Pine wilt disease as promising causal agent of the mass mortality of Pinus armandii Franch. var. amamiana (Koidz.) Hatusima in the field

The causal agent of the mass mortality of field populations of Pinus armandii Franch. var. amamiana (Koidz.) Hatusima (PAA) was investigated with special respect to the involvement of pine wilt disease. Wood chips, branches and/or increment cores for detecting the pinewood nematode, feeding marks of the vector insect and environmental stress in the past, respectively, were taken from live and dead PAA trees grown in three locations, Yaku-shima and Tanega-shima Islands and a plantation in Kagoshima City, from 1997 to 1998. Five trees died after the spring of 1996 and, of these, four were inhabited by the pinewood nematode. Feeding marks of the vector insects were found on the branches of all dead trees and most of the live trees investigated. These results suggest that the infection of pine wilt disease in PAA trees occurs in the field. Annual ring growth of the sample trees showed neither intervention nor growth reduction, which implies strong environmental stress that may cause mortality in PAA trees.