CE Specialist Anita Oberholster's research on Red Blotch Disease

Anita Oberholster

Grapevine red blotch-associated virus (GRBaV) was identified in grapevines in 2011 and has been associated with Red Blotch (RB) disease. Vines of red grape cultivars show symptoms similar to Leafroll disease. However, unlike Leafroll virus, vines with Red Blotch (RB) disease show red veins on the leaf undersides and no rolling. In Chardonnay, symptoms include subtle-to-obvious chlorotic regions within leaf blades. Very little is known about the impact of Red Blotch disease on grape and wine composition and quality. In a 2014 study, the impact of Red Blotch disease on three varieties (Chardonnay, Cabernet Sauvignon and Merlot) at 6 different sites in Sonoma and Napa Counties were investigated. A decrease in Brix was mostly obtained at harvest for grapes from RB infected grapevines irrespective of cultivar or clone compared to healthy grapevines. The impact of RB disease on the phenolic profiles of the grapes and wines were variable. There was a trend of decreased color in red RB positive (+) grapes where the impact on sugar accumulation was the most severe. One each of the Chardonnay and Cabernet Sauvignon sites were also evaluated in the 2015 season. We found that RB had a greater impact on grape composition in 2015 for both sites compared to 2014. There was also a consistent decrease in yield in Chardonnay and Cabernet Sauvignon with RB infection and RB (+) berries were significantly bigger in both seasons. 

Untargeted metabolomics clearly indicated a large impact on primary metabolites (sugars, organic acids, amino acids, polysaccharides). The next step will be to combine transcriptomics with targeted metabolomics to identify the metabolic pathways influenced by GRBaV. 

We also performed descriptive sensory analysis, which showed that the wines could be distinguished from each other based on disease status. Additionally, Cabernet Sauvignon wines, made with the inclusion of different concentrations of RB infected fruit, indicated that, for this vineyard and vintage, selective harvest should be used, if over 15% of the yield will be affected by RB. For this vineyard, RB infection resulted in a 3.6° Brix difference at harvest. In addition, our findings indicate that the impact of RB disease is influenced by both site and season. Thus selective harvest decisions should be based on Brix differences determined for the current season, at a minimum. Research is ongoing.