Chinese cabbage (Brassica rapa subsp. pekinensis Hanelt) is a leafy vegetable widely grown in China. In December 2012 to March 2013, a leaf rot disease was observed on the lower part of cabbage leaves in a field in Xianning, Hubei Province, China, with the incidence of 6.3% in that field. The diseased leaves showed water-soaked rot and brown symptoms at the top surface. White fluffy mycelia and small black sclerotia were produced on the lesion surface. Cabbage leaf tissues from the disease/healthy-bordering areas and the sclerotia from the lesions were separately surface-sterilized in 75% ethanol (v/v) for 30 s, followed by rinsing three times in sterilized water. The surface-sterilized tissues and sclerotia were placed on potato dextrose agar (PDA) and incubated at 22°C. Individual emerging fungal colonies from the leaf tissue pieces and the sclerotia were transferred to new 9-cm-diameter PDA plates and incubated for 15 days. A total of 40 isolates (20 from diseased tissue and 20 from sclerotia) were obtained. All the isolates grew rapidly on PDA with an average growth rate of 2.2 cm/day and produced abundant sclerotia on the colony surface (1,179 sclerotia/plate on average). None of the isolates produced conidia and any other spores in the PDA cultures. Mature sclerotia were black, irregular, spherical or elliptical, had a diameter of 0.5 to 1 mm, and easily detached from the colonies. The cultural and morphological characteristics of the isolates matched the description for Sclerotinia minor Jagger (3). Two isolates, A1 (from leaf tissue) and S2 (from a sclerotium), were further identified by analysis of the ITS region (ITS1-5.8S rDNA-ITS2) using the primer pair ITS1/ITS4. The resulting 540-bp DNA sequences (GenBank Accession Nos. KC836493 for A1 and KC836494 for S2) shared 100% identity S. minor isolate 62907 (JF279880). Pathogenicity of the isolates A1 and S2 was tested by inoculating detached cabbage leaves with mycelial agar plugs removed from the colony margin of the 3-day-old cultures. Isolates A1 and S2 were each inoculated on three leaves with three plugs per leaf. Three cabbage leaves inoculated with PDA plugs were treated as a control. The treated leaves were covered with plastic films to maintain high humidity (>90% RH) and incubated at 22°C for 72 h under the regime of 12 h light/12 h dark. Results showed that while the control leaves remained healthy, brown and water-soaked lesions appeared around the mycelial agar plugs of each isolate. Average lesion diameters were 47.5 mm for A1 and 47.8 mm for S2. Abundant small sclerotia were produced on necrotic leaf lesions after 7 days. The fungus in diseased leaf tissues was re-isolated and the morphological characteristics of the resulting fungus were the same as S. minor isolated from infected field-grown cabbages. Therefore, S. minor is the causal agent for the leaf rot disease on Chinese cabbage. S. minor has been reported to infect a few plant species in the genus Brassica, including B. rapa subsp. oleifera (3), B. oleracea var. gemmifera (3), B. napus (2), B. oleracea var. capitata (3), B. oleracea var. botrytis (3), and B. rapa (3). It was found on B. rapa subsp. pekinensis in Korea (1). To our knowledge, this is the first report of S. minor on B. rapa subsp. pekinensis in China.
References: (1) W. D. Cho and H. D. Shin. Page 779 in: List of Plant Diseases in Korea, 4th ed. Korean Society of Plant Pathology, 2004. (2) S. A. Gaetán and M. Madia. Plant Dis. 92:172, 2008. (3) M. S. Melzer et al. Can. J. Plant Pathol. 19:272, 1997.
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