Yeasts associated with plums and their potential for controlling brown rot after harvest.

Bacterial and yeast antagonists isolated from fruit surfaces have been effective in controlling various post-harvest diseases, and several microbial antagonists have been developed into commercial products. Our knowledge of the fruit microbial community, with the exception of grapes, apples and some citrus fruit, is rudimentary and the potential of the resident yeasts for biocontrol remains largely unknown. We determined the occurrence of yeasts on plum surfaces during fruit development from the pre-hardening stage until harvest for 2 years. A total of 16 species from 13 genera were isolated. Species from three genera, basidiomycetes Rhodotorula (29.5%) and Sporidiobolus (24.7%) and the dimorphic ascomycete genus Aureobasidium (24.7%), constituted 78.7% of all isolations and were recovered throughout fruit development, while Cryptococcus spp. constituted only 6.2% of the total plum isolates. The yeast community in the final sampling was significantly different from the first three samplings, reflecting a rapidly changing fruit habitat during the maturation of fruit. For example, Hanseniaspora, Pichia, Zygosaccharomyces and Wickerhamomyces occurred only on the most mature fruit. Screening of the yeasts for antagonistic activity against Monilinia fructicola, a fungus that causes brown rot, revealed a range of biocontrol activities. Several isolates provided complete control of the decay on plums, challenged with a pathogen suspension of 10(3) conidia/ml and > 90% of control on fruit inoculated with the pathogen at a concentration 10 times higher. Some of the best antagonists included A. pullulans and R. phylloplana. Populations of both of these antagonists increased rapidly by several orders of magnitude in wounds of plums incubated at 24ºC and 4ºC. Our results indicate that plum surfaces harbour several yeast species, with excellent potential for use in biological control of brown rot of stone fruits.