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Development of resistance in field populations of Botrytis cinerea following exposure to various fungicide programs

Scott Cosseboom: Strawberry Center, California Polytechnic State University

<div>Gray mold of strawberry, caused by <em>Botrytis cinerea</em>, is a very destructive fruit rot and is routinely managed by aerial fungicide applications. However, multiple chemical-class-resistant (CCR) populations reside in strawberry fields. We conducted two replicated field trials to examine the rate of resistance development to the active ingredients boscalid, fenhexamid, and fludioxonil. Treatments were applied weekly to plots for six weeks followed by a six-week period of no applications. Throughout this 12-week trial, 112 isolates of <em>B. cinerea</em> were collected from sporulating strawberry fruit five times. A mycelial growth assay using discriminatory dosages of fungicides differentiated sensitive from resistant isolates. Prior to the first fungicide application, low frequencies of resistance were observed to boscalid and fludioxonil, but 30% of all isolates were resistant to fenhexamid. Resistance to fenhexamid in control plots remained steady throughout the trial, yet treatments involving fenhexamid revealed a sharp increase in resistance to fenhexamid after the sixth application and an equally sharp decrease in resistance during the period of no applications. Our “resistance management” treatments (tank mixtures and chemical class rotation) offer insight into the effectiveness of these tactics. These trials indicate that the frequencies of resistance in a field population can drastically change within one season depending on the fungicides applied.</div>