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Characterization and Comparison of Isolates of Rhizoctonia solani AG-7 from Arkansas, Indiana, and >Japan, and Select AG-4 Isolates. R. E. Baird, Plant Pathology Department, University of Georgia, RDC, P.O. Box 1209, Tifton 31793. D. E. Carling, Agricultural and Forestry Experiment Station, University of Alaska Fairbanks, Palmer 99645, B. G. Mullinix, Computer Sciences Department, University of Georgia, Coastal Plain Experiment Station, Tifton 31793. Plant Dis. 80:1421-1424. Accepted for publication 24 September 1996. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/PD-80-1421.

Superficially, isolates of Rhizoctonia solani AG-7 and AG-4 appear to have similar cultural morphologies, and AG-7 isolates are often mistakenly identified as AG-4. It is important, therefore to develop methods for separation of AG-7 cultures from AG-4. A cultural characterization of R. solani AG-7 isolates from Arkansas, Indiana, and Japan showed only minor morphological differences between the groups but the Japanese isolates had areolate hyphae to woolly tufts of mycelium after growing for 21 days on potato dextrose agar (PDA). Also, isolates of AG-7 had pitted sclerotial clusters and a brownish exudate after 21 days on PDA whereas isolates of AG-4 did not. These cultural characteristics could be used to separate AG-7 from AG-4 as a preliminary test to determine the need for anastomosis pairing. Radial growth rates were greatest at the cardinal temperatures 30 to 35C for all isolates, and radial growth rates of the Japanese tester isolates were 1 to 2 cm greater than those of any other cultures of AG-7 or AG-4. Greenhouse and field studies were conducted to determine pathogenicity of R. solani AG-7 on watermelon, cotton (Gossypium hirsutum), and soybean (Glycine max) plants. Results from a greenhouse trial showed the R. solani AG-7 isolates significantly reduced (P < 0.05) stands of cotton and watermelon. Stands of soybean grown in infested and noninfested soil were similar although lesions caused by R. solani AG-7 were consistently found on roots of infested plants. Two field trials were conducted in 1994. Similar significantly different results occurred between the treatments in field trial 1 and in the greenhouse study, but no treatment differences were observed in field trial 2. A confounding factor during field trial 2 was the presence of the pathogen R. solani AG-4, which was isolated from 20% of the plant lesions, compared with 5% in field trial I. In field trial 3, AG-7 isolates 92.123.B (Arkansas), and 413 I-3F (Indiana) significantly reduced stands of cotton, compared with the noninfested control plots. Isolate RHS 109 (AG-4) was similar to 92.123.B in reducing the cotton plant stand.