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Fungicide resistance profiles of Botrytis cinerea isolated from berry crops in Oregon

Virginia Stockwell: USDA ARS, Horticultural Crops Research Unit

<div>The gray mold pathogen, <em>Botrytis cinerea</em>, causes pre- and post-harvest fruit rot and stem disease of berry crops. Disease control measures involve applications of fungicides in rotation or in combination from bloom to near harvest. <em>B. cinerea</em> can rapidly develop resistance to fungicides, which reduces control efficacy. Little was known about the fungicide resistance profiles of <em>B. cinerea</em> from berry crops grown in Oregon. We isolated <em>B. cinerea</em> from blackberry, blueberry, raspberry, and strawberry fruits in western Oregon fields from 2014 to 2017. Resistance to fungicides was determined by growth inhibition assays on fungicide-amended media. The media and discriminatory doses of formulated fungicides tested were: 1) potato dextrose agar amended with 0.5 ppm fenhexamid, 0.3 ppm fludioxonil, or 3 ppm iprodione, 2) yeast extract agar with 5 ppm boscalid, and 3) the defined medium, cyprodinil test agar (CTA), amended with 1 or 10 ppm cyprodinil. Among the 511 isolates evaluated, 61% were resistant to at least one fungicide. Boscalid resistance was common and resistance to fenhexamid or cyprodinil was frequently detected. Among the isolates with fungicide resistance, 64% were resistant to at least two fungicides and 36% were resistant to three or four of the fungicides. Although 39% of the isolates were sensitive to the fungicides tested, the emergence of multi-fungicide resistance may compromise the efficacy of chemical control of gray mold.</div>