First report of sclerotium rot in sword bean caused by Athelia rolfsii

Abstract

In 2018, sclerotium rot was observed in sword beans (Canavalia gladiata DC.) grown in a field at Gyeongsangnam-do Agricultural Research and Extension Services. The disease mainly affected the stems and pods of sword beans that touched the ground. The stems that touched the ground were water-soaked at the start of the infection and rotted, slowly withered, and eventually died. White mycelia spread across the lesions, and numerous sclerotia formed on the petiole near the soil line. The sclerotia were globoid, 1-3 mm in diameter, and white to brown in color. Hyphae on PDA were 4-8 mu m wide, and clamp connections were observed. Pathogenicity of isolated fungus on sword beans was confirmed according to Koch's postulates. For molecular identification, we sequenced the entire ITS rDNA sequence of the causative fungus and analyzed its phylogenetic relationships. The fungi isolated from sword beans were in a clade r-1 containing the reference strains of A. rolfsii and S. delphinii. The 540-bp PCR amplicons corresponding to A. rolfsii-specific amplification were produced in all three isolates. Mycological characteristics, ITS rDNA sequencing, the species-specific PCR analysis, and pathogenicity in the host plant identified this fungus as Athelia rolfsii (Curzi) C.C. Tu & Kimbr. This is the first report of sclerotium rot on sword bean caused by A. rolfsii.