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Detection and Quantification of Phytophthora capsici in Soil. Robert P. Larkin, Department of Plant Pathology, North Carolina State University, Raleigh, 27695-7616, Current address: USDA-ARS, Biocontrol of Plant Diseases Laboratory, Beltsville, MD 20705; Jean B. Ristaino, and C. Lee Campbell. Department of Plant Pathology, North Carolina State University, Raleigh, 27695-7616. Phytopathology 85:1057-1063. Accepted for publication 13 June 1995. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/Phyto-85-1057.

Several assay methods were compared for their efficacy in the detection and quantification of specific propagule types of Phytophthora capsici in soil. Zoospores, oospores, or sporangia and mycelial fragments were added to microwave-treated soil and nontreated field soil at densities from 1 to 1 104 propagules per g (ppg) of soil. Assay methods included standard soil dilution plating on selective medium without prior sample incubation, as well as dilution plating of soil, saturation water, or a pepper leaf disk bioassay after sample saturation, drainage, incubation for 5 days, and a 24-h resaturation period. Zoospore inoculum was detected at 10 ppg of soil or higher with standard soil dilution plating and the leaf disk bioassay, compared to >100 ppg of soil with dilution plating of soil or saturation water after sample incubation. Sporangial inoculum was detected at 1 ppg of soil with all assays when soil water matric potential was controlled during sample incubation. Oospores were detected at 1 ppg of soil with soil dilution and leaf disk assays after sample incubation. Maximum recovery rates were 10 and 100% of added zoospore and sporangial inoculum, respectively, with standard soil dilution plating (no incubation), and 30% of oospore inoculum with soil dilution plating after sample incubation. A constant soil water matric potential of 10 J/kg during the sample incubation period improved inoculum recovery with all assays, compared to incubation without controlled soil water matric potential. The sample incubation and saturation periods stimulated oospore germination and allowed detection and recovery of oospores added to soil. For most assay methods, recovery of all propagule types was lower in field soil than in microwave-treated soil; however, the lower limits of detection were comparable in both soils. Although no single assay method was suitable for the accurate detection and quantification of all propagule types of P. capsici, the leaf disk assay provided the best detection of all propagule types but was not sufficiently quantitative to estimate inoculum densities. Soil dilution plating without prior sample incubation was the best assay for the quantification of zoospores and sporangia, whereas soil dilution plating after sample saturation and incubation provided the best recovery of oospores.