Description and host susceptibility of Pseudocercospora euonymicola sp. nov. causing leaf spot disease on Euonymus japonicus in Taiwan

The Japanese spindle (Euonymus japonicus) is highly valued as an ornamental hedge and garden plant due to its dark green foliage and aesthetically pleasing shape. Additionally, its stem and leaf extracts possess medicinal potential. From 2019 to 2023, a previously unrecorded leaf spot disease was observed in Taichung and Taipei, Taiwan, severely impacting the ornamental value of E. japonicus. To identify the pathogen responsible for the leaf spot disease, 12 isolates from diseased samples were collected at different locations. Phylogenetic analysis using concatenated sequences of ITS, TEF1-α, ACT, RPB2, and LSU revealed that the 12 isolates formed a distinct clade within Pseudocercospora with high bootstrap support value. Morphological characteristics also distinguished these isolates from previously recorded species of Pseudocercospora and Cercospora on Euonymus. Thus, the pathogen was described herein as Pseudocercospora euonymicola sp. nov. Pathogenicity assays involving conidial inoculation on E. japonicus demonstrated that lesions developed only on wounded mature leaves, indicating that mature leaves are more susceptible than young leaves, and that wounds facilitate pathogen entry and lesion formation. Given the slow growth and sporulation difficulty of Pseudocercospora, this study represents the first successful inoculation experiment for Cercosporoid diseases on the genus Euonymus.