Susceptibility of adult trees of the endangered species Pinus armandii var. amamiana to pine wilt disease in the field

Adult trees of Pinus armandii var. amamiana (PAAm) and P. thunbergii grown in the field were inoculated with 100000 or 1000 of the nematode Bursaphelenchus xylophilus to evaluate their susceptibility to pine wilt disease. PAAm trees inoculated with 100000 nematodes started to show disease symptoms 2 weeks after inoculation, and all died within 29 weeks. Although the PAAm trees inoculated with 1000 nematodes tended to show delayed disease symptoms compared with those inoculated with 100000 nematodes, all of them died within 33 weeks after inoculation. All P. thunbergii trees inoculated with 1000 nematodes had died 6 weeks after inoculation. In the nematode-inoculated PAAm trees, death of branches distal to the nematode inoculation site was the first visible symptom, followed by the systemic discoloration of needles, whereas the whole tree wilted simultaneously in P. thunbergii trees. In nematode-inoculated PAAm trees, the period from inoculation to death was longer than that in P. thunbergii. These results suggest that adult PAAm trees are susceptible to pine wilt disease, but are less vulnerable than P. thunbergii.