A Rapid Microbioassay for Discovery of Novel Fungicides for Phytophthora spp.

ABSTRACT A microbioassay was developed for the discovery of compounds that inhibit Phytophthora spp. This assay uses a 96-well format for high-throughput capability and a standardized method for quantitation of initial zoospore concentrations for maximum reproducibility. Zoospore suspensions were quantifiable between 0.7 and 1.5 x 10(5) zoospores per ml using percent transmittance (620 nm). Subsequent growth of mycelia was monitored by measuring optical density (620 nm) at 24-h intervals for 96 h. Full- and half-strength preparations of each of three media (V8 broth, Roswell Park Memorial Institute mycological broth [RPMI], and mineral salts medium) and four zoospore concentrations (10, 100, 1,000, and 10,000 zoospores per ml) were evaluated. Both full- and half-strength RPMI were identified as suitable synthetic media for growing P. nicotianae, and 1,000 zoospores per ml was established as the optimum initial concentration. The assay was used to determine effective concentration values for 50% growth reduction (EC(50)) for seven commercial antifungal compounds (azoxystrobin, fosetyl-aluminum, etridiazole, metalaxyl, pentachloronitrobenzene, pimaricin, and propamocarb). These EC(50) values were compared with those obtained by measuring linear growth of mycelia on fungicide-amended medium. The microbioassay proved to be a rapid, reproducible, and efficient method for testing the efficacy of compounds that inhibit spore germination in P. nicotianae and should be effective for other species of Phytophthora as well. The assay requires relatively small amounts of a test compound and is suitable for the evaluation of natural product samples.