Resistance of Botrytis cinerea to Multiple Fungicides in Northern German Small-Fruit Production

During the vegetation period 2010, 353 isolates of Botrytis cinerea from 23 Northern German strawberry, raspberry, highbush blueberry, and redcurrant fields were examined for sensitivity to the benzimidazole derivative thiophanate-methyl and the dicarboximide iprodione, as well as five fungicides currently used against gray mold in Germany. Of all isolates, 40.5% were highly resistant to thiophanate-methyl, 64.0% to iprodione, 45.0% to fenhexamid, 76.8% to trifloxystrobin, 21.5% to boscalid, and 14.7% to cyprodinil. No high resistance to fludioxonil was observed but medium resistance was recorded to fludioxonil as well as cyprodinil (41.1 and 27.2% of all isolates, respectively). In all, 63 isolates were sensitive to all five of the currently registered botryticides whereas 43, 81, 94, 49, and 23 isolates were medium or highly resistant to one, two, three, four, and five fungicides, respectively. Isolates resistant to five fungicides in vitro were capable of causing fruit rot on wounded apple pretreated with any one of the three commercially available products containing fenhexamid, pyraclostrobin plus boscalid, or cyprodinil plus fludioxonil. These results question the sustainability of the current gray mold control strategy relying exclusively on fungicides with specific, single-site modes of action.