Comparison of Two Media Selective for Phytophthora and Pythium Species. S. N. Jeffers, Department of Plant Pathology, New York State Agricultural Experiment Station, Cornell University, Geneva 14456. S. B. Martin, Department of Plant Pathology, New York State Agricultural Experiment Station, Cornell University, Geneva 14456. Plant Dis. 70:1038-1043. Accepted for publication 5 July 1986. Copyright 1986 The American Phytopathological Society. DOI: 10.1094/PD-70-1038.

The standard selective medium for isolating pythiaceous fungi, P₁₀VP (cornmeal agar amended with 10 mg/ L pimaricin, 200 mg/L vancomycin, and 100 mg/L PCNB), was compared with a modified version of this medium, P₅ARP (pimaricin reduced to 5 mg/L and vancomycin replaced by 250 mg/L ampicillin + 10 mg/L rifampicin). Hymexazol (50 mg/L) was added (i.e., P₁₀VPH and P₅ARPH) to suppress unwanted Pythium colonies for direct enumeration of Phytophthora from soil. P₅ARP(H), when compared with P₁₀VP(H), 1) detected more germinable propagules of P. cinnamomi, P. parasitica var. nicotianae, and Pythium ultimum from naturally infested soils, 2) suppressed contaminating bacteria much better in the test soil, 3) was equally effective in isolating P. megasperma and P. cactorum from apple twigs, 4) was much less expensive to prepare, and 5) supported mycelium growth of 42 isolates of Phytophthora (13 species) and 10 isolates of Pythium (six species) equally well. PCNB was beneficial for reducing or eliminating background mycoflora on soil dilution plates. Hymexazol added to P₅ARP greatly inhibited P. cactorum and drastically limited the radial growth of one isolate each of P. megasperma f. sp. medicaginis, P. megasperma f. sp. glycinea, and P. capsici. P₅ARP(H) is an improvement over P₁₀VP(H) as a multipurpose selective medium for isolating Phytophthora and Pythium species from plant tissue and soil.