Taxonomic reevaluation of Raffaelea quercivora isolates collected from mass mortality of oak trees in Japan

In this study, we reevaluated isolates of Raffaelea quercivora associated with Japanese oak wilt in Japan to clarify their taxonomy and to help researchers diagnose the cause of the mass mortality of oak trees in Asian countries more accurately. We examined the morphological and molecular characteristics of 15 isolates of R. quercivora, including an ex-holotype strain, obtained from a wide range of areas of Japan. Light microscopy showed that all the isolates had wider ranges of conidial and conidiophore sizes than previously recognized, but that their sizes and shapes did not differ among the isolates. A phylogenetic tree generated from sequences for a partial large subunit of ribosomal DNA showed that the new isolates and the ex-holotype formed a single clade within the Raffaelea clade, with a high bootstrap value. Scanning electron microscopy revealed multimodal conidial development in the isolates: sympodial or annellidic-percurrent proliferation or both, with delayed secession. These results suggest that the isolates examined and the ex-holotype strain have a different genetic identity from other known Raffaelea species. The diverse conidiogenesis and subtle characteristics in the conidium morphology of R. quercivora reflected in the emended description.