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Resistance to SDHI fungicides in Botrytis cinerea from commercial strawberry fields in Spain

Dolores Fernandez-Ortuno: IHSM-UMA-CSIC La Mayora

<div>Gray mold, caused by <em>Botrytis cinerea</em> Pers., is one of the most economically important diseases of strawberries. Gray mold control involves the application of fungicides throughout the strawberry growing season; however, <em>B. cinerea</em> isolates resistant to multiple classes of site-specific fungicides have been recently reported in the Spanish <em>B. cinerea</em> population. Succinate dehydrogenase inhibitors (SDHIs) constitute a relatively novel class of fungicides registered for gray mold control representing new alternatives for strawberry growers. In the present study, 37 <em>B. cinerea</em> isolates previously characterized for their sensitivity to boscalid and amino acid changes in the SdhB protein, were used to determine the effective concentration that reduces mycelial growth by 50% (EC<sub>50</sub>) to fluopyram, fluxapyroxad and penthiopyrad. The EC<sub>50</sub> values ranged from 0.01 to >100 μg/ml for fluopyram, from <0.01 to 4.19 μg/ml for fluxapyroxad, and finally, from <0.01 to 59.65 μg/ml for penthiopyrad. The present study was also conducted to obtain discriminatory doses to monitor SDHI fungicide resistance in 580 <em>B. cinerea</em> isolates collected from 27 commercial fields in Spain during 2014-2016. Over the course of the three-year monitoring period, the overall frequencies of resistance to boscalid, fluopyram, fluxapyroxad and penthiopyrad were 56.9, 6.9, 12.9, and 24.6%, respectively. Four SDHI resistance patterns were observed in our population, which were associated with different aminoacid substitutions (H272R/Y, N230I) in the SdhB subunit. For gray mold management, it is suggested that the simultaneous use of boscalid and penthiopyrad should be limited to one application per season; however, the use of fluxapyroxad and, especially, fluopyram could be used as valid SDHI alternatives for gray mold control, but they should be applied with caution.</div>