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Isolation and Enumeration of Propagules of Sclerotium cepivorum from Soil. G. C. Papavizas, Plant Science Research Division, ARS, USDA, Beltsville, Maryland 20705; Phytopathology 62:545-549. Accepted for publication 20 December 1971. DOI: 10.1094/Phyto-62-545.

A selective medium and a technique were developed for the direct isolation of Sclerotium cepivorum from soil, and for the quantitative estimation of the inoculum density of the pathogen in soil. The selective agar medium contained inulin and NaNO3 as C and N sources, respectively, and pentachloronitrobenzene, chlortetracycline HCl, and streptomycin sulfate. Sclerotia in soil were mechanically concentrated by wet-sieving; competitive soil saprophytes were reduced by washing the soil residue retained on a 0.177-mm sieve and by exposing the concentrate to 0.25% NaClO solution. The average recovery of sclerotia by this technique from several artificially infested soil samples was approximately 80%. Soils artificially infested with the pathogen, cropped to onions, and subsequently assayed with this technique contained from 6 to 15 sclerotia/g soil. Naturally infested soils from various parts of the United States averaged 0-3 sclerotia/g soil.

Additional keywords: inoculum density, white rot.