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Title: Diversity and molecular determination of wild yeasts in a central Washington vineyard

item BOURRET, TYLER - Washington State University
item GROVE, GARY - Washington State University
item Vandemark, George
item HENICK-KLING, THOMAS - Washington State University
item GLAWE, DEAN - Washington State University

Submitted to: North American Fungi
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/21/2013
Publication Date: 11/29/2013
Citation: Bourret, T., Grove, G., Vandemark, G.J., Henick-Kling, T., Glawe, D. 2013. Diversity and molecular determination of wild yeasts in a central Washington vineyard. North American Fungi. 8:1-32. doi: 10.2509/naf2013.008.015.

Interpretive Summary: During the process of making wine, the sugars in grapes are converted to ethanol by the process of fermentation, which is typically performed by yeasts. The surface of wine grapes is colonized by a wide range of micro-organisms, including yeasts, other fungi, and bacteria. However, because the microbial populations on grapes can differ greatly across vineyards or different years, the use of wild yeasts to ferment grapes typically results in inconsistent fermentation and wine quality. Consequently, the yeasts that are used for making wine are commercial strains that have been selected for their ability to convert sugar from grapes into alcohol, and these yeasts are added to the mixture of crushed grapes. It is not well understood how wild yeasts present on grapes interact with the commercial yeasts, and it is also not known how many different wild yeasts can colonize wine grapes. The objective of this work was to collect and characterize wild yeasts from wine grapes harvested from a vineyard located in the Yakima Valley of Washington. We sampled grapes of the varieties “Riesling” and “Chardonnay” and identified 53 different yeast species, which was higher than expected based on earlier studies. Seventeen of these species had never before been isolated from wine grapes and 18 species had never before been reported from North America. Surprisingly, we found that classifying isolates based on how they grew and appeared in culture plates was as informative as classifying isolates based on differences in DNA sequences between isolates. This research suggests that viticulture conditions in the Yakima Valley promote the development of a diverse group of microbes on the surfaces of wine grapes. The awareness of this diversity is the first step towards determining how the wild yeasts impact wine production.

Technical Abstract: Yeasts were isolated from grapes collected from a research vineyard at the WSU IAREC, located at Prosser, WA. Species determination was based on cultural features, microscopic morphology, physiological tests and analysis of ITS and D1/D2 rDNA sequence data. 53 species were found distributed among five fungal subphyla, a greater number than expected based on similar published studies. Within Saccharomycotina, 13 species in the genera Candida, Hanseniaspora, Metschnikowia, Meyerozyma, Pichia, Wickerhamomyces and Yamadazyma were determined. Isolates within the Metschnikowia pulcherrima clade appeared to possess considerable diversity. Pucciniomycotina was represented by 12 species, in Curvibasidium, Rhodosporidium, Rhodotorula, Sporidiobolus and Sporobolomyces. Five phylogenetically distinct species in the subphylum could not be assigned to any described species. Isolates in Ustilaginomycotina were placed in Pseudozyma except for a single strain determined to be Rhodotorula bacarum. Within Agaricomycotina, 17 species of the genera Cryptococcus, Cystofilobasidium, Hannaella, Holtermanniella and Mrakiella were detected. Seven species of yeast-like Pezizomycotina were found, representing classes Leotiomycetes, Dothideomycetes and Sordariomycetes. Isolates of Aureobasidium pullulans represented three phylogenetically distinct subspecific lineages. Seventeen species were previously unreported from wine grapes and 18 species were unreported from North America. Several strains appear to represent undescribed species, including the recently described Curvibasidium rogersii.