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

最近，在日本海沿着的几个沿海地区发现了大规模死亡的橡树;一年内有5万多棵橡树死亡。许多枯树的树干曾受到食虫虻（Platypus quercivorus）的攻击，但生物和非生物死亡因素尚未得到解释。为了阐明致病真菌是否与群体死亡率有关，进行了田间和隔离试验。从变色边材，坏死的树皮和画廊墙壁死亡和枯萎的树木隔离测试。Raffaelea sp.是最常见的变色边材分离。该菌也从食栎假单胞菌的幼虫、蛹、成虫体表及雌虫的菌丝体中分离到。经鉴定，该真菌为栎属（Raffaelea quercivori sp. nov. Kubono et Ito）。接种试验表明，蒙古栎（Quercus mongolica var.）grosseserrata和Q. serrata被杀了。可见该菌对橡树具有致病性。在壳斗科树木中，Q.蒙古变种grosseserrata最敏感。另一方面，柳栎的敏感性较低。接菌后，树体水势迅速下降，最终萎蔫死亡。接种树木的木质部变得广泛变色，在这种变色区域的导管是不导电的。导管系统的电导率显着降低接种树。变色、形成层坏死、木质部脱水是导致树液上升受阻的主要原因。

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.