<i>Armillaria gallica</i> isolated from <i>Quercus crispula</i> attacked by <i>Platypus quercivorus</i> in Hokkaido, Japan
Аннотация
Japanese oak wilt (JOW) caused by <i>Dryadomyces quercivorus</i> and transmitted by <i>Platypus quercivorus</i> has rapidly expanded in northern Japan in recent years. In Hokkaido, JOW was first reported in 2023. Mortality was observed in trees shown remarkably lower levels of the beetle infestation compared with those reported in previous studies, suggesting the involvement of other factors weakening the trees prior to the beetle attacks. One of them was hypothesized to be <i>Armillaria</i> root rot. Therefore, this study investigated <i>Armillaria</i> infection and evaluated its effect on the nine JOW-affected <i>Quercus crispula</i> trees in Hokkaido in 2023, among which four died and five survived. All four dead trees were shown to be infected with <i>Armillaria</i>, while the five surviving ones were uninfected. The isolates obtained from the damaged trees were identified as <i>A. gallica</i>, which is known as a weak pathogen of hardwood species. The levels of sapwood discoloration were assessed to evaluate the blockage of water uptake. Some <i>Armillaria</i>-infected dead trees exhibited smaller discolored areas than the survival trees showing several wilted branches. It was considered that the JOW identified in 2023 in Hokkaido resulted from a decline in tree vigor and increased water stress induced by <i>Armillaria</i> root rot. Overall, this study showed that the presence of weakened trees may accelerate mortality at the JOW expanding front with very low densities of <i>P. quercivorus</i>.