Sensitivity of Penicillium expansum to diphenylamine and thiabendazole and postharvest control of blue mold with fludioxonil in ‘McIntosh’ apples

Penicillium expansum is one of the most important pathogens that cause blue mold in stored apples. Due to the development of resistance to the postharvest fungicide, thiabendazole (TBZ), an increase in blue mold has been observed in apple storages. The sensitivity of three TBZ-sensitive and three TBZ-resistant isolates of P. expansum to diphenylamine (DPA), scald inhibitor, was tested in vitro. Of the 94 isolates, collected later in the storage season, 41% were found resistant to both DPA and TBZ. To manage the fungicide resistance, a reduced-risk fungicide, fludioxonil, was tested against blue mold caused by TBZ-sensitive and -resistant P. expansum on ‘McIntosh’ apples treated with or without 1000 μg ml−1 of diphenylamine. Fruit were assessed for disease and scald incidence during storage. Diphenylamine controlled scald in treated fruit. Higher disease incidence of blue mold was observed in apples treated with diphenylamine and low concentrations of fludioxonil (3, 5, and 75 μg ml−1). DPA neither positively nor negatively affected the control of blue mold when DPA was applied together with 150, 300 and 600 μg ml−1 of fludioxonil during 12 weeks of storage at 4 °C.