Investigation of the impact of grapevine red blotch-associated virus (GRBaV) on grapevine health and subsequent grape and wine composition and style

Since its identification in 2011, grapevine red blotch disease has been found to be wide spread in the United States. This disease is caused by grapevine red blotch virus (GRBV, in 2017 the name changed from grapevine red blotch-associated virus (GRBaV)) infection of grapevines. Over the past four years, we have investigated the impact of GRBV on grape composition and resulting wine quality across varieties, rootstocks, seasons and sites. This investigation completed a two-year study on Cabernet Sauvignon (CS) and Merlot (ME) grape varieties, examining the impact of GRBV on grapes at harvest and on the resulting wine composition. Additionally, the effect of rootstock on disease expression was also investigated as the CS grapevines were grafted onto two different rootstocks: 110R and 420A.
Grapes through development were analyzed for sugar and anthocyanin content, as well as pH and TA levels. The phenolic profiles of harvested grapes and final wines were analyzed by RP-HPLC and a modified protein precipitation (PP) assay. Volatile profiles of harvested grapes and final wines were analyzed using HS-SPME-GC-MS. Results from these analyses were compared to the sensory evaluation of the four wine treatments through a trained panel.
Consistently through the four years, symptomatic grapevines (RB (+)) resulted in lower sugar content, less anthocyanin accumulation, and higher titratable acidity (TA) when compared to asymptomatic grapevines (RB (-)). Due to these observations, chaptalization was used to adjust the soluble sugars (SS) (and therefore ethanol content of final wines) of the diseased grapes (RB (+) S) at crush to be the same as the healthy grapes. Chaptalization could potentially determine if ethanol concentration differences between wines made from RB (-) and RB (+) grapes were contributing to phenolic differences in the wines. In another treatment, diseased grapes were harvested a second time (RB (+) 2H) when the SS reached similar values as RB (-) grapes.
The results of this investigation confirm previous findings, that disease expression is dependent on genotypic and environmental factors. Anthocyanin and sugar accumulation were significantly lower in RB (+) grapes than RB (-), yet, the level of disease impact varied from rootstock, variety and location. Vine physiological measurements and phenolic profile analysis on harvested grapes indicated a potential larger impact on RB (+) vines on 420A rootstocks. In general a second harvest was successful at increasing anthocyanin, some phenolic, and volatile concentrations in infected grapes. The phenolic extraction during fermentation was followed and revealed that longer hang time (RB (+) 2H) increased extraction more than chaptalization (RB (+) S). Chemical analysis of the final wines indicated that both mitigation strategies (chaptalization and sequential harvesting) alleviated some of the differences due to disease status. This indicate that both ethanol content and cell wall integrity may play a role in phenolic extractability during winemaking.