Sensitivity of Fusarium graminearum to metconazole and tebuconazole fungicides before and after widespread use in wheat in the United States

Fusarium head blight (FHB) caused primarily by Fusarium graminearum, is a major disease of wheat in the United States. FHB is managed in part by applications of demethylation inhibitor (DMI) triazole fungicides during anthesis. The objective of this study was to examine the sensitivity of U.S. populations of F. graminearum to the DMI triazole fungicides metconazole and tebuconazole. Isolates of F. graminearum collected from wheat between 1981 and 2014 were tested for fungicide sensitivity using mycelial growth assays to determine the effective concentration at which 50% of fungal growth was inhibited (EC₅₀). A total of 45 isolates were tested for metconazole sensitivity and 47 for sensitivity to tebuconazole. Isolates were analyzed in groups based on collection date. Groupings consisted of isolates collected prior to widespread fungicide use in wheat (designated as year 2000) or after fungicides became available for use in wheat. The mean EC₅₀ for isolates collected prior to 2000 was 0.0240 μg/ml for metconazole and 0.1610 μg/ml for tebuconazole. For both fungicides, isolates collected between 2000 and 2014 had significantly higher (P = 0.05) mean EC₅₀ values (mean EC₅₀ = 0.0405 and 0.3311 μg/ml for metconazole and tebuconazole, respectively) compared with isolates collected prior to 2000. Isolate, year, and state of collection all affected the mean EC₅₀ values of isolates collected between 2000 and 2014. A single isolate collected from Illinois in 2012 exhibited EC₅₀ values of 0.1734 mg/ml for metconazole and 1.7339 μg/ml for tebuconazole, indicating reduced sensitivity compared with the mean EC₅₀ of other isolates collected between 2000 and 2014. This study is the first step toward developing a fungicide sensitivity monitoring program for F. graminearum in the United States.