EVALUATION OF METHODS FOR ESTIMATING INOCULUM POTENTIAL OF APHANOMYCES-EUTEICHES IN SOIL

Three methods were evaluated for determining inoculum potential of Aph anomyces euteiches inoculum in soil and sand: rolled towel (RT), most probable number (MPN), and soil indexing (SI) bioassays. The RT and MP N bioassays were evaluated using clay loam, loam, and sand that were a rtificially infested with a series of oospore concentrations ranging f rom 2 to 500/g. Oospore numbers added to soil and estimated inoculum p otential measured using the RT and MPN were highly correlated (r = 0.9 9 and 0.85, respectively). These two bioassays, however, yielded highl y variable results among replications, and inoculum potential estimate s for sand and soil infested with equal numbers of oospores were dissi milar. When the three bioassays were compared using naturally infested clay loam soil, the SI method provided results with low variability c ompared with the MPN and RT bioassays. Estimates of inoculum potential obtained with the three bioassays were inconsistent between and withi n 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. eutei ches in soil.