Effects of inoculum density of pinewood nematode on the development of pine wilt disease in Japanese black pine seedlings pretreated with simulated acid rain

Six-month-old Japanese black pine seedlings (Pinus thunbergii) were exposed to simulated acid rain (SAR) at pH 3 and 2 three times a week. After treatment for 2 months, the seedlings were inoculated with a virulent isolate (S10) of the pinewood nematode (Bursaphelenchusxylophilus), at three inoculum levels (P-i = 50, 160 or 500 nematodes per seedling). In seedlings inoculated with 500 nematodes, both population growth of nematodes and disease development were accelerated by pretreatment with SAR at pH 3 or 2. In seedlings inoculated with 50 nematodes, population growth of the nematodes was suppressed and more time was needed for seedlings to die when pretreated with pH 3 SAR. This suggests that exposure to pH 3 SAR increased not only the progress of mortality, but also simultaneously enhanced the tolerance limit of the seedlings to the pinewood nematode - the critical value of physiological burden (represented as a product of time and initial nematode population) necessary to kill a seedling. Exposure to pH 2 SAR accelerated nematode reproduction in seedlings and increased seedling mortality irrespective of the number of nematodes inoculated.