Impact of red blotch disease on Cabernet Sauvignon and Merlot grape and wine composition and wine sensory attributes

Abstract

Context and purpose of the study: Grapevine Red Blotch disease (RB) is a recently discovered disease that has become a major concern for the viticulture and winemaking industry in California, USA. The causal agent, Grapevine Red Blotch Virus (GRBV) was identified in 2011 and its presence was confirmed in several states in the US, in Canada, and in Switzerland. It has been demonstrated that RB compromised the regulation of ripening by suppressing specific ripening events, altering the expression patterns of transcription factors and causing hormonal imbalances in Zinfandel. For the last 4 years, our research group have been focusing on the impact of RB on grape and wine composition and wine sensory properties. Our prior work demonstrated that RB decreases sugar accumulation and delayed color development in the berry, resulting in wines with lower ethanol and anthocyanin concentration, thus affecting sensory attributes. The aim of this study was to determine the impact of RB on grape and wine composition and sensory properties when grapes were harvested sequentially.

Material and Methods: Cabernet Sauvignon and Merlot vineyards from two traditional grape growing regions in California, Napa Valley and Paso Robles respectively were selected in the 2016 and 2017 season. Grape berries from infected (RB +) and healthy (RB -) grapevines were collected weekly from veraison to harvest. RB (+) grapevines were harvested sequentially at two-time points: (1) at the same time as healthy vines – but lower Brix, and (2) later when Brix was similar to those of the healthy grapes at harvest. Brix, pH, titratable acidity (TA), sugar loading, phenolic composition by protein precipitation assay and RP-HPLC and volatile composition by HS-SPME-GC-MS were determined on grapes. Wines were made in triplicate from healthy, RB symptomatic*, and second harvest RB symptomatic* grapes and analyzed for % EtOH v/v, volatile acidity, TA, free and bound SO2, phenolic composition by RP-HPLC and protein precipitation, and volatile composition by HS-SPME-GC-MS. Wine sensory properties were determined by descriptive analyses.

Results: Chemical analysis demonstrated that RB impacts berry composition by increasing TA and decreasing Brix, sugar loading, anthocyanins, altering phenolic composition and sensory attributes. Wines made from RB (+) grapes harvested later had higher pH than wines made from healthy and first harvested RB (+) fruit. On the other hand, wines made from second harvest grapes from symptomatic vines showed less impact of the disease, producing wines with chemical, phenolic and volatile profiles as well as sensory properties more similar to wines made from healthy fruit when compared to wines made from first harvest RB (+) fruit.

*Grapevines showing RB disease symptoms