A two year study in 2013 and 2014 evaluated the effect of grapevine red blotch disease on fruit produced on vines infected with Grapevine red blotch-associated virus (GRBaV). Vine growth was also monitored. In each of four sites, data vines were selected that tested positive or negative by qPCR for GRBaV and negative for several Grapevine leafroll associated viruses, vitiviruses and nepoviruses. Approximately 50%of the vines monitored were positive for Grapevine rupestris stem pitting-associated virus. Data vines were located in four vineyards; one site each of Chardonnay, Cabernet Sauvignon, Merlot and Zinfandel. Chardonnay and Cabernet Sauvignon vines were evaluated for two years and fruit was reduced in half of the GRBaV-positive vines to determine the effect of that practice on vine performance as compared to fruit from infected vines with a full crop load.
Foliar disease symptoms were evaluated in 2013 in Cabernet Sauvignon, Chardonnay and Merlot. Disease severity was an estimate of percent of leaf surface area expressing red color or chlorotic tissue associated with red blotch disease in red and white cultivars respectively. Across cultivars severity was greatest in the basal region of the canopies; however Chardonnay and Merlot canopies had a greater amount of leaf area affected by the disease compared to that of Cabernet Sauvignon. In Chardonnay severity was equally high in blades located in both the basal and middle regions of the canopy.
Red blotch disease consistently reduced sugar accumulation and increased malic acid in juice at harvest in Chardonnay, Cabernet Sauvignon and Merlot. Diseased vines had elevated titratable acidity in all cultivars and in Merlot differences were significant. Juice pH was increased in fruit from diseased Chardonnay and Zinfandel vines but not in Cabernet Sauvignon or Merlot. Crop reduction in diseased vines at early veraison in 2013 or just past fruit set on the same vines the following year did not significantly modify the effect of the red blotch disease on fruit composition.
Yield was reduced each year in diseased Chardonnay vines due to fewer clusters whereas Cabernet Sauvignon yield was not affected. In Zinfandel, GRBaV-positive vines produced fewer clusters with less mass than did GRBaV-negative vines although differences were not significant. Pruning weight was reduced in diseased Chardonnay in one of two years and not affected either year in Cabernet Sauvignon.