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Detection of Phytophthora parasitica from Soil and Host Tissue with a Species-Specific DNA Probe. P. H. Goodwin, Department of Environmental Biology, University of Guelph, Guelph, ONT N1G 2W1; J. T. English, D. A. Neher, J. M. Duniway, and B. C. Kirkpatrick. Department of Plant Pathology, University of California, Davis 95616. Phytopathology 80:277-281. Accepted for publication 12 September 1989. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/Phyto-80-277.

Phytophthora parasitica was detected with a species-specific DNA probe by employing several common techniques for isolating P. parasitica from soil, and then extracting DNA from the increased biomass. DNA of P. parasitica was detected with the probe following dilution plating of infested soil onto semiselective medium covered with a membrane, crushing portions of fungal colonies on a membrane, crushing tomato leaf disk baits on a membrane, and isolating DNA from colonized leaf disks. These methods were sufficiently sensitive to detect propagule concentrations in soil below those associated with yield loss in tomatoes. P. parasitica was also detected in infected tomato roots by modifying the DNA isolation method developed for leaf disk baits. To determine the feasibility of quantifying P. parasitica in soil with a DNA hybridization assay, the amount of P. parasitica DNA detected in leaf disk baits was compared to the number of propagules estimated by the most probable number method for the same soil samples. The relationship between the two sets of data, however, varied with sampling date. Although this assay method did not reliably quantify populations of P. parasitica, the results demonstrate that a DNA probe can detect P. parasitica in soil at population levels comparable to those detected by baiting methods.