Putative modes of action of Pichia guilliermondii strain R13 in controlling chilli anthracnose after harvest

The mode of action of an antifungal yeast, Pichia guilliermondii, strain R13, against the fungal pathogen of chilli anthracnose, Colletotrichum capsici, was examined both on agar plates and in chilli fruit wounds. Light microscopy revealed that strain R13 attached to the fungal pathogen, and this attachment apparently restricted the proliferation of C. capsici in the chilli fruit wounds. In chilli juice, strain R13 suppressed C. capsici spore germination and germ tube length, but the suppression was completely overcome by addition of 0.05% glucose, sucrose, or 1% of nitrate sources (NH4NO3, NaNO3, Ca(NO3)2·4H2O, Mg(NO3)2·6H2O, and KNO3), suggesting the yeast was competing with the fungus for these substrates. Strain R13 also produced hydrolytic enzymes, including β-1,3-glucanase, and chitinase, in both solid and liquid media. The activities of these enzymes were highest when the C. capsici hyphal cell walls, rather than laminarin or glucose, were the carbon source; the activities were approximately 2 and 15 times higher with hyphal cell wall than with laminarin or glucose. Unlike the other strains tested, strain R13 did not produce a lethal toxin when cultivated under similar conditions. This study provides evidence that attachment, competition for nutrients, and secretion of hydrolytic enzymes, at least partially, explain how P. guilliermondii strain R13 suppresses C. capsici.