Use of yeasts from different environments for the control of Penicillium expansum on table grapes at storage temperature.

A wide range of fresh fruits and vegetables is attacked by Penicillium species causing diseases during their postharvest handling. Many of these species are psychrotrophic and they are able to cause food spoilage at refrigeration temperature as happens with table grapes. After the harvest, grape bunches are stored inside boxes with SO2 generator pads to reduce the contamination with fungal conidia. However, SO2 residues are dangerous to people allergic to sulfites and they negatively affect the quality of fresh fruit. Biological control of phytopathogens with microbial antagonists naturally present on fruit surfaces could be helpful against postharvest diseases. The present study aimed to select native yeasts isolated from fermentation microenvironments and the surface of refrigerated grapes for their use in the biological control of P. expansum on table grapes stored in cold rooms. Non-pathogenic and pathogenic Penicillium species were isolated, and the four most aggressive pathogen isolates were identified as Penicillium expansum. Twenty yeast isolates identified as Aureobasidium pullulans, Cryptococcus magnus, Metschnikowia pulcherrima and Rhodotorula glutinis presented positive antagonistic activity against Penicillium expansum; they controlled the development of at least one of the fungi, significantly reducing the disease incidence. The results showed that three antagonistic yeasts (M. pulcherrima 22, 36 and 43) reduced the disease incidence and severity of all 4 P. expansum isolates. It was also found that the fruit surface is not the only source for isolation of biological control agents. Microenvironments with different stress conditions could be a promising source to isolate antagonistic microorganisms.

Antifungal modes of action of Saccharomyces and other biocontrol yeasts against fungi isolated from sour and grey rots.

The aim of this study was to determine the putative modes of action of 59 viticultural yeasts (31 Saccharomyces and 28 non-Saccharomyces) that inhibited fungi isolated from sour and grey rot in grapes. Inhibition of fungal mycelial growth by metabolites, enzyme activities (laminarinases, chitinases), antifungal volatiles, competition for nutrients (siderophores, Niche Overlap Index (NOI)), inhibition of fungal spore germination and decreased germinal tube length and induction of resistance were assayed. Biofungicide yeasts were classified into "antifungal patterns", according to their mechanisms of action. Thirty isolates presented at least two of the mechanisms assayed. We propose that inhibition of fungal mycelial growth by metabolites, laminarinases, competition for nutrients, inhibition of fungal spore germination and decreased germinal tube length, and antifungal volatiles by Saccharomyces and non-Saccharomyces viticultural yeasts is used as putative biocontrol mechanisms against phytopathogenic fungi. Twenty-four different antifungal patterns were identified. Siderophore production (N)and a combination of siderophore production and NOI>0.92 (M)were the most frequent antifungal patterns observed in the biofungicide yeasts assayed. Elucidation of these mechanisms could be useful for optimization of an inoculum formulation, resulting in a more consistent control of grey and sour rot with Saccharomyces and non-Saccharomyces biocontrol yeasts.