Comparative efficacy of oxathiapiprolin-based fungicides in controlling late blight in field-grown potatoes induced by mixed mefenoxam-sensitive and resistant genotypes of Phytophthora infestans

Potatoes are grown in Israel twice a year, in autumn and spring, contributing over $300 million to the state's economy in 2022. In both seasons late blight caused by the oomycete Phytophthora infestans is a major threat to yields. Fungicides are the most important means to control P. infestans. The classical fungicides mancozeb and chlorothalonil were banned by the regulator while mefenoxam, cymoxanil and infinito, suffer of reduced field efficacy. The new OSBPI fungicide oxathiapiprolin solo provides excellent control of the disease by foliar or soil applications but its proneness to resistance development enforces its application in mixtures with fungicides with a different mode of action. In three field trials done in Israel during 2021–2023, oxathiapiprolin solo and seven oxathiapiprolin-premixed fungicides were applied to potato crops Sifra or VR808 at early season. Plants were thereafter inoculated with mixed genotypes of P. infestans, sensitive and resistant to mefenoxam, and disease progress was monitored along 35 days after inoculation. All oxathiapiprolin products provided excellent control of the disease all along the season, significantly better than the fungicides mancozeb, mefenoxam, cymoxanil, ranman or infinito. Disease control was dependent on the dose applied, the proportion of oxathiapiprolin in the mixture, the partner fungicide, and the experiment. At the recommended dose of 0.1 % (50 g/ha in 500 L), the mixtures with mefenoxam or mandipropamid were more effective than the mixtures with chlorothalonil or azoxystrobin. At 0.2 %, all seven mixtures were highly effective all along the season. Tuber yields were significantly higher in plots treated with oxathiapiprolin mixtures as compared with control fungicide-free plots. Monitoring assays done along the season failed to detect oxathiapiprolin-insensitive mutants of P. infestans in all three trials, including plots treated with 0.2 % oxathiapiprolin solo (MIC = 0.3 ppm ai). We conclude that a single preventive application of oxathiapiprolin-based fungicides to potato crops at early season can provide effective control of late blight under heavy pressure of P.infestans for as long as 5 weeks without selecting for insensitive mutants.