Biological control of Penicillium sp. causing postharvest blue rot of table grape by viticultural yeasts

Abstract

Table grapes are highly perishable and non-climacteric fruits susceptible to severe changes during postharvest. Low temperatures stimulate the development of fungal diseases caused by different species of Penicillium, as blue rot. In cold storage, prior to export, sulfur dioxide (SO2 ) is used as the most common method for controlling postharvest fungal decay in table grapes. However, SO2 residues are dangerous to people allergic to sulfites, and this compound is highly injurious to fresh fruits, causing bleaching of the berries and browning of the rachis in grapes. Among different biological approaches suggested in the literature, the use of yeast as biocontrol agents shows great potential as an alternative method of postharvest disease control. There is little background on the control of Penicillium using yeast at low temperature, and there are no reports about biocontrol of these fungi in table grapes in cold storage conditions. The main objective of this study was to evaluate the ability of viticultural yeasts for the biocontrol of pathogenic Penicillium isolates, at cold storage conditions. Yeasts were isolated from fermenting musts at 12°C and healthy berries stored at low temperature (2 ± 1°C). Four virulent Penicillium sp. were previously isolated from rotten grapes. Yeasts were assayed in vivo for biocontrol activity at 2 ± 1°C during 4 weeks. A single wound was made at the equator of berries of Superior Seedless table grapes using the tip of a sterile dissecting needle. Twenty microliters of each yeast suspension in water (106 cfu/mL) were pipetted into each wound. After 2 h, 20 μL of 104 Penicillium sp. conidia/mL of sterile destilled water were poured into each wound. Treated grapes were air-dried and placed in plastic bags (with wet paper towels to maintain high humidity). Positive and negative controls were included. Percentage of disease incidence [% = (number of decayed wounds/number of total wounds) ×100] was calculated at the end of the experiment. A randomized complete block design was used (10 berries per replicate and three replicates per treatment). Yeasts that at least reduced 60% disease incidence were considered antagonistic. Ninety-six yeasts were isolated from viticultural environments. In vivo experiments showed that 18 isolates belonging to 4 non-Saccharomyces species (4 Rhodotorula glutinis, 9 Cryptococcus laurentii, 4 Cryptococcus humicola, 1 Debaryomyces hansenii) significantly reduced at least one phytopathogenic fungus. From all antifungal selected yeasts, 14 were isolated from fermenting musts, and 4 from surface of healthy grapes. It can be considered that several non-Saccharomyces species mainly from viticultural environments can inhibit fungi involved in blue rot. This study suggests that these yeasts may be novel important agents of biocontrol during the cold storage of table grapes for extended periods of time.