Yacón, or ground apple (Smallanthus sonchifolius), is a perennial plant grown in the northern and central Andes from Colombia to northern Argentina for its crisp, sweet, and tuberous roots. In 2012, yacóns cultivated in the research field of the Gyeongsangnam-do Agriculture and Extension Services, Jinju, South Korea, suddenly died. The characteristic symptoms consisted of rotting, wilting, and blighting. Initial symptoms included water-soaked lesions on lower stem tissue near the soil line. Infected plants gradually withered, and white mycelial mats and sclerotia appeared on the surface of roots and stems at the soil line. The sclerotia were collected and disinfested by immersion in 1% sodium hypochlorite solution for 30 s and in sterilized distilled water for 1 min. Then, the sclerotia were placed on potato dextrose agar (PDA) and incubated at 30°C. Numerous globoid sclerotia were formed on PDA after 18 days of growth. The sclerotia (1 to 3 mm in diameter) were white at first and then gradually turned dark brown. Aerial mycelia usually formed many narrow hyphal strands 4 to 9 μm wide. The white mycelium formed a typical clamp connection after 5 days of growth. To fulfill Koch's postulates, 50-day-old healthy seedlings were transplanted individually into pots (10 × 10 × 9 cm) containing autoclaved soil. After 7 days, five seedlings were inoculated with colonized agar discs (6 mm in diameter) directly on the base of the plant, and five yacón seedlings were inoculated similarly with PDA discs as the control treatment. The inoculated and non-inoculated plants were incubated in a humid growth chamber at 28°C for 24 h and then maintained in a greenhouse. Eight days after inoculation, the yacón seedlings inoculated with the fungus died, whereas those treated with PDA discs were symptomless. The fungus was re-isolated from the artificially inoculated plants. To confirm the identity of the causal fungus, the complete internal transcribed spacer (ITS) rDNA region of the fungus was amplified using the primers ITS1 (5′-TCCGTAGGTGAACCTGCGG-3′) and ITS4 (5′-TCCTCCGCTTATTGATATGC-3′) (2), and the PCR amplicons were cloned into the pGEM-T Easy vector. The resulting plasmid (pOR119) was sequenced in both directions with the primers M13F and M13R. The resulting 684 bp of ITS rDNA sequence was deposited in GenBank (Accession No. KJ944395). A DNA analysis revealed that it was 100% identical to Sclerotium rolfsii (HM355751). Cultures of S. rolfsii have been deposited with the Korean Agricultural Culture Collection (KACC 47750), National Academy of Agricultural Science, Suwon, South Korea. On the basis of symptoms, fungal colonies, the ITS sequence, and pathogenicity to the host, this fungus was identified as S. rolfsii Saccardo (1). This is the first report of sclerotium rot on yacón caused by S. rolfsii in South Korea.
References: (1) J. E. M. Mordue. CMI Descriptions of Pathogenic Fungi and Bacteria, No. 410, 1974. (2) T. J. White et al. Page 315 in: PCR Protocols: A Guide to Methods and Applications. M. A. Innis et al., eds. Academic Press, San Diego, 1990.