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<div>Gray mold caused by <i>Botrytis cinerea</i> is an emerging postharvest disease affecting stored mandarin fruit in California. To develop effective control programs, phenotypes of resistance to four citrus postharvest fungicides were determined. One hundred <i>B. cinerea</i> isolates each in 2015 and 2016 were obtained from decayed fruit collected from packinghouses and tested for resistance to the fungicides. Sensitivity to azoxystrobin was examined based on the point mutation in the <i>cyt b</i> gene using PCR, while sensitivities to fludioxonil, pyrimethanil and thiabendazole were examined on fungicide-amended media. For azoxystrobin, 83% and 98% of the isolates were resistant in 2015 and 2016, respectively. Based on the in vitro fungicide tests, 71% and 93% were resistant to pyrimethanil, and 63% and 68% were resistant to thiabendazole in 2015 and 2016, respectively. No fludioxonil resistance was detected in both years. Five fungicide resistant phenotypes were detected, and the most common phenotype was multiple resistance to the three fungicides, accounting for 59 and 65% in 2015 and 2016, respectively. Of the 200 <i>B. cinerea</i> isolates, 5%, 23.5%, and 62% were resistant to 1, 2 or 3 classes of fungicides, respectively. Efficacy tests were conducted to evaluate if the fungicides at label rates control various resistant phenotypes on fruit. Most fungicides failed to control gray mold on mandarin fruit inoculated with respective fungicide resistant phenotypes. Our results suggest that alternative control methods need to be integrated into existing decay control programs to target this emerging disease on mandarin fruit.</div>