Spatial and temporal dynamics of fungicide resistance in <i>Botrytis cinerea</i> in California strawberries

Spatial and temporal effects of fungicide inputs on resistance selection were evaluated by conducting two field studies and by leveraging publicly available pesticide application data along with previously published fungicide resistance survey results. The trials were conducted to evaluate the short-term impact of fungicide programs on the development of resistance in Botrytis cinerea, the causal agent of Botrytis fruit rot in strawberries, where strawberry plots were treated with various single and rotational fungicide regimens over a six-week period. Resistance in B. cinerea isolates was assessed across five sampling dates using discriminatory dose assays. After the consecutive applications, resistance frequencies tended to remain stable to boscalid and fludioxonil but increased sharply for fenhexamid. After a 6-week period with no fungicide inputs, resistance frequencies to fenhexamid tended to decrease, while resistance frequency to pyraclostrobin was stable. The findings suggest varying responses in resistance selection and fitness costs depending on the mode of action that the populations were exposed to. The public fungicide application data showed that intensive strawberry production regions with high fungicide inputs were hot spots for fungicide resistance selection. The findings emphasize the need for strategic fungicide use and resistance management.