Evaluation of an Assay for Quantifying Populations of Sclerotia of Macrophomina phaseolina from Soil. C. Lee Campbell, Associate Professor, Department of Plant Pathology, North Carolina State University, Raleigh 27695. Larry A. Nelson, Professor, Department of Statistics, North Carolina State University, Raleigh 27695. Plant Dis. 70:645-647. Accepted for publication 29 January 1986. Copyright 1986 The American Phytopathological Society. DOI: 10.1094/PD-70-645.

A semiselective assay procedure for quantitative isolation of Macrophomina phaseolina from soil was evaluated for reliability and precision. Prior sieving of soil through mesh with 2-mm-diameter openings did not significantly affect recovery of sclerotia of M. phaseolina. The assay recovered a constant proportion of inoculum from soil over a range of inoculum densities (1–34 and 2–127 propagules per 10 g of soil). The optimum number of subsamples to assay per soil sample is one when variation among samples, subsample variation, and costs are considered. If costs are completely ignored, a sample size of two appears appropriate.