Mechanisms of action and efficacy of four isolates of the yeast Metschnikowia pulcherrima active against postharvest pathogens on apples

The mechanisms of action and efficacy of four isolates (GS37, GS88, GA102, and BIO126) of the yeast Metschnikowia pulcherrima against Botrytis cinerea, Penicillium expansum, Alternaria sp., and Monilia sp., all postharvest pathogens of apple fruit, were studied in vitro and on apples, in controlled and semi-commercial conditions. An application of a cell suspension (10⁸ cells per ml) of the antagonists in artificial wounds of apples reduced growth of B. cinerea and P. expansum after storage at 23 °C. A complete suppression of the pathogen was obtained against Monilia sp., stored at 23 °C, and against B. cinerea and P. expansum, stored at 4 °C. The results against Alternaria sp. were more variable. Applications of culture filtrates and autoclaved cells of the isolates were ineffective in reducing the diameter of the lesions on the fruit, supporting the hypothesis that living cells are necessary for biocontrol. In experiments of antagonism in vitro, on different solid substrates, a reduction in the mycelium growth of the pathogens resulted, so that, at least in vitro, the antagonists could produce some diffusible toxic metabolites. In co-cultivation in vitro on a synthetic medium, B. cinerea spore (10⁵ ml⁻¹) germination was completely inhibited by the presence of 10⁸ cells of the antagonists, while culture filtrates and autoclaved suspensions were not able to reduce germination. Dipping boxes of apples cv. Golden delicious in a suspension of 10⁷ antagonist cells per ml and storing for 8 months in controlled atmosphere at 1 °C, showed levels of control against B. cinerea and P. expansum similar to those from thiabendazole.