The Role of Pre-formed Antifungal Substances in the Resistance of Fruits to Postharvest Pathogens

Abstract

Plants contain secondary metabolites with antifungal properties. In fruits they are mostly concentrated in the peel at immature stage and decline during ripening in coincidence with fungal rot development. The information on antifungal systems in immature avocado and mango, reviewed here, suggests that they play a role in natural disease resistance. Immature mangoes have evolved a formidable antifungal system comprising several resorcinols, gallotannins and chitinases. Resorcinols and gallotannins are inhibitory to major postharvest pathogens, Colletotrichum gloeosporioides causing anthracnose and Botryodiplodia theobromae causing stem-end rot. Their levels are generally higher in resistant cultivars than in susceptible ones. Mango latex, distributed in a fine network of canals in the fruit peel, contains chitinases which have the ability to rapidly digest conidia of C. gloeosporioides. Gallotannins and resorcinols decline progressively during ripening and the latex disappears when ripe rot development begins. Retention of latex in the harvested fruit reduces anthracnose and stem-end rot development during ripening. Treatment of harvested fruit with CO₂ or inoculation with certain non-pathogenic fungi increased antifungal resorcinol concentration. Immature avocado fruits possess a pre-formed antifungal system comprising at least five antifungal compounds. The quiescence of C. gloeosporioides in the immature fruit has been attributed to the pre-formed antifungal activity of the peel. Lipoxygenase activity increases during fruit ripening, while epicatechin levels decline, suggesting that these events are linked to the decrease in di-ene concentrations. Inhibition of lipoxygenase activity results in retention of antifungal di-ene during ripening increasing fruit resistance. In freshly harvested avocados, the di-ene concentration can be further enhanced by treatment with biotic and abiotic agents.