Residual Activity of Fludioxonil and Pyrimethanil Against Penicillium expansum on Apple Fruit

At harvest, apple fruit are placed into wooden or plastic bins (approximately 400 kg of fruit per bin), which are then transported to storage and packing facilities. Prior to storage, fruit may be drenched with a fungicide to control postharvest diseases (10). Fruit are stored in the bins in either regular atmosphere (RA) or controlled atmosphere (CA) (1 to 2% O2 and 0.5% CO2) at –1 to 3°C (9). Depending on the volume of harvested crops and cultivar, fruit may be stored for up to 10 to 12 months before packing. Blue mold, caused by Penicillium expansum Link, is a major postharvest fruit rot disease of apple worldwide (11). Punctures, bruises, or any physical damage on the fruit that occur during harvest and postharvest handling are the main avenues for the pathogen to infect apple fruit (11). Protecting wounds from infections by P. expansum is essential to blue mold control. Fungicide applications after harvest are commonly used to control blue mold on stored apple fruit. Since the introduction into the market in the late 1960s for postharvest use on pome fruits, thiabendazole (TBZ) has been commonly used for control of postharvest diseases of apple, but resistance to TBZ in P. expansum often results in the failure of blue mold control (5,7). In 2004, fludioxonil and pyrimethanil were registered for postharvest use on pome fruits in the United States. Fludioxonil and pyrimethanil belong to phenylpyrrole and anilinopyrimidine classes, respectively. Both fungicides are highly effective in inhibiting conidial germination and germ-tube elongation of P. expansum (7). The P. expansum populations from apple in Washington State prior to the commercial use of these two fungicides consisted of a mixture of phenotypes varying in sensitivity to fludioxonil and pyrimethanil, but both thiabendazolesensitive (TBZ-S) and -resistant (TBZ-R) isolates of P. expansum can be effectively controlled by the two new fungicides (7). Currently, fludioxonil and pyrimethanil are increasingly being used as alternatives to TBZ for control of blue mold and other postharvest diseases in pome fruits, and particularly as a tool for control of blue mold caused by TBZ-R strains of P. expansum (5,7). The three apple-postharvest fungicides can be applied as either a prestorage drench treatment or a line-spray or dip treatment at packing. Fludioxonil and pyrimethanil are highly effective in controlling blue mold when used as a prestorage drench treatment (5,7). However, it is unknown how long the residues of the two new postharvest fungicides on/in apple fruit can last during cold storage and whether or not the residues of these two fungicides on/in fruit can protect fruit at packing from infection by P. expansum, which is commonly present in the water system and on the packingline equipment during packing (14). The objectives of this study were to determine residual activity of fludioxonil and pyrimethanil in fungicide-drenched apple fruit against P. expansum and the distribution of fludioxonil and pyrimethanil residues in the flesh of apple fruit at levels that are effective in inhibiting blue mold development.