Selective Isolation and Enumeration ofLaetisaria arvalisfrom Soil

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Abstract
Three MBC [2-benzimidazolcarbamic acid methyl ester] fungicides (benomyl, thiabendazole, thiophanate-methyl) at 25 .mu.g a.i. [active ingredient]/ml of potato-dextrose agar completely inhibited linear growth of Rhizocionia solani but not that of Laetisaria arvalis (Corticium s.l.), 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 [2-(4-thiazolyl)benzimidazole] contained, per liter: agar, 15 g; streptomycin SO4, chlortetracycline 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 7 field soils ranged 0-7.0 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.