Investigation of the Impact of Grapevine Red Blotch-associated Virus on Grape and Wine Composition and Quality
Grapevine red blotch-associated virus (GRBaV) is the latest virus to be identified in grapevines. Since discovery in 2011, its widespread presence has been confirmed in several states and in different white and red wine grape varieties. At this stage very little is known about the effect of the red blotch virus on both grape development and composition at harvest as well as the long term effect on wine composition and quality. Several varieties should be studied to determine if the effects of the virus are variety specific. Preliminary data from the currently funded AVF investigation into the impact of GRBaV on grape and wine composition and quality show significant differences in basic chemical data for red blotch positive and negative grapes sampled at harvest. Berry samples were taken from vines at harvest from multiple sites investigated (three Chardonnay sites in Sonoma County, two Cabernet Sauvignon sites and one Merlot site in Napa County). A decrease in Brix was mostly obtained at harvest for grapes from red blotch infected grapevines irrespective of cultivar or clone compared to healthy grapevines. This decrease was up to 6%in Chardonnay grapes and respectively a maximum of 20 and 16%in Cabernet Sauvignon and Merlot grapes. Differences in pH were minor with TA values mostly higher in grapes from red blotch diseased grapevines.
The first year’s data indicate that red blotch disease resulted in a decrease in total phenols and tannins in Chardonnay grapes of up to 25%. Phenolic results for the different Cabernet Sauvignon and Merlot sites were more variable. There was only a clear decrease in tannin concentration for one of the Cabernet Sauvignon sites (18%). However, the total anthocyanin concentration was 20 to 38%lower in all red blotch infected red grapes. This indicates variability in response to red blotch infection within a variety. Future analyses to determine the individual phenolic profiles and tannin compositions of red blotch infected and non-infected grapes may shed more light on these findings. Wines could only be made from one of the Chardonnay sites in addition to the two Cabernet Sauvignon sites described due to the fact that we were unable to obtain enough GRBaV negative grapevines for winemaking. All analyses will be repeated on the completed wines to coincide with descriptive sensory analysis. Final results will enable us to relate grape and wine composition as well as potentially find a correlation between differences in mouthfeel properties of the wines and changes in phenol and tannin compositions due to red blotch disease. At least one more year’s data is needed to confirm results and determine the potential impact of seasonal variability on the expression of GRBaV. It is important to determine the impact of GRBaV on grape and wine composition so that recommendations can be made to the industry regarding the future of infected vines.