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Precision and Bias of Three Quantitative Soil Assays for Verticillium dahliae. P. C. Nicot, Research assistant, Department of Plant Pathology, University of Wisconsin, Madison 53706, Present address: Department of Plant Pathology, Beijing Agricultural University, People’s Republic of China; D. I. Rouse, Associate professor, Department of Plant Pathology, University of Wisconsin, Madison 53706. Phytopathology 77:875-881. Accepted for publication 26 November 1986. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/Phyto-77-875.

Three widely used techniques to quantitatively assay field soil for Verticillium dahliae were compared for precision, bias, and time required to assay a sample. The techniques compared were dilution plating, wet sieving, and the Andersen sampler technique. Precision was measured by the standard error of the mean for repeated assays of 10 samples of naturally infested soil. Bias was measured by the deviation between the inoculum density estimates obtained with each of the techniques and the true number of microsclerotia introduced into samples of steamed silica sand. Wet sieving was the most precise method, but also the most biased and time consuming, and gave the lowest recovery of the fungus from field soil. On the average, only 64% of the microsclerotia introduced into samples of steamed silica sand were detected with the wet sieving assay, and estimates of inoculum levels in naturally infested soil samples were nearly two to three times higher with the Andersen sampler and dilution plating than with wet sieving. The Andersen sampler technique was unbiased and efficient in detecting the fungus in field soil, but it was the least precise technique. Finally, the dilution plating technique was the least biased with a recovery rate of nearly 100% in steamed silica sand amended with a known number of propagules. It was the least time consuming, had the highest recovery rate of the fungus in naturally infested soil, and was moderately variable.