A grapevine disease known as leaf roll virus is proving to be a major problem for South African wineries, especially for those in Stellenbosch, the country's premier wine region. The virus, which reduces vine yields and results in unripe grapes, has vintners worried that, if the problem goes unchecked, South Africa's reputation could be hurt by the production of too many "green," bitter wines.
In recent years, South African vineyards have gone through substantial changes. Old and diseased vines, as well as grape varieties that are no longer fashionable, have been torn out and replanted en masse. According to Wines of South Africa, a government-sponsored trade group, 38 percent of the country's vineyards have been replanted in the past seven years.
During this period, the prevalence of leaf roll virus, which is typically spread through the use of infected plant material during grafting, appears to have increased.
"It is bad here, probably worse than anywhere else in the world, and it's particularly bad in Stellenbosch," said Dr. Johan Burger, a senior lecturer at the University of Stellenbsoch, who researches grapevine resistance to various diseases, including leaf roll.
The virus does not kill a vine, but it does stifle a vine's production and ripening ability by varying degrees. Leaf roll, characterized by bright-red leaves rolled up at the edges, is more easily identifiable in red varieties.
The industry does not yet have any hard figures on the total number of vineyard acres that have been affected. But Gyles Webb of Thelema Vineyards, one of Stellenbosch's top producers, confirmed: "Leaf roll virus is a major problem in South African vineyards, probably more widespread than most producers here will acknowledge. I'd say that all our vineyards on Thelema are infected to some degree."
But not all South African vintners are in agreement about the severity of the problem, and some question whether the disease is truly spreading or is just more visible.
"Leaf roll has always been present in South African vineyards," said Gawie Kriel, manager of plant improvement for KWV International, South Africa's largest wine producer. "The perception that there has been an increase in the appearance of leaf roll can be ascribed to the fact that many more red-wine varieties [in which it's more visible] have been planted recently."
However, for the first time, an insect has been linked to the transmission of leaf roll virus, leading to fears that it could be spreading the disease to uninfected plants. The mealy bug, an aphid that feeds on vines' rootstock, has proven difficult to control through biological or chemical means as it burrows into the cracks in the soil created by plowing.
The South African Wine Industry Trust has provided 2.5 million rand (about $272,000) to research the virus this year. Among other things, the research will look into other possible vectors, as the mealy bug is probably not alone in its ability to spread the disease.
"We simply do not have hard facts," said Burger. "I think [the spread of the virus] has increased recently, and the modern farmers are starting to feel the effects. In the past, the economic pressure was not there to deal with it, but today's wine industry has to compete."
Yet some producers also debate the severity of the virus' effects. "The biggest influence is that it produces 'green' wines," noted Jean Englebrecht, owner of Stellenbosch's Rust en Vrede Estate.
Because the virus causes the leaf to roll up early in the growing season, it hampers photosynthesis. Thus, red varieties infected with the leaf roll virus tend to produce grapes with less color and tannins that are out of balance, resulting in green, bitter flavors. There is also a tendency for wines to lack varietal aromatics as well.
Conversely, although the disease lowers a vine's production, it is still possible to ripen quality fruit on a diseased vine, while maintaining economically viable yields.
"Acute leaf roll causes plant decline with a corresponding drop in quality and quantity," explains Mike Dobrovic, winemaker at Mulderbosch Vineyards, one of Stellenbosch's top wineries. "However, certain vineyards are known to give good-quality wines irrespective of their virus status. The slower ripening [caused by leaf roll] allows Cabernet Sauvignon to ripen in the cooler time of the year. Virus-free Cabernet can ripen in early March or even late February -- the hottest time of the year." Cooler weather generally allows for slower, more even ripening, than hot weather.
Bruwer Raats, winemaker for Delaire Winery, is outright skeptical of all the concern over leaf roll. "Can anybody at this stage put their hand up and confidently say that viruses are necessarily negative or positive?" he asked. "As far as I have read, there are still many viruses that haven't even been identified. To take leaf roll, one of many viruses, and blow it out of proportion is premature and not scientifically based. I have made award-winning wines from so-called virus-free plant materials, as well as from virus-infected vines, and therefore have no preference in which one is better."
At Mulderbosch, Dobrovic has actually seen some of his virus-infected vines produce some of his best grapes.
"My best production vineyards are old-clone [not virus-purified] Sauvignon Blanc vines that yield approximately 5 tons a hectare," said Dobrovic. That translates to about 2 tons per acre, a yield that would be considered low for Sauvignon Blanc by current standards. Low yields generally mean more concentrated berries and thus more concentrated flavors in the wines.
While the virus may have some benefits, its negative effects and the ease with which it spreads seem to outweigh them. To combat the problem, producers are taking greater care in the propagation and use of healthy plant material. Several independent nurseries have been set up in recent years in an attempt to answer winemakers' calls for greater access to healthy material.
"As a producer, I'm horrified by the quality of material available to us. It's very frustrating," said Webb, who recently invested in a new nursery. "I'm sure people will disagree with me, but no nursery can guarantee virus-free material. They can test for certain viruses, but it's an imperfect science."
Dobrovic echoed his frustration. "As far as the reds are concerned, all my vines have been planted from so-called virus-free stock; however, they all show leaf roll symptoms," he said.
While a laboratory test can confirm a vine's virus-free state, the moment the vine leaves the lab, anything can happen, explained Burger. "I've seen young vineyards -- just 2 or 3 years old and not yet in production -- showing signs of infection. That's very troubling." But he goes on to note that "if you're practicing sensible vineyard management, then it is a problem that can be controlled."
While controlling the problem is a viable short-term plan, eliminating the problem is the more desirable long-term solution. That looks to be difficult though.
Engelbrecht, who was forced to replant his entire Rust en Vrede Estate in a short period of time due to the virus, said, "In my opinion, there is no cure for the negative effect the virus has. The only way forward is for producers to rip out all infected vineyards and replant them as quickly as possible."
Another option, which has received little consideration so far, is genetic engineering, which so far has limited acceptance among consumers and is opposed outright by some vintners, particularly in France. "We know of no breeding that we can do to provide a vine with resistance to leaf roll," said Burger. "The only option left [to create resistance] is genetic engineering. While there is no reason why we shouldn't be able to find a solution through genetic engineering, GMOs have about as much acceptance here as they do elsewhere."