MODE OF ACTION OF THE POSTHARVEST BIOCONTROL YEAST, PICHIA-GUILLIERMONDII .1. CHARACTERIZATION OF ATTACHMENT TO BOTRYTIS-CINEREA

An isolate (87) of the yeast Pichia guilliermondii, protects apples from postharvest fruit rotting fungi Botrytis cinerea and Penicillium expansum. In order to examine the yeast-pathogen interaction, B. cinerea was grown on agar plates overlayed with cellophane. Effective and non-effective yeast isolates were applied near the young hyphal growth. Samples were taken 24 h later from the area where the fungi and yeast had intersected. Light microscopy revealed a general attachment of the effective biocontrol agent P. guilliermondii (isolate 87) and a non-effective isolate (117) of Debaryomyces hansenii. Low temperature scanning electron microscopy (LTSEM) indicated that both species of yeast attached to the fungal hyphae, but the 87 isolate attached fastidiously. Twenty-four hours after applying the 87 isolate to B. cinerea, pitting and collapse of the hyphae were observed. These observations were confirmed using transmission electron microscopy. These features were not observed with the ineffective isolate of D. hansenii. Further experiments indicated that attachment of P. guilliermondii to hyphae of B. cinerea could be blocked by agents that alter protein integrity (salts, proteases, etc.) and certain sugars. Isolates of both species produced β-(1-3) glucanase when cultured in various carbon sources and on cell walls of fruit rotting pathogens. Culture supernatants from P. guilliermondii, however, yielded two- to five-fold more β-(1-3) glucanase activity compared with D. hansenii. Data indicate that tenacious attachment, along with secretion of cell wall degrading enzymes, may play a role in the biocontrol activity of this yeast antagonist. © 1991.