Abstract
Grape sour (bunch) rot is a polymicrobial disease of vineyards that causes millions of dollars in lost revenue per year due to decreased quality of grapes and resultant wine. The disease is associated with damaged berries infected with a community of acetic acid bacteria, yeasts, and filamentous fungi that results in rotting berries with high amounts of undesirable volatile acidity. Many insect species cause the initial grape berry damage that can lead to this disease, but most studies have focused on the role of fruit flies in facilitating symptoms and vectoring the microorganisms of this disease complex. Like fruit flies, social wasps are abundant in vineyards where they feed on ripe berries and cause significant damage, while also dispersing yeasts involved in wine fermentation. Despite this, their possible role in disease facilitation and dispersal of grape rots has not been explored. We tested the hypothesis that the paper wasp Polistes dominulus could facilitate grape sour rot in the absence of other insect vectors. Using marker gene sequencing we characterized the bacterial and fungal community of wild-caught adults. We used a sterilized foraging arena to determine if these wasps transfer viable microorganisms when foraging. We then tested if wasps harboring their native microbial community, or those inoculated with sour rot, had an effect on grape sour rot incidence and severity using a laboratory foraging arena. We found that all wasps harbor some portion of the sour rot microbial community and that they have the ability to transfer viable microorganisms when foraging. Foraging by inoculated and uninoculated wasps led to an increase in berry rot disease symptom severity and incidence. Our results indicate that paper wasps can facilitate sour rot diseases in the absence of other vectors and that the mechanism of this facilitation may include both increasing host susceptibility and transmitting these microbial communities to the grapes. Social wasps are understudied but relevant players in the sour rot ecology of vineyards.
Highlights
Sour rot is one of the most important diseases that affects wine grape quality worldwide (Barata et al, 2011; Steel, Blackman & Schmidtke, 2013)
Sour rot causes millions of dollars in losses to vineyards annually (Steel, Blackman & Schmidtke, 2013), yet we know little about the ecology of this disease, including what insects are relevant to the dispersal and establishment of the sour rot microbial community
Using data from multiple foraging experiments and pairing this with data on the microbial community of P. dominulus, we found that paper wasps can directly influence sour rot and black mold disease (Aspergillus rot) in the absence of additional vectors and that they may be vectoring this microbial community to grapes when foraging
Summary
Sour rot (bunch rot) is one of the most important diseases that affects wine grape quality worldwide (Barata et al, 2011; Steel, Blackman & Schmidtke, 2013). This polymicrobial disease involves acetic acid bacteria, Ascomycota yeasts, and filamentous fungi that attack ripe, thin-skinned cultivars in the late summer (Barata, Malfeito-Ferreira & Loureiro, 2012a; Nally et al, 2013). Infection leads to berry decomposition, the overgrowth of contaminating saprophytic yeast species, acetic acid bacteria resulting in the production of acetic acid, and in some cases, if certain strains of Aspergillus species are present, accumulation of carcinogenic mycotoxins such as Ochratoxin A (Bisiach, Minervini & Zerbetto, 1986; Zoecklein, Williams & Duncan, 2000; Varga & Kozakiewicz, 2006; Barata et al, 2008). Current management strategies rely on reducing environmental conditions that facilitate sour rot, the use of chemical fungicides to control prior fungal infections that facilitate sour rot development (UC IPM Pest Management Guidelines: Grape, 2014), and the reduction of known vectors (Bisiach, Minervini & Zerbetto, 1986)
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