CHARACTERIZATION AND COMPARISON OF ISOLATES OF RHIZOCTONIA-SOLANI AG-7 FROM ARKANSAS, INDIANA, AND JAPAN, AND SELECT AG-4 ISOLATES

Superficially, isolates of Rhizoctonia solani AG-7 and AG-4 appear to have similar cultural morphologies, and AG-7 isolates are often mistak enly identified as AG-4. It is important, therefore to develop methods for separation of AG-7 cultures from AG-4. A cultural characterizatio n of R. solani AG-7 isolates from Arkansas, Indiana, and Japan showed only minor morphological differences between the groups but the Japane se isolates had areolate hyphae to woolly tufts of mycelium after grow ing 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 dete rmine the need for anastomosis pairing. Radial growth rates were great est at the cardinal temperatures 30 to 35 degrees C 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. sola ni AG-7 on watermelon, cotton (Gossypium hirsutum), and soybean (Glyci ne max) plants. Results from a greenhouse trial showed the R. solani A G-7 isolates significantly reduced (P less than or equal to 0.05) stan ds of cotton and watermelon. Stands of soybean grown in infested and n oninfested 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 occur red between the treatments in field trial I and in the greenhouse stud y, but no treatment differences were observed in field trial 2. A conf ounding factor during field trial 2 was the presence of the pathogen R . solani AG-4, which was isolated from 20% of the plant lesions, compa red with 5% in field trial 1. In field trial 3, AG-7 isolates 92.123.B (Arkansas), and 413 1-3F (Indiana) significantly reduced stands of co tton, compared with the noninfested control plots. Isolate RHS 109 (AG -4) was similar to 92.123.B in reducing the cotton plant stand.