Post-Infection Application of Fenhexamid at Lower Doses in Conjunction with Captan Slowed Fungicide Resistance Selection in Botrytis cinerea on Detached Grape Berries

Abstract

Fungicide resistance is a limiting factor in sustainable crop production. General resistance management strategies such as rotation and mixtures of fungicides with different modes of action have been proven to be effective in many studies, but guidance on fungicide dose or application timing for resistance management remains unclear or debatable. In this study, Botrytis cinerea and the high-risk fungicide fenhexamid were used to determine the effects of fungicide dose, mixing partner, and application timing on resistance selection across varied frequencies of resistance via detached fruit assays. The results were largely consistent with the recent modeling studies which favored the use of the lowest effective fungicide dose for improved resistance management. In addition, even 10% resistant B. cinerea in the population led to about 40% reduction of fenhexamid efficacy. Overall, our findings showed that application of doses less than the fungicide label dose, mixture with the low-risk fungicide captan, and application post-infection seem to be the most effective management strategies in our controlled experimental settings. This somewhat contradicts the previous assumption that preventative sprays help resistance management.