Interaction of antagonistic yeasts against postharvest pathogens of apple fruit and possible mode of action

The interactions between two antagonistic yeasts (Pichia membranefaciens and Cryptococcus albidus) and three fungal pathogens (Monilinia fructicola, Penicillium expansum and Rhizopus stolonifer) were examined both on apple juice agar plates and in apple wounds. Light microscopy and scanning electron microscopy (SEM) observations indicated that P. membranefaciens had a stronger capability of attaching to the fungal hyphae than C. albidus did. The attachment was blocked thoroughly by SDS and β-mercaptoethanol. Addition of nutrients had no visible effect on the interaction between yeasts and pathogens. Culture extract of P. membranefaciens had higher β-1,3-glucanase and exo-chitinase but less endo-chitinase activity than that of C. albidus in Lilly-Barnett medium supplied with cell wall preparation (CWP) of pathogens as the sole carbon source. This indicated that tenacious attachment, along with the secretion of extracellular lytic enzymes, may play a role in the biocontrol activity of yeast antagonists, and the interaction between yeasts and pathogens was hampered by a protein denaturant at low concentrations.