Selective Isolation of Species of Phytophthora from Natural Soils on an Improved Antibiotic Medium

Abstract

DIRECT isolation from soil on many of the common agar media used for isolating soil fungi has long been unsuccessful for species of Phytophthora, one of the most destructive fungal pathogens. Until recently^1–5, the most successful methods for isolating these species from soil involved susceptible plant hosts or fruit traps^6–8. The selective antibiotic media developed a few years ago—the 3P medium⁹ and the PV medium¹⁰—are satisfactory for isolating Phytophthora from infected plant tissues. The antifungal polyene antibiotic pimaricin, used at 100 p.p.m. (µg/ml.) in both media, inhibits the growth of almost all fungi except the pythiaceous members (Pythium and Phytophthora species)⁹. But neither of these media has been effective for isolating Phytophthora from infested soils. Reasons for this difference in recovery between the use of infected tissue and infested soil were unknown for several years. It was later discovered^3,4,11,12 that spores and mycelia of many species of Phytophthora have a differential sensitivity to pimaricin. At 100 p.p.m., the concentration used in both the 3P and PV media, pimaricin did not inhibit mycelial growth but strongly or completely inhibited the germination of the resistant chlamydospores, sporangia and zoospores of many species tested. It was partially inhibitory to these spores at 25 p.p.m., but was non-inhibitory at 12 p.p.m. or less. Most species of Phytophthora do not exist in soil as mycelia, but chiefly as non-mycelial propagules (for example, chlamydospores), and so it became apparent why Phytophthora could not be recovered from soil whenever selective media containing pimaricin at 100 p.p.m. were used. It also explained the recovery of P. megasperma variety sojae from soil by Haas¹ on a modified 3P medium containing only 2 p.p.m. of pimaricin.