Comparison of Media for Isolating Rhizoctonia solani from Soil. P. C. Vincelli, Assistant Professor, Department of Plant, Soil and Insect Sciences, University of Wyoming, Laramie 82071. C. M-S. Beaupré, Research Associate, Department of Plant, Soil and Insect Sciences, University of Wyoming, Laramie 82071. Plant Dis. 73:1014-1017. Accepted for publication 25 July 1989. Copyright 1989 The American Phytopathological Society. DOI: 10.1094/PD-73-1014.

Three media used for isolating Rhizoctonia solani from soil were evaluated by comparing in vitro growth of R. solani AG-1 through AG-5, suppression of indigenous soilborne microflora, and efficiency of recovery of R. solani from soil. Growth of isolates of AG-1 was most rapid on water agar (WA), somewhat reduced on Ko and Hora’s medium amended with 5 µl/L of prochloraz 40EC (KH_p), and greatly reduced on ethanol-potassium nitrate medium containing 2% ethanol (EPN₂). Isolates of AG-2-1, AG-2-2, AG-3, AG-4, and AG-5 grew equally well on WA and KH_p but grew very slowly on EPN₂. KH_p and EPN₂ were both highly effective in inhibiting soilborne aerobic fungi and bacteria. At least 97.9 and 98.8% of fungi detected in three soils using peptone-glucose agar (for quantifying total aerobic fungi) were inhibited on KH_p and EPN₂, respectively, after 7 days at 19–22 C; 99.9% of soilborne aerobic bacteria were inhibited on both media. At least 70% of aerobic bacteria failed to grow on WA after the same period, but all fungi detected in soil grew on WA. No difference was observed among media in estimates of populations of R. solani from three soils. Although efficiency of recovery of R. solani from soil was equal among media, it was much easier to locate and identify colonies of R. solani on the two selective media than on WA. Although materials for EPN₂ are costly, this medium would be useful for quantifying soilborne populations of R. solani when restricted growth of R. solani is desirable, such as when making standard plate counts of serially diluted organic matter extracted from soil. KH_p was a preferable medium when using a most-probable-number technique to quantify propagules in serially diluted organic matter, because of the low cost of materials and rapid growth of R. solani on the medium.