Fludioxonil Does Not Antagonize Propiconazole Activity Against Geotrichum candidum

Fludioxonil and propiconazole are frequently used fungicides for managing fungal diseases in specialty crops. They are often used in mixture to manage postharvest brown rot caused by Monilinia fructicola and sour rot caused by the yeast-like fungus Geotrichum candidum on stone fruits. Previous research indicated, however, that fludioxonil can increase the expression of the ATP-binding cassette transporters CDR1 and CDR2 in the yeast Candida albicans, leading to increased resistance to demethylation inhibitor (DMI) fungicides. In this study, mycelial growth assays and detached fruit assays were conducted with G. candidum isolates of varying sensitivity to propiconazole to investigate potential antagonism between the two active ingredients. Isolates sensitive, reduced-sensitive, and resistant to propiconazole were able to grow with or without 20 μg/ml of fludioxonil on potato dextrose agar containing up to 0, 3, and 30 μg/ml of propiconazole and on water agar containing up to 7.5, 7.5, and 30 μg/ml of propiconazole, respectively, and no antagonistic interactions were detected. Propiconazole controlled disease with or without 20 μg/ml of fludioxonil equally on detached cherry fruit caused by isolates with different DMI sensitivity phenotypes. Our results indicated no detectable antagonism between fludioxonil and propiconazole that would interfere with the mixture's ability to manage sour rot of cherry.