Status of sensitivity of Northern German Botrytis populations to the new SDHI fungicide fluopyram prior to its release as a commercial fungicide

In a survey of Botrytis populations in Northern Germany in 2012–2014, an average of 67.7% and 73.8% of isolates from strawberry and raspberry fields (respectively) were resistant to QoI fungicides whilst 40.2% and 64.9% (respectively) were resistant to the SDHI boscalid. There was a sporadic occurrence (2.8% and 3.3%, respectively) of strains resistant to the new SDHI fungicide fluopyram, due to be used in German soft fruits from 2015 onwards. Both moderate (EC₅₀ = 0.5–5.0 mg l⁻¹) and high (EC₅₀ > 10 mg l⁻¹) levels of resistance to fluopyram were observed in a spore germination assay in which the baseline sensitivity was 0.02–0.2 mg l⁻¹. Among several amino acid substitutions in the SDHI target site found in Botrytis isolates from fields of strawberries, raspberries and other horticultural crops, P225F and N230I, based on mutations in the sdhB gene, as well as a type of resistance of unknown basis, caused positive cross-resistance to boscalid and fluopyram. Further, N230I and the unknown type of resistance were detected in strains with multiple resistance to all other currently used fungicides, i.e. fenhexamid, cyprodinil, fludioxonil and QoIs. For resistance management purposes, it is suggested that the number of fluopyram applications should be limited to one per season.