Biological control of Botrytis cinerea in apple by yeasts from various habitats and their putative mechanisms of antagonism

Twenty-eight yeasts isolated from habitats other than apple were screened for their potential to protect wounds of Golden Delicious apples during storage from gray mold caused by Botrytis cinerea. All isolates reduced (P < 0.05) decay severity and 23 reduced decay incidence after 7 days at 22-24 degrees C when applied at 5 x 10(6) CFU per wound 1-2 h earlier than 8 x 10(3) conidia of B. cinerea (P < 0.05). When selected isolates were tested at 5 x 10(5) CFU per wound on apples stored for 30 days at 4 degrees C, Cryptococcus humicola NRRL Y1266, Filobasidium floriforme NRRL Y7454, and Rhodosporidium toruloides NRRL Y1091, previously unreported gray mold antagonists, reduced both incidence and decay severity as well as chlorothalonil and Sporobolomyces roseus FS-43-238, a reported biocontrol agent. Experiments at 22-24 degrees C indicated that, with the exception of R. toruloides, protection generally required yeast inoculation into wounds before conidia. Nine yeasts protected wounds at 5 x 10(6) CFU per wound, whereas only three of nine were effective at 5 x 10(2) CFU per wound, a density calculated to be too low to preemptively carpet the wound, even though selected yeasts increased at least one log unit in the first 24 h following inoculation into wounds. In vitro utilization of C-14-labeled sucrose by the yeasts was greater than that by the conidia after 12-48 h incubation, supporting nutrient competition as a mechanism of antagonism. Cell-free, dilute sucrose solutions that were preincubated with effective biocontrol yeasts for 24 h significantly inhibited conidium germination; however, a solution preincubated with a sucrose-utilizing but less effective yeast did not. Results suggest that nutrient competition may have played a significant role in biocontrol, but the contribution of preemptive physical exclusion and antifungal metabolites to a complex biocontrol process cannot be dismissed. (C) 1996 Academic Press, Inc.