Effect of grapevine leafroll disease on grape composition and wine quality
Burger, J T
University of Stellenbosch. Department of Genetics
Blancquaert, E H
Maree, H J
Du Toit, W
Strever, A E
Vivier, M A
Objectives and Rationale
We investigated the effects of GLRaV-3, as the main agent of GLD, its impact on grapevine physiology, fruit development and wine quality in five commercial wine cultivars (Chardonnay, Pinotage, Merlot, Shiraz and Cabernet Sauvignon) under South African conditions.
Trial blocks of Chardonnay, Pinotage, Merlot, Shiraz and two from Cabernet Sauvignon commercial vineyards were identified. Vineyards were visually evaluated in 2017 for typical GLD symptoms by an experienced team of field virologists and viticulturists from SU and VinPro. Thirty infected and 30 healthy plants of each cultivar were selected in 2017, and an additional 30 more vines were identified in the Merlot, Shiraz and Cabernet Sauvignon vineyards to harvest two weeks later than the commercial date. Selection of infected plants was based on severe symptom expression, while completely asymptomatic plants were selected for the healthy controls. All plants selected for the project were subjected to ELISA to confirm the GLRaV-3 status of these plants before the start of the project in February 2017, and again ELISA-screened just before harvest in 2018. Grape berries were sampled and subjected to “classical” (Total soluble solids (TSS), pH and titratable acidity) analysis. Experimental wines were made on a small scale, and under controlled conditions. Two repeats were made from grapes from healthy vines, as well as from infected vines. The wines were subjected to sensorial evaluation in 2017 by a trained panel at the Department of Viticulture and Oenology, as well as by a group of wine experts at an information day. In 2018, the wines were pre-selected by a panel which were involved in the project and three of the wines (Chardonnay, Merlot and one Cabernet Sauvignon were selected for sensorial evaluation).
For the 2017 season, ELISA results confirmed the visual assessments almost perfectly (98%) for all red cultivars, but showed that from the 60 Chardonnay samples collected (based on symptomology), only a single plant tested negative. ELISA results for the 2018 season, however, were radically different – a 55% average correlation between symptoms and ELISA results for “healthy” red cultivars was obtained, suggesting that either many vines from the previous year became infected, or that virus titres increased to levels where they became detectable by ELISA. Symptomatic vines had lower stomatal conductance compared to the asymptomatic vines. Main shoot and secondary shoot growth were negatively impacted in symptomatic vines. Overall, asymptomatic vines had a higher crop load (bunch number) and higher bunch weights at harvest. Lower Brix, lower pH and higher titratable acidity were observed in all five of the cultivars at harvest. Grapes harvested two weeks after the commercial harvest date did show an increase in Brix to acceptable levels for red wines in South Africa, for both seasons. Alcohol levels were higher in the wines made from the asymptomatic vines, this was also the case for anthocyanin levels, colour density and total phenolics, in the red grape cultivars, for both seasons. Wines made from symptomatic vines had lower phenolics for both seasons. Grapes from symptomatic plants left longer on the vines never yielded wines having the same elevated phenolics levels than those made from asymptomatic vines. All wines were subjected to blind sensorial evaluations by both expert and trained panels. The Chardonnay wines made from the symptomatic vines were often described as being more light-bodied than those made from the asymptomatic vines. The same trends were also seen for the Cabernet Sauvignon wines, with wines made from the symptomatic vines often described as being less full than those made from the asymptomatic vines. In the case of the Cabernet Sauvignon treatments, wines made from the second harvest infected (red) vines were often described as being unbalanced and over-ripe and jammy, although their alcohol levels were lower than those made from the uninfected (green) vines.
Key Conclusion of Discussion
Typical GLD symptoms are not a reliable indicator of the presence of the disease in white cultivars. Season-to-season increases in GLD incidence in individual vineyards can be dramatic where a leafroll management protocol is not implemented. Grapevine physiological functioning and grape berry metabolites were affected by the presence or absence of GLD in the experimental vines. The wine quality was also impacted since panellists (trained and experts) were able to distinguish between grapes made from symptomatic and asymptomatic GLD symptoms.
Recommendation to Industry / Key take-home message
Clear differences in wine qualitative characteristics were indicated in sensorial evaluations of infected and non-infected vines for all cultivars included in this project. If practically possible, asymptomatic and symptomatic vines should be harvested separately and evaluated by the winemaker.