Establishment of baseline sensitivity and fungicide sensitivity profiles of Fusarium graminearum from wheat grown in the United States to pydiflumetofen fungicide

Fusarium head blight (FHB), primarily caused by Fusarium graminearum, is a prominent disease across United States (U.S.) wheat producing regions, contributing to yield losses and grain contamination with mycotoxins. For many years, fungicide products that contained demethylation inhibitor (DMI) active ingredients were the only fungicide products that included FHB as a target disease on their labels. In 2019, the first succinate dehydrogenase inhibitor (SDHI) fungicide, pydiflumetofen, was registered for use on wheat for FHB management. In vitro fungicide-amended plate mycelial assays were performed to determine baseline fungicide sensitivity levels and sensitivity profiles of F. graminearum, isolated from wheat, to pydiflumetofen. As part of the US Wheat and Barley Scab Initiative, 177 isolates were collected from 16 states and screened for fungicide sensitivity, including 14 historic isolates (1991 to 2014 with no known exposure to pydiflumetofen) used to establish baseline fungicide sensitivity levels, 98 isolates obtained in 2020, and 65 isolates obtained in 2021. The effective concentration to reduce mycelial growth by 50% (EC 50 ) was determined for each isolate using fungicide-amended growth medium. The average EC 50 baseline value of historic isolates was 0.289 μg/mL. In 2020-2021 isolates, mycelial growth EC 50 values averaged by state and year(s) ranged from 0.156 to 0.555 μg/mL, cumulatively averaging 0.338 μg/mL across all 2020-2021 isolates. The established baseline fungicide sensitivity level and 2020-2021 sensitivity profiles will aid fungicide research by allowing for monitoring of changes in sensitivity levels in F. graminearum populations as use of SDHI active ingredients expands across U.S. wheat and rotational crops.