Anthracnose of Lemon Balm Caused by Colletotrichum spp. in Korea

Anthracnose symptoms were observed on lemon balm (Melissa officinalis) plants growing in fields at locations Cheongju and Okcheon, Korea, during investigation of the crop disease occurrence in August 2024. The symptoms initially appeared as small, circular or irregular dark brown to black spots on the leaves, petioles, and stems. In the later stages, the symptoms expanded to irregular spots on the plants, and severely diseased plants blighted and died. The disease incidence on the plants in the fields ranged from 20% to 40%. Four single-conidium isolates obtained from the diseased plants were found to belong to Colletotrichum sp. based on their morphological characteristics. The Colletotrichum sp. isolates were tested for their pathogenicity on lemon balm leaves through artificial inoculation. Among the four isolates tested, three induced lesions on lemon balm leaves by artificial inoculation, but one did not. The lesions on the leaves induced by the inoculation tests with the three isolates were similar to those observed in the investigated fields. Two out of the pathogenic isolates were identified as Colletotrichum gigasporum, and one as Colletotrichum truncatum based on phylogenetic analyses and morphological characteristics. This is the first report of C. gigasporum and C. truncatum causing anthracnose in lemon balm in Korea.