INFLUENCE OF SOIL CHARACTERISTICS AND ASSAY TECHNIQUES ON QUANTIFICATION OF VERTICILLIUM-DAHLIAE IN OHIO SOILS

Samples of soil from 45 commercial potato production fields in Ohio we re analyzed for physical characteristics. Subsamples from 21 soils wer e either infested with two levels of Verticiilium dahliae or left unin fested, 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 h igher when soil was dried for 2 or 4 wk. Recovery from artificially in fested soils was highly variable among samples, even within similar te xtural groupings. Recovery from coarse-textured soils was variable acr oss 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 s oils. Organic matter was not generally associated with low recovery of V. dahliae propagules, while gravel content and pH were inversely ass ociated with recovery efficiency. The influence on recovery of V. dahl iae of glass vs. plastic petri plates, soil concentration in a direct- plate assay, and the total sail sample volume were also examined. Reco very of V. dahliae was higher with glass than with plastic petri plate s in two of three soils tested. Soil dilution had no effect on colony- farming units per cubic centimeter of soil. Increasing the number of s oil 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.