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Phenotypic and molecular characterization of Botrytis cinerea isolates from strawberry to isofetamid and cross-resistance with other SDHI fungicides.

Adrian Zuniga: Gulf Coast Research and Education Center; University of Florida

<div><em>Botrytis cinerea</em>, the causal agent of Botrytis fruit rot on strawberry, has rapidly selected resistance to the SDHI fungicides. In this study, we identified mutations conferring resistance to isofetamid, a SDHI newly registered for strawberry, and determined resistance frequencies of 565 Botrytis isolates to five SDHI fungicides, i.e. boscalid, penthiopyrad, fluopyram, benzovindiflupyr, and isofetamid. Isolates were collected during two consecutive seasons (2015-16 and 2016-17), and used for fungicide sensitivity assays. Molecular characterization to isofetamid was based on mycelial growth inhibition assays of previously characterized isolates with B<sup>H272R</sup>, B<sup>H272Y</sup>, B<sup>N230I</sup>, and B<sup>P225F</sup> mutations known to confer resistance to SDHI fungicides, but only B<sup>N230I</sup> and B<sup>P225F</sup> were found in isolates moderately and highly resistant to isofetamid, respectively. Among the <em>B. cinerea</em> isolates collected during the first season, 95, 33, 21, 25, and 0% were resistant to boscalid, penthiopyrad, fluopyram, benzovindiflupyr, and isofetamid, respectively. The respective resistance frequencies for the second season were 91, 95, 44, 27, and 1.3%. Three moderately resistant isolates to isofetamid were found during 2016-17 and the B<sup>N230I</sup> mutation was confirmed. Isolates moderately resistant to isofetamid were also highly resistant to the other SDHI’s tested, suggesting alternative disease control strategies are needed to avoid cross-resistance and preserve efficacy of newer SDHI fungicides.</div>