Aggressiveness of Rhizoctonia solani AG 2-2 on sugar beet and rotation crops

Rhizoctonia crown and root rot of sugar beet caused by Rhizoctonia solani AG 2-2 intraspecific groups (ISGs) IV and IIIB is increasing in Minnesota and North Dakota. Of 1,000 cultures of AG 2-2 isolated from diseased sugar beet, 24 of each ISG were selected to represent a wide geographic area and previous crops. They were tested for aggressiveness on adult sugar beet roots and seedlings of sugar beet and rotation crops. Adult sugar beet roots were inoculated when 8-wk old; in seedling tests, a commercial greenhouse soil was infested with inoculum before planting. Disease was assessed at 12 days after inoculation. Both ISGs were equally aggressive on adult sugar beet roots; root rot indices (RRI = 0-7 scale) averaged 5.0 (3.3-5.6) for IV and 4.9 (3.8-5.9) for IIIB. On seedlings of sugar beet and rotation crops, range of aggressiveness for both ISGs overlapped, but IIIB caused significantly more disease than IV. Sugar beet RRI (0-100 scale) averaged 78 (42-100) for IIIB and 51 (5-100) for IV. Corn RRI (1-5 scale) averaged 3.1 (1.8-4.1) for IIIB and 2.1 (1.2-3.0) for IV. Pinto bean RRI (1-5 scale) averaged 4.4 (3.5-5.0) for IIIB and 2.7 (1.8-4.5) for IV. Soybean RRI (1-5 scale) averaged 4.1 (2.7-5.0) for IIIB and 3.2 (1.9-4.6) for IV. Aggressiveness of AG 2-2 cultures on adult sugar beet roots was unrelated to pathogenicity on seedlings of any crop but there were significant correlations (P < 0.001) for aggressiveness of cultures on seedlings between crops.