Characterization and pathogenicity of sweet potato (Ipomoea batatas) black rot caused by Ceratocystis fimbriata in Korea

Black rot disease was observed in sweet potato in Korea during a disease monitoring survey in 2017. The symptoms were black, small, circular, and slightly sunken lesions in the initial stages, leading to enlarged black spots and perithecia with long necks that appeared as dark bristles in the later stages. The causative agent was isolated on potato dextrose agar (PDA) and stored at the Sweet Potato Research Laboratory, Bioenergy Crop Research Institute, RDA, Muan (SPL17100 and SPL17101). The cultured organism produced cylindrical single-celled conidia after 5 days of incubation at 25 °C with a conidial size of 8.5–65.2 × 2.6–7.3 μm. Brownish, globose, and thick-walled aleuroconidia were produced with an 11.9–17.0 × 8.3–12.2 μm size. The perithecia were globose and dark brown with long thin necks tapering from the base to the apex. Morphologically, the fungus was identified as Ceratocystis fimbriata, which was well supported by the combined phylogenetic analysis of the sequences of the internal transcribed spacer (ITS), large subunit (LSU), and elongation factor 1-α (EF-1α) genes. Pathogenicity tests were conducted, and Koch’s postulates were confirmed.