Peter Ayogu

Grape berry is an ecological niche for a myriad of microbes, whose interactions with one another modulate fruit health and can play a role in fermentation, imparting character and distinctiveness to wines. With the growing concerns about and awareness of the risks associated with the overuse of chemical pesticides in viticulture, microbial-based pest control is increasingly encouraged as a more sustainable and environmentally friendly strategy. The use of yeasts from grape berries is a promising alternative for the control of vineyard diseases and their increasing acceptance is rapidly changing our perception of fungicides. In this paper, we provide an overview of the latest methodologies for characterising the dynamics of grape berry yeasts in the context of grape disease management, and discuss the prospects for their effective use as biocontrol agents in viticulture. Most research has focused on the control of fruit rots produced by Botrytis, Aspergillus, Colletotrichum and Penicillium spp. using formulations that comprise single strains of specific yeast genera, including Aureobasidium, Metschnikowia, Saccharomyces, Pichia, Candida and Rhodotorula. However, the challenging disparity between successful biocontrol outcomes obtained in vitro and their low applicability in the field is a major limitation for the large-scale implementation of these strategies. Novel research approaches for maximising the stability and efficiency of yeast-derived bioactives are discussed in this review.

During their lifespan, grapes are exposed to conditions that can endanger plant health, including fluctuating climate and infection by pests and insects. In particular, attacks by filamentous fungi represent huge annual losses for grape farmers. Present concerns about rising antimicrobial resistance, fungicide prohibitions and shifting consumer tastes are driving the search for less harmful ways of preventing damage to grapevine by pathogenic fungi. In this study, the antagonist activity of yeasts isolated from grape berry surface microbiota was assessed by in vitro and in vivo assays on an assortment of grapevine fungal pathogens, including Botrytis cinerea and Aspergillus niger. The results show that W. anomalus, A. pullulans, C. intermedia, R. glutinis, Z. meyerae and H. takashimae were able to inhibit by 75 to 100 % mycelia growth and sporulation of the filamentous fungi in vitro. Strains of M. pulcherrima and S. bacillaris also showed 50 to 75 % efficacy in reducing mould growth. The strains were further assessed on incised leaves of 2-month-old grapevines for their ability to reduce disease incidence by the filamentous fungi. W. anomalus, A. pullulans, M. pulcherrima, C. intermedia and R. glutinis significantly reduced infection symptoms in the leaves, indicating their potential as effective biocontrol yeasts in the vineyard.

The relevance of non-Saccharomyces native yeasts for improving the complexity of wine aroma is increasingly recognised, besides their use in a range of biotechnological applications. Previously, we used a metagenomic approach to assess the effects of vineyard calcium sprays on the microbiota of grape berries cv. Vinhão and Loureiro. In this study, we aimed at assembling a catalogue of the cultivable yeasts from grape berries of these cultivars and to investigate the direct effect of calcium and other elicitors on the isolated taxa. Seventeen unique colony morphologies were identified, and the sequencing of the region spanning the rDNA internal transcribed spacers or the rDNA D1/D2 domain revealed that they are distributed into 13 different genera. Although the cultivar and vintage influenced the taxa abundance and diversity, Aureobasidium pullulans and Hanseniaspora uvarum were the most abundant species, ubiquitously populating the fruits together with Pichia terricola, Rhodotorula glutinis, Starmerella bacillaris and Vishniacozyma heimaeyensis. The rarest genera comprised Candida, Trichosporon and Zygoascus. Analysis of yeast growth in the presence of specific elicitors revealed that Pichia spp. are the most sensitive to exogenous calcium, while Sporobolomyces roseus is the most sensitive to copper exposure. Moreover, A. pullulans, H. uvarum and S. bacillaris displayed the highest sensitivity to osmotic stress. In turn, Wickerhamomyces anomalus, Metschnikowia pulcherrima, P. fermentans, P. kluyveri and P. kudriavzevii showed mild resistance to ethanol, unlike the remaining non-Saccharomyces yeasts. Correlation networks expressing the overall yeast performance showed that H. uvarum and S. bacillaris share the strongest positive interspecies correlation, followed by the pairs P. fermentans–P. kluyveri, B. albus–R. glutinis and A. pullulans–V. heimaeyensis. The strongest negative correlations were found for the pairs P. kluyveri–S. roseus, P. terricola–W. anomalus, P. kudriavzevii–V. heimayensis, P. kluyveri–V. heimayensis, and A. pullulans–P. kluyveri. This study exposed microbial niches with a predictably common response to abiotic stress.