Characterization and Pathogenicity of a New Anastomosis Subgroup AG-2-3 of Rhizoctonia solani Kühn Isolated from Leaves of Soybean

Abstract

During July to August in 1992, a unusual foliar blight disease was observed on soybean plants intercropped between rows of winter wheat in an upland field converted from a paddy field at Tohoku National Agricultural Experiment Station in Morioka, northern part of Japan. The symptoms appeared as primary lesions consisting of small, circular necrotic spots, 1-mm or less than 1-mm in diameter, followed by secondary lesions showing circular to irregularly-shaped and large-sized areas of necrosis around the primary lesions under humid conditions. All the isolates of Rhizoctonia solani Kühn consistently recovered from leaves with the primary and secondary lesions (hereinafter referred to as leaf spot isolates) formed anastomoses in a high frequency (>75%) with the tester isolates of the anastomosis subgroup AG-2-1 but in a low frequency (<16%) with those of the AG-2-2 IIIB and IV and anastomosis group AG-BI. Among the 66 leaf spot isolates, 64 were auxotrophic for thiamine, whereas the isolates of the AG-2-1 were autotrophic for thiamine. The remaining 2 isolates could not grow even in the presence of thiamine. Culture appearance and optimum growth temperature of the leaf spot isolates were similar to those of the AG-2-1 rather than to those of the AG-2-2 IIIB and IV subgroup. Inoculation tests revealed that the leaf spot isolates were highly pathogenic to soybean, adzuki bean and kidney bean and caused severe pre-emergence and post-emergence damping-off, but were not pathogenic to rape and radish. The isolates caused foliar blight on soybean. These results indicated that most of the leaf spot isolates of AG-2 from soybean did not fit to either the AG-2-1 or AG-2-2 subgroup. Hence, we assigned these isolates to a new subgroup 3 in AG-2 (designated as AG-2-3).