Suppression of Monilinia fructicola cutinas production by peach fruit surface phenolic acids

Chlorogenic and caffeic acids are major phenolic acids in the epidermis and subtending cell layers of peach fruit (Prunus persica). Their concentrations are especially high in peach genotypes with a high level of resistance to the brown rot fungus, Monilinia fructicola, and decline as fruit mature with a corresponding increase in disease susceptibility. Chlorogenic acid and caffeic acid at concentrations up to 5 mM did not inhibit M. fruticola spore germination or mycelial growth in culture. In cultures amended with either of these phenolic acids, cutinase activity was reduced with a sharp decrease apparent at 0.1 mM. The presence of caffeic acid in cultures during growth on cutin prevented the appearance of two major cutinase isoforms as well as additional esterases in the filtrates. Cutinase activity was not directly inhibited by chlorogenic acid or caffeic acid, nor by oxidized caffeic acid. A series of cinnamic and benzoic acids with various degrees of hydroxylation and methoxylation of the aromatic ring also suppressed cutinase levels in culture, indicating the effect was not specific to the naturally abundant chlorogenic and caffeic acids. Cinnamic acid, with an unsubstituted aromatic ring, was least suppressive, and 2.3-dihydroxybenzoic acid, a known chelating agent that can facilitate iron transport, was most suppressive. We propose that the high concentration of chlorogenic acid present in immature fruit and in fruit from genotypes with high levels of disease resistance may contribute to the brown rot resistance of the tissue by interference with the production of factors involved in degradation of host polymers rather than by direct toxicity to the pathogen. Chlorogenic acid and related phenolics in combination with other factors such as iron could function to arrest M. fructicola in quiescent infections associated with immature and ripening fruit.