Impacts of grapevine leafroll disease on fruit yield and grape and wine chemistry

Article review by Lucinda Heyns - 29 May 2016

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Washington State University did a three year study from 2009 – 2011 to look at the impact of grapevine leafroll disease (GLD) on grapevine fruit yield as well as grape and wine chemistry. An own rooted, commercial Merlot vineyard, planted in 1998 in Yakima Valley (eastern Washington state) was used for the study. Vines showing symptoms and that tested positive for GLDaV-3 were compared to adjacent non-symptomatic vines that tested negative. Each year, the vines were retested to confirm their status. Grapes were harvested at different stages of berry development and juice analysis done regularly throughout ripening.

Across all three seasons, it was clear that GLD decreased vigour (concluded from cane pruning weights) and yield in infected vines. These vines had less clusters per vine and the cumulative weight of the clusters was less.

Juice analysis of grapes from infected vines indicated significantly lower concentrations of total soluble solids (an indication of sugar content). This is probably due to viral induced interference of phloem translocation. These grapes also had lower pH and higher titratable acids than juice from vines that are GLD negative. These differences became significant after the onset of veraison and became more pronounced throughout ripening until harvest. Similarly, berry anthocyanins were significantly less in grapes from infected vines at the linear phase of berry ripening. At the time of harvest, the difference in anthocyanins was however less pronounced.

Analysis of the wines indicated that wines from GLD infected grapes had consistently lower alcohols. The effect of GLD on wine acidity, pH, tannins and polymeric pigment concentration was however season dependent. Significantly higher concentrations of anthocyanins were present in wines from healthy vines, each year - despite the fact that the difference in anthocyanins in grape juice from healthy and infected vines appeared to dissipate at the time of harvest.

Evaluation of sensory attributes showed that wines from GLD positive vines had a more earthy colour and brown-red color. Wines from healthy vines had more saturated, purple colour and red fruit aroma. The negative impact of GLD was more evident in cooler seasons.

In South Africa, Gerhard Pietersen and his team at University of Pretoria, has done a tremendous amount of research on leafroll virus. All this information along with guidelines for a leafroll control strategy are available in a series of fact sheets on the IGWS website.

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