Effect of DPA on Penicillium expansum Growth and on Postharvest Control of Blue Mold with Fludioxonil in 'McIntosh' Apples

Penicillum expansum is one of most the important pathogens that cause blue mold in stored apples. Due to the development of resistance to the postharvest fungicide, thiabendazole, an increase of blue mold has been observed in apple storages. The sensitivity of three thiabendazole-sensitive and three thiabendazole-resistant isolates of P. expansum to diphenylamine (DPA), an antioxidant scald inhibitor, was tested in vitro. All six isolates were found to be resistant to DPA. To manage the fungicide resistance, a reduced-risk fungicide, fludioxonil, was tested against blue mold caused by thiabendazole-sensitive and -resistant P. expnasum on ‘McIntosh’ apples treated with or without 1000 ppm of DPA. Fruit were assessed for disease and scald incidence during storage. Fludioxonil was effective against blue mold caused by thiabendazole-sensitive and -resistant isolates of P. expansum. DPA controlled superficial scald in treated fruit. Higher disease incidence of blue mold was observed in apples treated with DPA and low concentrations of fludioxonil (3, 5, and 75 μg ml¹). DPA neither positively nor negatively affected the control of blue mold when DPA was applied together with 600 μg ml^-1 of fludioxonil.