwide. Studies on the pine wilt mechanism have been reported on intensively by many researchers [1], but adequate explanations as to the cause of wilting of pines have not yet been obtained [2]. The objectives of this paper are to clarify the wilting mechanism of the disease and to consider control measures.

Wilting mechanism and environmental conditions
Pine wood nematodes introduced into the shoot of pines during maturation feeding of Japanese pine sawyer, Monochamus alternatus, migrated rapidly into the whole trunk of pine with a maximum speed of 40-50 cm a day. The nematode population density is very low at early stages of disease development, often as low as a few nematodes per 100 g fresh wood, even following high concentration inoculations. A slight reduction of oleoresin exudation flow is a unique phenomenon at an early stage of disease development, which is not related to the water status of pines. Symptom development may then be diveded into two stages, an early stage and an advanced stage. In the early stage, cytological changes in the xylem parenchyma cells and a certain number of embolisms in tracheids are observed in pine seedlings as well as ethylene production in the wood. However, the growth of population of nematodes is not confirmed in the living wood. In the advanced stage, cambial death and embolism in the outer xylem result in a water deficiency in leaves, resulting in the cessation of photosynthesis. Two factors, the physiological status of water potentials and the nematode population density, are supposed to be decisive factors in the development of the disease. As to the wilting mechanism, the following hypotheses have been suggested; (a) water stresses, and (b) air pollution including acid rain as promoting agencies, and (1) toxins, (2) cellulase, and (3) cavitation as the cause of wilting. The problems with these hypotheses are discussed.

Control measures and threat of epidemics
Control measures against pine wilt disease are aimed at breaking the pine-pine wood nematode-pine sawyer relationship triangle. Present control measures consist for the most part of aerial spraying of insecticides against pine sawyer as a preventive of disease, spraying of insecticides on timber already damaged by infestation to prevent spread and trunk injection of chemicals active against pine wood nematodes. However, in spite of various efforts, the total amount of pine timber lost to the disease is not decreasing. This indicates a lack of understanding of the disease by the local people. Pine wilt disease has the potential to become a major threat if it were to be exported to other European countries, particularly as increasingly warm and unusual weather conditions are expected in the near future.

1. Fukuda K, 1997. J. For. Res. 2, 171-181.
2. Suzuki K, 1992. Forest Pests 41, 59-64.