Field evaluation of fluazinam fungicide in dollar spot populations confirmed in vitro insensitivity.

Abstract

Fluazinam, a fungicide widely used in agriculture and turf management, was traditionally thought to pose a low risk of resistance. However, our in vitro sensitivity test conducted in 2021 revealed reduced sensitivity of fluazinam in dollar spot, highlighting the need for more careful field monitoring. In 2022 and 2023, we evaluated the field responses of four Clarireedia jacksonii isolates with different in vitro sensitivity to fluazinam. Fluazinam was used at a full labeled rate (0.5 oz/1,000 ft2) and a half-rate (0.25 oz/1,000 ft2) to evaluate the effectiveness of isolate-inoculated plots in the field. In 2022, natural and sensitive isolates showed significantly better control than insensitive isolates in both half- and full-rate treatments. However, in 2023, half-rate fluazinam demonstrated limited control in high disease pressure, providing relative disease control of dollar spot less than 45% across all treatments. In contrast, full-rate fluazinam maintained significantly better control of natural and sensitive isolates compared to insensitive isolates. Our results showing in vitro insensitivity leading to field insensitivity under inoculated field conditions suggest the development of fluazinam insensitivity in the C. jacksonii population. This highlights the need for judicious use of the fungicide fluazinam and the establishment of continuous resistance monitoring. Furthermore, the loss of control observed when applied at half-rates under high disease pressure highlights the importance of careful use of fungicides.