Sensitivity of the causal agent of northern leaf blight of corn, Exserohilum turcicum, to the demethylase inhibiting fungicide flutriafol

Fungicides including active ingredients in the demethylation inhibitor group (Fungicide Resistance Action Committee Group 3) are used to manage corn diseases such as northern corn leaf blight, caused by Exserohilum turcicum. This study assessed the level of sensitivity to the demethylation-inhibiting fungicide flutriafol on a population of 81 Exserohilum turcicum isolates collected from across the United States in 2020. Sensitivity to flutriafol for E. turcicum was assessed using fungicide-amended media to determine the effective concentration needed to limit hyphal growth by 50%. Mean absolute EC50 values of the overall population calculated using the 4-parameter log-logistic and Weibull2 4-parameter models were 2.1881 and 3.2643 µg/ml, respectively. Growth of most isolates (n=70, 86.4%) was completely inhibited by 1 µg/ml flutriafol. However, growth of seven isolates (8.6%) was only completely inhibited at 10 or 100 µg/ml and for four isolates (4.9%) complete inhibition was not observed at 100 µg/ml. These results suggest that there may be isolates of E. turcicum in the U.S. that are resistant to flutriafol. Additional fungicide sensitivity screening is critical to monitoring for the development of fungicide resistance in U.S. populations of E. turcicum.