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Selective Isolation and Enumeration of Laetisaria arvalis from Soil. G. C. Papavizas, Plant pathologist, Soilborne Diseases Laboratory, Plant Protection Institute, Agricultural Research Service, U.S. Department of Agriculture, Beltsville, MD 20705; B. B. Morris(2), and J. J. Marois(3). (2)(3)Research associate, and plant pathologist, respectively, Soilborne Diseases Laboratory, Plant Protection Institute, Agricultural Research Service, U.S. Department of Agriculture, Beltsville, MD 20705, (2)Present address: Department of Genetics, University of Wisconsin, Madison 53706. Phytopathology 73:220-223. Accepted for publication 28 July 1982. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1983. DOI: 10.1094/Phyto-73-220.

Three MBC fungicides (benomyl, thiabendazole, thiophanate-methyl) at 25 μg a.i./ml of potato-dextrose agar completely inhibited linear growth of Rhizoctonia solani but not that of Laetisaria arvalis (Corticium sensu lato), an antagonist of R. solani and Pythium ultimum. Using this information, a semiselective medium was developed for the direct isolation and enumeration of L. arvalis from soil. The medium, designated as WAA-HC+TBZ, contained, per liter: agar, 15 g; streptomycin SO₄, chlorotetracycline HCl, and thiabendazole, 100, 50, and 25 mg, respectively; and Hunt and Cobb solution, 5 ml. The average percentage recovery of the antagonist from artificially infested soil with a multiple pellet soil-sampler (MPSS) was 66%. The table beet seed colonization method of isolation was more sensitive than the MPSS technique in recovering L. arvalis from soil but did not yield quantitative data on numbers of propagules per gram of soil. Isolation of L. arvalis was also obtained on the WAA-HC+TBZ medium by the dilution-plate method, but the numbers recovered were smaller than those obtained with the MPSS. Numbers of propagules of L. arvalis in seven field soils ranged from 0 to 7.0 per gram of soil. No significant difference of population density of L. arvalis was observed on the medium from soils infested or uninfested with R. solani.