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Quantitative Estimation of Rhizoctonia solani AG-3 in Soil. C. Castro, Former graduate student, Division of Plant Pathology, Department of Plant, Soil and Entomological Sciences, University of Idaho, Moscow 83843, Present address: Embrapa-CNPFT, CP 403-96, 100 Pelotas, RS-Brazil; J. R. Davis, and M. V. Wiese. Professors, Division of Plant Pathology, Department of Plant, Soil and Entomological Sciences, University of Idaho, Moscow 83843. Phytopathology 78:1287-1292. Accepted for publication 22 April 1988. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/Phyto-78-1287.

An improved procedure for quantitative estimation of propagules of Rhizoctonia solani anastomosis group 3 (AG-3) in soil was developed. The procedure involved incubating soil pellets on Ko and Hora’s medium after both the soil and medium are amended with 5 ppm of prochloraz. After incubation for 24–72 hr, colonies developing from the pellets were examined microscopically for characteristics of R. solani and separated into anastomosis groups by examining hyphal anastomosis in paired culture with tester isolates. Isolates of R. solani showing hyphal anastomosis with AG-3 tester isolates were further tested for characteristic brown colony color on Stewart’s medium. With these techniques, the relative efficiency of recovery of R. solani AG-3 from an artificially infested field soil was 75%, and less than 2% of all plated soil pellets yielded interfering contaminant fungi. Without prochloraz in the soil and medium, recovery of R. solani AG-3 was reduced 17%, and more than 96% of the soil pellets gave rise to fast-growing contaminant fungi. With prochloraz-amended soil pellets on prochloraz-amended Ko and Hora’s medium (KHP), up to 20 propagules of R. solani, predominantly AG-3, were recovered per 100 g of potato field soil having a soil moisture of 10–12% (w/w). The assay method permitted recovery of R. solani AG-2, AG-4, AG-5, and R. frazariae in addition to AG-3 from artificially infested soil.