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Comprehensive research on leafroll virus in South African vineyards is at an advanced stage and should shed light on various significant aspects of this widespread disease. However, the exact extent of this disease will remain a mystery. This has emerged from interviews with various role players in the relevant industry organisations with a view to determining the progress made with research and measures to combat the disease, following reports in the international media two years ago, suggesting that up to half the country's vineyards were infested. In September 2001 a Winetech press statement said that measures were being considerably improved and a sound foundation which should dramatically decrease the incidence and spread of leafroll in the next five years was being laid. It now appears that good progress with research and a combined strategy to curb the disease has been made over a wide front, but nobody is able (or willing?) to give statistics on the incidence of the disease. Not even an educated estimate could be solicited. The approach appears to be that measuring the total incidence of leafroll does not serve much of a purpose and that all efforts should be concentrated on ensuring that newly established vineyards are clean and kept clean. Also that it is just about impossible to measure the extent for various practical reasons e g that a white grape vineyard does not necessarily show the symptoms, unlike a red grape vineyard. According to Dr Johan Burger of the University of Stellenbosch, who heads the reorganised virus programme - in collaboration with Winetech - an important study is well under way and preliminary results will be published before the end of 2003. This is on the incidence and spread of leafroll disease in mother blocks and will, however, not show the bigger picture. "It would be dangerous to claim that the incidence and spread of leafroll have diminished," explains Dr Burger. "It's simply unrealistic to expect this to have happened in two years. Keep in mind that once a vineyard is infected, it stays infected until it's replaced and that it will always serve as a potential virus source for secondary spread." Dr Burger, however, concludes that "most producers are very serious about using only virus-free plant material and are implementing some of the prescribed 'management' measures. We are confident that this trend will ensure medium term benefits for the industry. "Our short-term strategy is to 'manage' the current situation. A number of research projects aim to yield practical measures to prevent the planting of infected material, to prevent secondary spread in vineyards, to control natural vectors of the viruses and give guidelines on how to 'handle' infected grapes. Popular publications (leaflets) on leafroll disease and on the monitoring and control of the mealy bug vector have been distributed and are still available from ARC-Infruitec/Nietvoorbij." WineLand asked Dr Burger to respond to the fact that the foreign press reports were slated as negative and exaggerated, if - even today - there are no reliable statistics available to show the contrary. "As far as I know, a number of local producers were interviewed and these people gave their personal opinions about virus incidence and spread. None of these comments were based on any scientific study," says Dr Burger. Although some experts say that the virus has no detrimental effect whatsoever on the consumer - saying that top wines of international quality have had their origin from affected vines - Dr Burger explained, however, that the virus does influence the quality of the grape, and therefore wine quality. "It has been shown that severe virus infection has detrimental effects on both sugar and acid levels of the fruit. Negative effects on plant and root growth have also been observed." Meanwhile on a positive note, Jan Booysen, executive director of Winetech, says virus related research funding from the South African Wine Industry Trust has been increased from R6,5 million to R10 million over a ten year period (which started three years ago). Existing research projects are also continuing, especially the focus on the following areas:
On being asked about progress being made with these projects, Dr Burger said that excellent progress in most research projects has been made over the past two years. "Especially the studies on epidemiological aspects of leafroll disease and its insect vectors." Research results and progress are communicated during their annual Grapevine Virus Workshop when researchers report back on the previous year's progress. "We've also had a roadshow at which a number of researchers reported back on selected topics in the programme," said Dr Burger. "This year we had two such meetings, the first was specifically aimed at nurserymen and the second at viticultural consultants. In previous years, producers from various regions were targeted." According to Booysen, communication also takes place through the Distell and VinPro viticulturists, guidelines and protocol in WineLand, VinPro/Winetech information days, as well as the South African forum for viticulturists (SAFV). "Although research in all the above named areas is actively being pursued, two main research thrusts have evolved," explains Dr Burger. "These are 'management practices', which include studies on the incidence and spread of the virus, all aspects of vector involvement and control, practical measures to prevent infection, and to 'manage' the disease in infected vineyards. They are short to medium term projects. "The other thrust is the development of virus resistant grape cultivars, which we believe is the only realistic way to ultimately get rid of virus diseases. Most of these projects rely on transgene technology, which place them in a longer-term category. This is not only because of the complex nature of the work, but also because of GMO acceptance issues, especially in Europe." And a breakthrough: "Well, maybe not a breakthrough in our industry yet, but a very interesting and exciting development is the discovery of gene silencing technology. This holds immense potential as a highly effective way to fight viruses. It has the added benefit that potentially only rootstock material needs to be genetically modified, i e the fruit (and wine) would still be GM-free. We have started a project using this technology and hope to start testing the system in transgenic tobacco later this year." For detailed guidelines on leafroll and protocol to restrict the spread of mealybug in a vineyard block, contact Anel Andrag on (021) 807-3387 or e-mail andraga@kwv.co.za. Research progress
International picture Leafroll occurs worldwide in all grape-growing areas. Germany suffered an 80% leafroll infection in 1936. In Nuriootpa, Southern Australia, natural spreading of leafroll symptoms in Pinot Noir clone material was observed. In 1993, 6% of vines displayed leafroll symptoms, which escalated to 21 and 36% in 1994 and 1995 respectively. The incidence of leafroll infection over a period of five years in New Zealand increased from 11-100%. In France some vineyards are reported to be 80% infected with leafroll. - Guidelines by Winetech
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