Influence of Soil Characteristics and Assay Techniques on Quantification ofVerticillium dahliaein Ohio Soils

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Abstract
Samples of soil from 45 commercial potato production fields in Ohio were analyzed for physical characteristics. Subsamples from 21 soils were either infested with two levels of Verticiilium dahliae or left uninfested, then dried for 0, 2, 4, or 6 wk and assayed for colony-forming units of V. dahliae per cubic centimeter of soil. Recovery efficiency from naturally infested fine-textured soils was similar for all four drying periods, while recovery in peat and coarse-textured soils was higher when soil was dried for 2 or 4 wk. Recovery from artificially infested soils was highly variable among samples, even within similar textural groupings. Recovery from coarse-textured soils was variable across all drying times. Recovery was highest at 2 and 4 wk of drying for peat soils and least variable at all drying times for fine-textured soils. Organic matter was not generally associated with low recovery of V. dahliae propagules, while gravel content and pH were inversely associated with recovery efficiency. The influence on recovery of V. dahliae of glass vs. plastic petri plates, soil concentration in a direct-plate assay, and the total sail sample volume were also examined. Recovery of V. dahliae was higher with glass than with plastic petri plates in two of three soils tested. Soil dilution had no effect on colony-farming units per cubic centimeter of soil. Increasing the number of soil cores that made up a composite soil (up to 20) from which a single subsample was assayed resulted in less deviation between observed and expected colony-farming units per cubic centimeter.
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