Biocontrol by yeasts of blue mould of citrus fruits and the mode of action of an isolate of Pichia guilliermondii

Nineteen yeast isolates, from different fruits, were tested for antagonistic activity against blue mould (Penicillium italicum) on artificially wounded citrus fruits. Strains 1A, 5A, and 29A of Pichia guilliermondii strains 21D, 22D and 30F of Candida famata and strain 13 L of C. sake reduced infection from 86 to 98% compared with the control. Isolate 5A of P. guilliermondii was the most effective antagonist and its mode of action was investigated further. Its activity against the pathogen declined when the number of yeast cells inoculated in the wound was reduced although it showed good ability to colonize the wound site and grow rapidly, cell numbers increasing by up to 60-fold within 24 h. Scanning electron microscopy of the wound also showed attachment of the yeast cells to the pathogen hyphae. The high antagonistic activity of isolate 5A appeared to be related to its capability to occupy space and use nutrients available in the wound, thus depriving the pathogen of them. It also induced in the wound the biosynthesis of the phytoalexins (scoparone and scopoletin); however this mode of action seems not to play an essential role in the biocontrol of the pathogen.