First report of the occurrence of multiple resistance to Flutianil and Pyriofenone in field isolates of Podosphaera xanthii, the causal fungus of cucumber powdery mildew

Two new fungicides, flutianil and pyriofenone were introduced into the Japanese market in 2013 and 2014, to control powdery mildew on cucumber. Isolates of Podosphaera xanthii, the causal agent of powdery mildew, were collected in Ibaraki Prefecture, Japan, between 2017 and 2019 from cucumber greenhouses with a history of flutianil and pyriofenone usage. They were then tested for sensitivity to both fungicides by the leaf disc test. First, the sensitivity of three baseline reference isolates to each fungicide was determined. Minimum inhibitory concentrations values were 0.0063 mu g/ml for flutianil and 10 mu g/ml for pyriofenone, while 50% effective concentration (EC50) values of these fungicides were 0.00013-0.00035 mu g/ml and 0.39-0.70 mu g/ml, respectively. To determine the current sensitivity of P. xanthii in detail, 23 single-spore isolates were then sampled from five greenhouses and tested. Nineteen isolates showed high resistance (Flu-HR/Pyr-HR) with EC50 values of >100 mu g/ml for flutianil and > 1000 mu g/ml for pyriofenone. The low sensitivity of Flu-HR/Pyr-HR isolates was stable even after 46 subcultures on fungicide-untreated cotyledons. Additionally, two isolates showed moderate resistance (Flu-MR/Pyr-MR) to flutianil (EC50: mean 0.8 mu g/ml) and pyriofenone (mean 58.7 mu g/ml). The results from foliar inoculation tests on potted cucumber plants confirmed low efficacies of flutianil and pyriofenone against resistant isolates. Among a total of 122 isolates sampled from 13 greenhouses, 89 isolates (73.0%) were categorized as Flu-HR/Pyr-HR and two isolates (1.6%) as Flu-MR/Pyr-MR. This is the first report on flutianil resistance in any pathogen.