Carrot (Daucus carota var. sativa DC.), an important root vegetable, is cultivated widely because of its dietary fiber and beta carotene. In June 2009 and June 2010, a disease suspected as root rot of carrot caused by Sclerotium rolfsii occurred in a 5-ha field in Jinju, Korea. Early symptoms consisted of water-soaked lesions on root and lower stem tissue near the soil line. Infected plants gradually withered and white mycelial mats appeared on the surface of roots. Numerous sclerotia were often produced on stem and root surfaces in contact with the soil. The heavily infected carrots became rotted and blighted and the whole plant eventually died. The freshly isolated pathogenic fungus was grown on potato dextrose agar (PDA) and examined microscopically. Optimum temperature for mycelia growth or sclerotia formation was 25 to 30°C. Numerous globoid sclerotia formed on the PDA after 18 days of mycelial growth. The sclerotia (1 to 3 mm in diameter) were white at first and then gradually turned dark brown. Aerial mycelia usually formed, consisting of many narrow hyphal strands 3 to 9 μm wide. The white mycelium formed a typical clamp connection after 5 days of growth at optimum temperature. To fulfill Koch's postulates, 10 carrot seedlings were inoculated with colonized agar discs (6 mm in diameter) of the causal fungus directly on the root and incubated in a humid chamber at 25°C for 24 h. Ten carrot seedlings were inoculated similarly with agar discs as the control treatment. After this period, the inoculated and noninoculated plants were maintained in a greenhouse. Eight days after inoculation, the disease symptoms seen in the field were reproduced and the fungus was reisolated from the artificially inoculated plants. To confirm identity of the causal fungus, the complete internal transcribed spacer (ITS) rDNA region of the causal fungus was amplified using the primers ITS1 and ITS4 (2) and sequenced. The resulting sequence of 684 bp was deposited in GenBank (Accession No. JF342557). The sequence was 99% similar to sequences of Athelia rolfsii (Sclerotium rolfsii) in GenBank. Cultures of S. rolfsii have been deposited with the Korean Agricultural Culture Collection (KACC 45154), National Academy of Agricultural Science, Korea. On the basis of symptoms, fungal colonies, the ITS sequence, and the pathogenicity test on the host plant, this fungus was identified as S. rolfsii Saccardo (1). To our knowledge, this is the first report of root rot of carrot caused by S. rolfsii in Korea. This disease is highly dependent upon environmental conditions, including warm weather and high humidity. Recent occurrence of the disease suggests that S. rolfsii could spread widely.
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, Inc., New York, 1990.
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