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A “Most Probable Number” Method for Estimating Inoculum Density of Aphanomyces euteiches in Naturally Infested Soil. W. F. Pfender, Graduate research assistant, Department of Plant Pathology, University of Wisconsin, Madison 53706; D. I. Rouse(2), and D. J. Hagedorn(3). (2)(3)Assistant professor, and professor, respectively, Department of Plant Pathology, University of Wisconsin, Madison 53706. Phytopathology 71:1169-1172. Accepted for publication 22 February 1981. Copyright 1981 The American Phytopathological Society. DOI: 10.1094/Phyto-71-1169.

A bioassay was developed to determine the effective inoculum density of Aphanomyces euteiches in naturally infested field soil. Infested soil was mixed with steamed soil to produce a series of dilution levels. Each dilution level was divided into aliquots, and each aliquot was tested for the presence of the pathogen using a host bioassay under optimal conditions for infection. The proportion of aliquots found to be infested at each dilution level was used to calculate the infective inoculum density of A. euteiches in the original soil by means of the “most probable number” (MPN) method. A computer program was written to calculate the MPN for any series of up to five dilutions and any number of aliquots per dilution. In addition, the program calculates confidence limits for the MPNs and the statistical acceptability of the MPN. MPN estimates of infective inoculum density in field soils were consistent with the results of a standard root rot potential test and were correlated with epidemiologic and yield measurements from field plots of diseased peas.