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Detection and Quantification of Phytophthora cactorum in Naturally Infested Soils. M. K. Rahimian, Postdoctoral project associate, Department of Plant Pathology, University of Wisconsin, Madison 53706; J. E. Mitchell, Emeritus professor, Department of Plant Pathology, University of Wisconsin, Madison 53706. Phytopathology 78:949-952. Accepted for publication 11 February 1988. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/Phyto-78-949.

The optimum temperature for release of zoospores of Phytophthora cactorum grown on a culture medium was 8 C. In contrast, the most frequent Pythium isolated from naturally infested ginseng soils did not release zoospores at 4, 8, and 12 C. Based on these results, a procedure was developed to detect and quantify P. cactorum in naturally infested soil. Air-dried soil was moistened 1–3 days and flooded 2 days at 24 C. Soil was then exposed to 8 C for 2 hr to induce zoospore release. The soil supernatant was then plated onto a selective medium. Duration of the moistening period of 1–3 days did not affect recovery of the fungus. Temperatures of 16–24 C during this period did not significantly affect the recovery of the fungus, but a temperature of 28 C significantly reduced the recovery of the fungus. A soil sample from around an apple tree showing symptoms of crown rot and a soil sample from around a maple tree yielded 1,078 and 53 colony-forming units (cfu) of P. cactorum per gram dry soil, respectively. Thirty soil samples from ginseng gardens yielded a range of 7 to 61 cfu/g dry soil. Colonies of Pythium species were rarely observed. The number of colony-forming units obtained from the apple soil was not significantly different when the assay was replicated 15 times. The procedure developed here is simple, relatively quick, and a reproducible method for detection and quantification of P. cactorum in naturally infested soils.