Baseline Sensitivities of New Fungicides and Their Toxicity to Selected Life Stages of Phytophthora Species from Citrus in California

Abstract

Phytophthora citrophthora, P. syringae, P. nicotianae, and P. hibernalis are important pathogens of citrus in California but few chemical treatments are currently available. In vitro toxicities of four new fungicides to isolates of Phytophthora spp. from California were determined. Mean effective concentration values to inhibit mycelial growth by 50% for ethaboxam, fluopicolide, mandipropamid, oxathiapiprolin, and mefenoxam were 0.068, 0.04, 0.004, 0.0003, and 0.039 µg/ml, respectively, for 62 isolates of P. citrophthora; 0.005, 0.045, 0.003, 0.0001, and 0.008 µg/ml, respectively, for 71 isolates of P. syringae; 0.016, 0.057, 0.005, 0.0005, and 0.183 µg/ml, respectively, for 31 isolates of P. nicotianae; and 0.030, 0.018, 0.005, <0.0003, and ≤0.001 µg/ml, respectively, for two isolates of P. hibernalis. Mean values for ≥90% inhibition of sporangia formation of four isolates of P. citrophthora were 0.1, 0.28, 0.026, 0.005, and 55 µg/ml for ethaboxam, fluopicolide, mandipropamid, oxathiapiprolin, and mefenoxam, respectively. Zoospore cyst germination of P. citrophthora was most inhibited by oxathiapiprolin and mandipropamid. Chlamydospore formation of P. nicotianae was most sensitive to oxathiapiprolin, with a mean ≥90% reduction (EC>90) of 0.002 µg/ml, moderately sensitive to mandipropamid (EC>90 = 0.2 µg/ml) and mefenoxam (EC>90 = 0.6 µg/ml), and least sensitive to ethaboxam and fluopicolide (EC>90 = 1 µg/ml). Oospore formation of P. nicotianae was inhibited by ≥90% using oxathiapiprolin at 0.0004 µg/ml, mandipropamid at 0.02 µg/ml, ethaboxam at 0.1 µg/ml, or fluopicolide at 0.4 µg/ml, whereas 62% inhibition was obtained by mefenoxam at 40 µg/ml.