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First Report of Myrothecium Leaf Spot of Common Bean in China Caused by Myrothecium roridum

Common bean (Phaseolus vulgaris L.) is an economically important crop in China. In June 2008, a new foliar disease was observed on beans in Shunyi District, Beijing, China. The disease occurred in approximately 15% of the plants in a commercial field. Leaf spots were circular to irregular, reddish brown, zonate, and 8 to 20 mm in diameter. Black sporodochia with white tuffs were present on older lesions and black spore masses were present in moist weather. Ten isolates recovered from lesions produced white, floccose colonies and spore masses after 4 days on potato dextrose agar. The rod-shaped, hyaline conidia had rounded ends and averaged 6.8 × 2.5 μm. All characteristics were consistent with the description of Myrothecium roridum Tode ex Fr. (1). The internal transcribed spacer regions of one isolate were sequenced and deposited in GenBank (Accession No. GQ 381291). Sequences of the isolate from bean in China were 98% similar to sequences of M. roridum in GenBank. To determine pathogenicity, 30 healthy seedlings of common bean were inoculated by spraying a 1 × 10⁵ conidia ml^--1 suspension of M. roridum onto the foliage. Ten seedlings were sprayed with sterile water and served as controls. Plants were kept in a humid chamber at 27°C overnight and then placed in a growth chamber. After 6 days, the symptoms described above were observed on leaves in all inoculated plants, whereas symptoms did not develop on the control plants. The pathogen was reisolated from inoculated leaves, fulfilling Koch's postulates. There is one report of M. roridum on soybean in Korea (2). To our knowledge, this is the first report of Myrothecium leaf spot on common bean in China.

References: (1) M. Fitton et al. CMI Mycol. Pap. No. 253, 1970. (2) K. J. Yum et al. Plant Pathol. J. 6:313, 1990.