Study on a causal agent and mechanisms of the disease development in mass mortality of oak trees.

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Recently, mass mortality of oak trees has been discovered in several coastal areas along the Japan Sea ; more than fifty thousand oak trees were killed in one year. Many of the trunks of the dead trees had been attacked by ambrosia beetles, Platypus quercivorus, but the biotic and abiotic mortality factors have not yet been explained. To clarify whether a pathogenic fungus was associated with mass mortality, field and isolation tests were conducted. From isolation tests from the discolored sapwood, necrotic inner bark and gallery walls of dead and wilting trees. Raffaelea sp. was most frequently isolated from the discolored sapwoods. This fungus also isolated from body surfaces of larvae, pupae and adult of P. quercivorus and from the mycangia of females. This fungus was identified as Raffaelea quercivori sp. nov. Kubono et Ito. From inoculation tests using this fungus, Quercus mongolica var. grosseserrata and Q. serrata were killed. So it was clear that this fungus had pathogenicity to oak trees. Among the fagaceous trees, Q. mongolica var. grosseserrata was most susceptible. On the other hand, Q salicina was less susceptible. The water potential of the inoculated trees decreased rapidly and then the trees wilted and died. The xylem of inoculated trees became widely discolored and vessels in such discolored areas were non-conductive. Conductivity of the vascular system was markedly less in inoculated trees. It was clear that discoloration, cambial necrosis and xylem desiccation began for the complete blockage of sap-ascent.