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

Abstract

During Jul.-Aug. 1992, an unusual foliar blight disease was observed on soyabean plants intercropped between rows of winter wheat in an upland field converted from a paddy field at Tohoku National Agricultural Experiment Station in Morioka, Honshu, Japan. The symptoms appeared as primary lesions consisting of small, circular necrotic spots ≤1 mm in diam., followed by secondary lesions showing circular to irregularly-shaped and large-sized areas of necrosis around the primary lesions under humid conditions. All isolates of R. solani consistently recovered from leaves with the primary and secondary lesions (leaf spot isolates) formed anastomoses in high frequency (\textgreater75%) with the tester isolates of the anastomosis subgroup AG-2-1 but in a low frequency (\textless16%) 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 AG-2-1 isolates were autotrophic for thiamine. The remaining 2 isolates could not grow even in the presence of thiamine. Culture appearance and opt. growth temp. of the leaf spot isolates were similar to those of AG-2-1 rather than to the AG-2-2 IIIB and IV subgroup. Inoculation tests revealed that the leaf spot isolates were highly pathogenic to soyabean, Vigna angularis and Phaseolus vulgaris and caused severe pre-emergence and post-emergence damping-off, but were not pathogenic to rape and radishes. The isolates caused foliar blight on soyabeans. It is concluded that most of the leaf spot isolates of AG-2 from soyabean did not fit to either the AG-2-1 or AG-2-2 subgroup. These isolates were therefore assigned to a new subgroup 3 in AG-2 (designated AG-2-3).