Impact of grapevine leafroll disease on grape and wine composition

Grapevine leafroll disease (GLD) is the most common grapevine disease, which is present in grape-growing regions globally. It has been proven that GLD impacts grapevine physiology and the subsequent berry composition.

Very little information is available on GLD impact on grapevine physiology, grape ripening and the wine composition under South African conditions.

Introduction to leafroll

To date 70 viruses and five viroids have been identified to infect grapevines.¹ Virus and virus-like diseases are a great threat to grapevine vineyards globally.¹ Grapevine leafroll disease (GLD) has been identified as one of the most important viral diseases.²

Within the South African context, grapevine leafroll disease is widely accepted to be the most damaging grapevine virus disease, with grapevine leafroll-associated virus 3 (GLRaV-3), the most prevalent leafroll-associated virus.^3,4

This virus is transmitted by vine mealybug Planococcus ficus which is the predominant mealybug in South African vineyards, as well as the more restricted Pseudococcus longispinus and a number of scale insect species.^5,6

The spread of a systemic virus such as GLRaV-3 is influenced by the direction of the carbohydrate movement in the phloem.⁷ The sieve elements are blocked and ruptured in the parenchyma cells in the leaves which will result in a delay of the carbohydrate translocation.⁸

Subsequently, carbohydrates cannot be mobilised and transported further from the leaves to other grapevine organs which subsequently result in the downward rolling leaves and become brittle.⁸

Red cultivars express red spotting which turn red purple colour and a green vein-banding symptom and interveinal yellowing of leaves and leafrolling in white cultivars.^2,8

Previous studies on GLD in red grape varieties reported significant losses in both yield and fruit quality.^9-13 The wine properties were also impacted by the presence of GLD.

Wines made from non-symptomatic vines were perceived as being more purple in colour, with less brown and with a more saturated colour.¹¹ The aim of this study was to:

• evaluate the occurrence of GLD and non-GLD-infected grapevines in commercial vineyards,
• monitor GLD infection rate in a commercial vineyard over a two season period,
• grapevine physiological responses to GLD, and
• evaluate the impact of fruit and wine chemistry.

Materials and methods

The study was conducted during the 2016/2017 and 2017/2018 growing seasons in two commercial Cabernet Sauvignon vineyards (hereinafter Cabernet Sauvignon A – clone CS 12/R110 and Cabernet Sauvignon B – clone CS 45/101-14 Mgt) with a north-south and east-west row orientation in the Stellenbosch wine region.

In 2017/2018 a commercial Merlot noir vineyard (clone MO343/R99) in the Constantia wine region was also evaluated. Vineyards were visually evaluated for typical GLD symptoms. Thirty symptomatic (SV) and 30 non-symptomatic (AS) grapevines were selected in each vineyard.

An additional 30 symptomatic vines were selected which were harvested two weeks later than the commercial harvest date (SVLH).

The 2016/2017 season was aimed at identifying SV- and AS-infected grapevines in commercial vineyards through virus diagnosis. Grape berries were sampled at the commercial harvest date and subjected to classical analysis, total soluble solids (TSS), pH and titratable acidity (TA).

The bunch number and bunch weights were recorded. Wines were made from the harvested grapes according to the experimental cellar protocol from the Department of Viticulture and Oenology, Stellenbosch University.

In 2017/2018 the same grapevines which were used in 2016/2017 were subjected to virus diagnosis to ensure that the AS were not infected during the course of the season.

Destructive shoot measurements were performed at pea-size. Stomatal conductance was recorded and grape ripening was monitored throughout the season from véraison until commercial harvest and post commercial harvest for the SVLH treatment. The same harvesting and winemaking protocols followed in 2016/2017 were repeated 2017/2018.

Statistical analysis

One-way and two-way analyses of variance (ANOVA) were performed on the grape chemical and wine chemical data using Statistica, Version 13.5 (Statsoft, Tulsa, OK, U.S.A).

Significant differences were judged on a 95% significance level (p≤0.05) using Fischer’s LSD test. All stated uncertainty is the standard deviation of three replicates of one treatment.

Results and discussion

A 25%, 35% and 2.5% increase in infection was noticed in the asymptomatic grapevines in the respective Cabernet Sauvignon A, Cabernet Sauvignon B and Merlot vineyards.

Overall shoot length (main shoots and lateral shoots) were negatively impacted by the presence of GLD irrespective of the vineyard or clones (Table 1). Clonal differences or environmental variation appeared to have an impact on the virus response. Stomatal conductance (g_s) was significantly higher (p≤0.001) in the SV grapevines in the Cabernet Sauvignon and Merlot vineyards (Table 1).

No significant differences were observed in the Cabernet Sauvignon B vineyards. Grapes from the SV treatment harvested two weeks later (SVLH) showed an increase in TSS from the commercial harvest date two weeks earlier in all the cultivars.

This phenomenon was observable irrespective of the cultivar/clone (Table 2). TSS was significantly impacted by the vineyard, presence of the virus and the interaction between the two parameters. Juice pH was lower in SV when compared to the AS grapevines (Table 2). SV had significantly higher TA than the AS grapevines in 2016/2017.

Table 1: Vineyard measurements and harvest data obtained in 2017/2018.

Table 2: Grape juice analysis at harvest.

A two week-extended hanging time of the symptomatic grapevines (SVLH) fruit proved to increase berry weight (Table 1). Bunch numbers from the respective treatments were not impacted by the presence or absence of GLD (Table 1).

The presence of GLD resulted in lower Brix levels and extended hanging time of the fruit showed to increase the Brix in SV fruit (Table 2). Wine pH and TA showed in some cases significant treatment effects in both seasons when the wines made from AS and VS were compared (Table 3).

