Spatial pattern of pine wilt disease caused byBursaphelenchus xylophilus (Nematoda: Aphelenchoididae) within aPinus thunbergii stand

Summary To understand the mechanism of spread of pine wilt disease caused by the pinewood nematode,Bursaphelenchus xylophilus, which is vectored by a cerambycid,Monochamus alternatus, the spatial distribution of trees weakened by the nematode was examined within aPinus thunbergii stand from June to October for 4 years. The weakened trees were distributed in a clumped pattern in 1980 and 1981, at an early stage of infestation. In many cases, they showed a double-clumped pattern. The degree of aggregation was higher in June or July than after August. They were uniformly distributed in June or July 1982 and in June 1983 whereas they showed a double-clumped pattern after August. The trees were frequently weakened in June or July when they were near the trees weakened during the previous year. At quadrat sizes of more than 25 m², spatial overlapping was pronounced between trees weakened during June–July of the current year and those weakened in the previous year. The seasonal changes in spatial distribution of weakened trees were explained by the interaction amongM. alternatus, B. xylophilus andPinus trees.