Differential responses of <i>Botrytis</i> isolates to Pristine, Luna Tranquility, Switch, and their active ingredients

<i>Botrytis cinerea</i> is an adaptable, high-risk pathogen capable of developing resistance to several site-specific fungicides. To manage gray mold, blueberry growers in Washington state and Oregon follow spray regimes that include rotation of site-specific fungicides, tank-mixing site-specific fungicides with a multisite fungicide such as captan, and using premix fungicides to slow down selection of <i>Botrytis</i> populations with resistance to site-specific fungicides. In this study, we conducted conidial germ tube growth assays on media to assess responses of <i>Botrytis</i> isolates to discriminatory doses of three premix fungicides (Pristine, Switch, and Luna Tranquility) and their active ingredients (boscalid and pyraclostrobin, cyprodinil and fludioxonil, pyrimethanil and fluopyram, respectively). Of the 676 <i>Botrytis</i> isolates tested, 430, 493, and 369 isolates showed in vitro resistance to boscalid, pyraclostrobin, and Pristine, respectively. Interestingly, 6% of the 657 <i>Botrytis</i> isolates tested showed in vitro resistance to Luna Tranquility, but 329 (50%) and 131 (20%) isolates exhibited resistance to the individual active ingredients fluopyram and pyrimethanil, respectively. None of the 345 <i>Botrytis</i> isolates germinated on sucrose agar amended with 1 µg/ml of Switch or the active ingredient fludioxonil. Detached apple fruit inoculation assays supported the responses of <i>Botrytis</i> isolates to fungicides obtained from conidial germ tube growth assays on agar media. In addition to confirming the in vitro efficacy of Luna Tranquility and Switch, we discuss an array of responses exhibited by <i>Botrytis</i> isolates when exposed to premix fungicides or their individual active ingredients.