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Selective Isolation of Phytophthora capsici from Soils. G. C. Papavizas, Plant pathologist, Soilborne Diseases Laboratory, Plant Protection Institute, Agricultural Research, Science and Education Administration, U.S. Department of Agriculture, Beltsville, MD 20705; J. H. Bowers(2), and S. A. Johnston(3). (2)Research assistant, Soilborne Diseases Laboratory, Plant Protection Institute, Agricultural Research, Science and Education Administration, U.S. Department of Agriculture, Beltsville, MD 20705; (3)Assistant professor, Department of Plant Pathology, Cook College—New Jersey Agricultural Experiment Station, Rutgers University, New Brunswick 08903. Phytopathology 71:129-133. Accepted for publication 1 August 1980. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1981. DOI: 10.1094/Phyto-71-129.

A selective medium and a dilution-plate technique were developed for the direct isolation of Phytophthora capsici from soil and from infected pepper plants, and for the quantitative estimation of the inoculum density of the pathogen in soil. The selective agar medium, a modification of Tsao’s P₁₀VP medium, contained Difco cornmeal agar (17 g/L) acidified to pH 3.8–4.0 with 3.5 ml of 1.0 N HCl added after autoclaving. The medium, designated as P₅VPP-BH, contained the following antimicrobial agents (μg/ml); pimaricin, 5; vancomycin, 200; pentachloronitrobenzene, 100; penicillin G, 100; benomyl, 2.5; and hymexazol, 20. Aliquots from concentrated soil dilutions (1 in 2, 1 in 4, 1 in 8, and 1 in 10) were placed on the surface of the P₅VPP-BH medium, incubated for 3–4 days at 20 C in the dark, washed with a cotton swab under running water, and examined with oblique fluorescent light when the agar surface was still wet. The average recovery of sporangia and zoospores on this medium from artificially infested soil was about 60 and 22%, respectively. Recovery of mycelial inoculum from soil on the P₅VPP-BH medium was satisfactory. Although P. capsici also could be recovered from soil with the most probable number technique, quantitation by this technique is limited to soils with ≥46 propagules per gram of soil. The number of propagules per gram of soil recovered from an artificially infested field plot declined rapidly from 6 days after infestation until no propagules could be recovered at about 40 days after infestation. Naturally infested soils from commercial pepper, squash, and eggplant fields in southern New Jersey averaged 0–24 propagules per gram of soil. Both compatability types of P. capsici were found in commercial fields and in several cases in the same field or on the same plant.