Evaluation of Methods for Estimating Inoculum Potential of Aphanomyces euteiches in Soil. D. K. MALVICK, Research Fellow, Department of Plant Pathology, University of Minnesota, St. Paul 55108. J. A. PERCICH, Professor, F. L. PFLEGER, Professor, J. GIVENS, Undergraduate Research Assistant, and J. L. WILLIAMS, Graduate Research Assistant, Department of Plant Pathology, University of Minnesota, St. Paul 55108. Plant Dis. 78:361-365. Accepted for publication 22 December 1993. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/PD-78-0361.

Three methods were evaluated for determining inoculum potential of Aphanomyces euleiches inoculum in soil and sand: rolled towel (RT), most probable number (MPN), and soil indexing (SI) bioassays. The RT and MPN bioassays were evaluated using clay loam, loam, and sand that were artificially infested with a series of oospore concentrations ranging from 2 to 500/g. Oospore numbers added to soil and estimated inoculum potential measured using the RT and MPN were highly correlated (r = 0.99 and 0.85, respectively). These two bioassays, however, yielded highly variable results among replications, and inoculum potential estimates for sand and soil infested with equal numbers of oospores were dissimilar. When the three bioassays were compared using naturally infested clay loam soil, the SI method provided results with low variability compared with the MPN and RT bioassays. Estimates of inoculum potential obtained with the three bioassays were inconsistent between and within samples tested. The results from these bioassays must be interpreted with caution, and more accurate and sensitive methods are needed for quantitative studies of the inoculum potential and ecology of A. euteiches in soil.

Keyword(s): peas, Pisum sativum, root rot