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.

Evaluation of New Oomycota Fungicides for Management of Phytophthora Root Rot of Citrus in California

“…The isolates of P. nicotianae and P. citrophthora in the current study were used as representatives of each species with EC 50 values for each fungicide within the baseline range of sensitivity. In the Gray et al (2018) study, values for oxathiapiprolin were the lowest among the four compounds for inhibition of five Phytophthora life stages including mycelial growth, sporangium formation, zoospore cyst germination, and chlamydospore and oospore formation. This high toxicity was reflected by low effective field rates (i.e., 35 g/ha) that we identified in the field study with P. citrophthora.…”

“…The high efficacy of ethaboxam, fluopicolide, mandipropamid, and oxathiapiprolin in reducing disease and Phytophthora soil populations correlated with their low in vitro effective concentration (EC) values against mycelial growth of several Phytophthora species from citrus that we determined previously (Gray et al 2018). The isolates of P. nicotianae and P. citrophthora in the current study were used as representatives of each species with EC 50 values for each fungicide within the baseline range of sensitivity.…”

“…Thus, the reduced effectiveness of mefenoxam in our study, a treatment that has been used successfully in commercial applications for managing Phytophthora root rot of various tree crops for many years, may have been due to using inadequate rates. Furthermore, trees were inoculated with an isolate of P. nicotianae with an EC 50 value for mycelial growth of 0.24 mg/liter that was in the midrange among 31 isolates from California evaluated previously (Gray et al 2018). Because baseline sensitivities before commercial use of mefenoxam were never established for P. nicotianae from citrus, and the phenylamide class of fungicides has been used since the 1980s in California citriculture, this isolate may be part of a lesssensitive subpopulation of the species that cannot be easily managed with mefenoxam applications.…”

“…The thiazole carboxamide (FRAC code 22) ethaboxam inhibits mitosis and cell division (Uchida et al 2005), the benzamide (FRAC code 43) fluopicolide destabilizes plasma membrane formation by delocalizing a spectrin-like protein (Toquin et al 2006), the carboxylic acid amide (FRAC code 40) mandipropamid targets the cellulose synthase-like gene and inhibits pathogen cell wall synthesis (Blum et al 2010), and oxathiapiprolin, the first fungicide in the isoxazoline class (FRAC code 49), inhibits an oxysterol binding protein and thereby inhibits many cellular processes (Pasteris et al 2016). We previously determined that the four fungicides were highly inhibitory to mycelial growth of P. nicotianae, P. citrophthora, P. syringae, and P. hibernalis isolates from citrus in California (Gray et al 2018). Furthermore, oxathiapiprolin and mandipropamid also inhibited sporangium and oospore formation as well as zoospore cyst germination using low concentrations, and oxathiapiprolin was also highly active against chlamydospore formation (Gray et al 2018).…”

“…We previously determined that the four fungicides were highly inhibitory to mycelial growth of P. nicotianae, P. citrophthora, P. syringae, and P. hibernalis isolates from citrus in California (Gray et al 2018). Furthermore, oxathiapiprolin and mandipropamid also inhibited sporangium and oospore formation as well as zoospore cyst germination using low concentrations, and oxathiapiprolin was also highly active against chlamydospore formation (Gray et al 2018).…”