In some cases longer hanging time negated these differences. Alcohol content (% ethanol v/v) were significantly higher in the AS treatments than the SV treatments (Table 3). SVLH wine showed to often have a higher alcohol content when compared to the SV treatment, although these levels did not reach those found in the wines made from AS vines.

Anthocyanin and colour density showed significantly higher concentrations in wines produced from AS wines compared to the SV wines (Table 3).

Despite the extended hanging time of the SVLH treatment anthocyanin and colour density did not increase significantly, except in the case of colour density in the Cabernet Sauvignon B in 2017/2018.

Methyl cellulose precipitable (MCP) tannin in wine were in some cases significantly different between treatments over the two seasons (Table 3), but inconclusive results were obtained to GLD response to tannins in the wine.

Table 3: Wine chemical analysis in the study.

Conclusions

From this study it is clear that there is a link between the presence and/or absence of GLD on grapevine functioning. The latter is primarily linked to the shoot length, sugar accumulation, titratable acidity and the wine ethanol (% v/v).

SV had consistently lower TSS and higher TA content. The GLD impact on the wine composition was variable and dependent on the cultivar, clone, site and season. Extended hanging time resulted in an increase in the TSS and % alcohol v/v.

Phenolics increased in these later-harvested (SVLH) fruit, but it was not consistent over the two seasons. Additional studies are needed to identify the specific influence of clone, cultivar and site on GLD grapes and the resulting wines.

References

1. Martelli, G. P. An overview on grapevine viruses, viroids, and the diseases they cause. In Grapevine Viruses: Molecular Biology, Diagnostics and Management, eds B. Meng, G. P. Martelli, D. A. Golino, and M. Fuchs (Cham: Springer International Publishing), 31–46, 2017. doi: 10.1007/978-3-319-5.
2. Maree, H.J., Almeida, R.P.P., Bester, R., Chooi, K.R., Cohen, D., Dolja., V.V., Fuchs, M.F., Golino, D.A., Hooste, A.E.C., Martelli, G.P., Naidu, R.A., Rowhani, A., Saldaerelli, P., Burger, J.T. Grapevine leafroll-associated virus 3. Front Microbiol. 2013 4: 82. doi: 10.3389/fmicb.2013.00082
3. Pietersen, G. Spread of grapevine leafroll disease in South Africa: A difficult, but not insurmountable problem. Wynboer June 2004 (www.wynboer.co.za/recentarticles/0406leaf.php3
4. Pietersen, G. Spatio-temporal dynamics of grapevine leafroll disease in Western Cape vineyards. In Extended Abstracts of the 15th Meeting of the International Council for the Study of Virus and Virus-like Diseases of the Grapevine, pp. 126-127, 2006. Stellenbosch, South Africa.
5. Douglas, N., Krüger, K. Transmission efficiency of Grapevine leafroll-associated virus 3 (GLRaV-3) by the mealybugs Planococcus ficus and Pseudococcus longispinus (Hemiptera: Pseudococcidae). Eur. J. Plant Path. 2008 122:207-212
6. Walton, V., Dreves, A.J., Skinkis, P., Kaiser, C., Buchanan, M., Hilton, R., Martin, B.R., Castagnoli, S., Renquist, S. Grapevine Leafroll Virus and Mealybug Prevention and Management in Oregon Vineyards. 2009, https://catalog.extension.oregonstate.edu/em8990
7. Hull, R. Matthews’ plant virology, 2002. 4th edn. Academic Press, San Diego
8. Montero, R., El aou ouad, H., Flexas, J., Bota, J. Effects of grapevine leafroll associated virus 3 (GLRaV-3) on plant carbon balance in Vitis vinifera L. cv. Giro´ Ros. Theor. Exp. Plant Physiol. 2016, 28:1–10.
9. Golino, D.A., Wolpert, J., Sim, S.T., Benz, J., Anderson, M., Rowhani, A. Virus effects on vine growth and fruit components of three California ‘Heritage’ clones of Cabernet Sauvignon. Extended abstracts 16th Meeting of ICVG, Dijon, France, 31 Aug—4 Sep, 2009a. Pp 243-244.
10. Golino, D.A., Wolpert, J., Sim, S.T., Benz, J., Anderson, M., Rowhani, A. Virus effects on vine growth and fruit components of Cabernet Sauvignon on six rootstocks. Extended abstracts 16th Meeting of ICVG, Dijon, France, 31 Aug—4 Sep, 2009b. Pp 245-246.
11. Lee, J., Martin, R.R. Influence of grapevine leafroll associated viruses (GLRaV-2 and -3) on the fruit composition of Oregon Vitis vinifera L. cv. Pinot noir: Phenolics. Food Chem. 2009, 112: 889–896.
12. Endeshaw, S.T., Sabbatini,P., Romanazzi, G., Schilder, A.C., Neri, D. Effects of grapevine leafroll associated virus 3 infection on growth leaf gas exchange, yield and basic fruit chemistry of Vitis vinifera L.cv.Cabernet Franc. Sci.Hortic. 2014, 170,228–236.
13. Alabi, O.J., Casassa, L.F., Gutha, L.R., Larsen, R.C., Henick-Kling, T., Naidu, R.A. Impacts of Grapevine Leafroll Disease on Fruit Yield and Grape and Wine Chemistry in a Wine Grape (Vitis vinifera L.) Cultivar. PLoS ONE, 2016, 11(2): e0149666. doi:10.1371/journal.pone.0149666.

– For more information, contact Erna Blancquaert at ewitbooi@sun.ac.za.

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