Print PageVast expanses of oak forests, particularly the stands of Quercus crispula, are defoliating because of the oak wilt disease (OWD). The OWD is a fungal infection caused by Raffaelea quercivora, which is carried by the vector beetle Platypus quercivorus. The disease was first reported in the 1930s, and its emergence has been sporadically reported thereafter; however, it began spreading rapidly since the mid-1980s at the rate of 2,000 ha/year mainly in the middle of the Honshu Island, Japan. We extensively collected field survey records of OWD defoliations in the regions along the Japan Sea coast at a resolution of 1.0 km x 1.0 km mesh data. Using this data of fine scale, we aimed at determining the epicenter (the local origin of the defoliation) and analyzing the propagation patterns of the oak defoliation.
Results/Conclusions
We noticed a general trend of defoliation starting from the southwest and moving toward the northeast, destroying the oak trees as it progressed. However, we also found several local epicenters in local scale. Oak defoliation started from individual epicenters and propagated to the neighboring areas, and ultimately all these areas merged to form a wide defoliation area. We tried to determine the cause of the multiple-epicenter formation using the accessory information in addition to our field records. We found that in some areas, defoliation was caused by human introduction of infested logs from other infected areas and/or an extremely long-distance dispersal, whereas in other areas, it was caused by a local P. quercivorus outbreak albeit the insect population was regulated to below endemic level until recently. We think that changes in the management of oak forests are the main causes of the sudden OWD outburst. It has been reported that older, large-sized oaks are more susceptible to OWD than the younger and thin ones. Oak trees were traditionally coppiced for daily use and used as woodchips for papermaking. However, oak trees have been growing abandoned since the mid-1960s because coppicing was unnecessary in modern Japan, and imported woodchips were used instead of the domestic ones because of their low price.
