The effects of simulated acid rain on the development of pine wilt disease

Six-month-old Japanese black pine seedlings were exposed to simulated acid rain (SAR) at pH 3 and 2, three times a week. After this treatment for 2 months, the seedlings were inoculated with virulent (S10) or avirulent (C14-5) isolates of pine wood nematode at three inoculum densities (50, 160 or 500 nematodes per seedling). In the seedlings inoculated with 500 virulent nematodes, disease development was accelerated by the pretreatment with SAR at pH 2 or 3. In the seedlings inoculated with 50 virulent nematodes, however, the appearance of dead seedlings was retarded by the pretreatment with pH 3 SAR. SAR at pH 2 increased seedling mortality irrespective of the number of virulent nematodes inoculated. When inoculated with 500 avirulent nematodes, SAR at pH 2 or 3 slightly increased seedling mortality. When inoculated with 50 or 160 avirulent nematodes, almost all seedlings survived throughout the experimental period except for the seedlings pretreated with SAR at pH 2. The relationship between the cumulative mortality of the seedlings and the load on the host given by nematodes, the product of the logarithm of the number of nematodes inoculated (Pi) and the time after inoculation (t), was analysed by linear regression. The results suggest that SAR at pH 3 not only raised the rate of increase in seedling mortality, but also increased resistance of pines to the virulent nematodes. SAR at pH 3 is suggested to sustain little damage to the resistance of pines to the avirulent nematodes. According to these results, we conclude that acid rain at pH 2 decreases the resistance of pines to pine wood nematodes, and acts as a promoting factor of pine wilt disease even if pines were not directly killed by the impacts of acid rain. Acid rain at the current ambient level in Japan, however, may not increase the mortality of pine trees infected by pine wood nematodes, but rather retard the appearance of dead trees.