Physical Factors that Influence the Recovery of Microsclerotium Populations of Cylindrocladium crotalariae from Naturally Infested Soils

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Abstract
Air-drying of soil (0.12-0.38% water or about -2000 bars) resulted in no recovery of microsclerotia from soils, but rewetting soils to near field capacity for 1-4 wk before assay resulted in partial recovery from the deleterious effect of drying. Incubation of soil at low temperature (6.degree. C) greatly reduced recovery of microsclerotia from soils. No microsclerotia were recovered from one soil incubated at 6.degree. C for 1 mo. Incubation at 26.degree. C had little or no effect on microsclerotium recovery. Many small microsclerotia were present in Virginia [USA] peanut-field soils. The median 90% of microsclerotia recovered had a width in the range 25-103 .mu.m; use of a sieve with a pore size of 25 .mu.m or slightly smaller is recommended for recovery of microsclerotia. Little or no interference from conidium-or ascospore-size propagules in soil was found in assays using this sieve. Soil dispersion by blending or shaking was not essential for assaying sandy peanut-field soils containing most microsclerotia free of organic matter.