Comparison of Two Media Selective forPhytophthoraandPythiumSpecies

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Abstract
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, P3ARP (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 P5 ARPH) 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 Phytophthora 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 Phytophthora 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. P5 ARP(H) is an improvement over P10VP(H) as a multipurpose selective medium for isolating Phytophthora and Pythium species from plant tissue and soil.