Selective Medium for Isolation and Enumeration of Phialophora gregata from Soybean Straw and Soil. Alemu Mengistu, Postdoctoral Research Associate, USDA-ARS, Department of Plant Pathology, Iowa State University, Ames 50011. H. Tachibana, and C. R. Grau. Professor, USDA-ARS, Department of Plant Pathology, Iowa State University, Ames 50011; and Professor, Department of Plant Pathology, Russell Laboratories, University of Wisconsin, Madison 53706. Plant Dis. 75:196-199. Accepted for publication 12 August 1990. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/PD-75-0196.

Phialophora gregata, the cause of brown stem rot of soybean (Glycine max), can be isolated readily from physiologically active host tissue but is difficult to recover from senescent soybean tissue and soil. A selective medium was developed for the direct isolation and quantitative estimation of propagules of P. gregata from soybean straw and organic debris from soil. The medium, designated as PGM, contained 20 g of Difco Bacto agar and 129 g of homogenized green beans per liter of distilled water. After autoclaving, 0.8 g of CuSO₄ and 10 mg of pentachloronitrobenzene were added, and the medium was adjusted to pH 5.5 with lactic acid. Background colonies of bacteria and other fungi were suppressed significantly on PGM. Colonies of P. gregata were identified on PGM after 6 days of incubation. Recovery of P. gregata was 8 × 10² colony-forming units (cfu) per gram of wet sieved soil, which was significantly lower than recovery from samples that combined surface straw and debris. Populations of P. gregata detected on PGM ranged from 4 × 10⁵ to 1 × 10⁶ cfu/g of air-dried and ground plant debris. The sample of plant debris consisted of surface soybean straw and plant debris recovered from sieved soil sampled directly beneath the soybean straw. A lower detection threshhold was established for PGM than for a modified water agar medium and a selective medium designed for the recovery of the adzuki bean (Vigna angularis) strain of P. gregata. More propagules of P. gregata were recovered from surface straw and soil debris collected from plots continuously cropped to soybean cultivars susceptible to brown stem rot than from soil collected from fields previously planted to a resistant soybean cultivar. Disease severity, based on the percentage of internal stem browning of soybean plants, was highly correlated (r = 0.93) with the number of colonies of P. gregata recovered from soybean straw and soil. The more aggressive pathotype I was recovered on PGM at a higher frequency than the less aggressive pathotype II.