Characteristics of Lithocarpus edulis tree death caused by Japanese oak wilt

Abstract

ABSTRACT In the southern part of the Boso Peninsula in Chiba prefecture, Japan, Japanese oak wilt has been spreading since the first dead trees were found in 2017. In this area, primarily Lithocarpus edulis trees died, but the mechanisms by which the ambrosia beetle Platypus quercivorus and the associated fungus Raffaelea quercivora kill L. edulis trees have yet to be clarified. We investigated the relationships among tree trunk diameter, xylem discoloration, and wilt symptoms as well as attack density by the ambrosia beetle. Dead and declining trees increased as the attack densities increased; however, the attack densities were lower than those previously reported in Quercus crispula and Q. serrata. Beetle galleries were distributed mainly in the heartwood. Xylem discoloration caused by R. quercivora expanded throughout the heartwood and into the inner sapwood. At the places where beetle galleries extended to middle parts of the sapwood, the discoloration frequently expanded radially close to the cambium. In the dead and declining trees, the healthy sapwood narrowed at a certain height, above which many parts of xylem were dead and stained dark, suggesting that death of foliage and branches resulted from restriction of water uptake at a particular height. In conclusion, L. edulis trees were killed even at low attack densities and the contribution of P. quercivorus and R. quercivora to the tree death was different from that of other oak species.