Active defense as a mechanism of resistance in pine against pine wilt disease.

Resistance mechanisms of pine against pine wilt disease were studied using the pine wood nematode and 2-year-old pine seedlings of which resistance to this disease being different.The pathogenic nematode multiplied most vigorously in the most susceptible pine, Pinus thunbergii, followed by P. densiflora.The increment was low in the moderately resistant pine, P. taeda, and no increment was found in highly resistant pine, P. rigida.The inoculated nematodes passed through cut stems of 2.5-cm length of both susceptible (P.thunbergii) and resistant (interspecific hybrid between P. taeda and P. rigida) seedlings.When stems were cut into 5-cm length, the inoculated nematodes passed through that of P. thunbergii, but not through stems of resistant interspecific hybrid.The heat treatment at 55C for 5min destroyed the resistance of the resistant stems.Thus, the resistance of interspecific hybrid seemed to be induced during nematodes migrated through at the point between 2.5 and 5.0 cm from the inoculated site.The inoculated stems contained higher amount of nematode paralyzing substance than uninoculated ones.