Why table-grape growers should care about leafroll

The problem quantified

Pietersen has led two SATI-funded projects investigating grapevine leafroll in table grapes. The first was a survey of table-grape viruses conducted from 2017–2020. A total of 145 vines from 25 vineyards representing five major table-grape production areas was sampled and tested for 41 different grapevine viruses, including types 1 to 4 of the grapevine leafroll-associated viruses. GLRaV-3 comprised a third or more of the viruses identified in each region. “It occurred in every single one of the 25 vineyards we sampled, and 114 of our 145 samples tested positive,” recalls Pietersen. GLRaV-2 was found in about a third of grapevines and GLRaV-1 in only five vines. Vines often had more than one type of leafroll virus. Pietersen notes that GLRaV-2 is interesting because insects do not spread it. “It’s only transmitted by plant material – that indicates a part of our problem.” The survey confirmed Pietersen’s suspicions that grapevine leafroll-associated viruses are rife in table-grape vineyards. He then embarked on a second project to assess their impact, focussing on the problem of poor colour development in Crimson Seedless. His team mapped the position of vines with poor bunch colouring and vines with signs of leafroll annually in seven vineyards from 2019–2021. Five of the seven vineyards showed a similar spatial pattern whereby the colouring problem spread over time, and the positions of poorly colouring and leafroll-affected vines overlapped. The other two vineyards had been esta-blished with a virus-free Crimson Seedless clone. Plant material was produced through heat therapy and meristem tip culture and tested free of GLRaV-3. The grape colour was initially superior in these vineyards. “Of course, this could just be a selection of darker and more even-ripening Crimson Seedless,” concedes Pietersen. “But then we started finding some individual vines with the same problem of bunches not colouring up.” Affected vines clustered in specific areas of the vineyard where vines also showed signs of leafroll disease in autumn. Subsequent laboratory testing confirmed infection with GLRaV-3. “We think we have fairly firm evidence that the Crimson Seedless bunch colouring problem is associated with GLRaV-3,” says Pietersen. “We think this colour problem is just a different manifestation of grapevine leafroll disease in this particular cultivar.”

Leafroll control in four steps

According to Pietersen, controlling grapevine leafroll-associated viruses is based on four simple principles: planting certified material, removing infected vines, controlling vectors, and preventing vector dispersal. Infected plant material significantly contributes to the spread of many viruses affecting horticultural crops. Establishing a vineyard with even a few leafroll-infected plants literally sets the virus rolling. Pietersen has monitoring data demonstrating a 10- fold increase in infected vines in a block over four seasons. Grapevine certification in SA falls under the Vine Improvement Association, part of Plant Improvement South Africa. The scheme for grapevines covers table and wine grapes. To relieve some of the cost pressures of virus testing for certification, Pietersen is conducting SATI-funded research to identify Ramsey plants that are GLRaV-3 resistant. Ramsey is the most common table-grape rootstock in SA. Regarding vector control, Pietersen believes table-grape growers have a head start, as mealybugs are more significant in table grapes than wine grapes. But he’s concerned that the proximity of many table-grape units may make it hard to stop insects carrying leafroll viruses between them. “And then there’s a much greater reluctance among table-grape growers to remove infected vines,” he adds. “In the wine industry, they have no qualms about removing infected vines – it’s a commonly accepted practice.” Based on Pietersen’s research, it’s clear that grapevine leafroll disease is no less a problem in table than in wine grapes. GLRaV-3 is ubiquitous and has a significant impact on grape quality. He speculates that the association between poor colour and leafroll disease has gone unnoticed because bunches and leaves are affected at different times. “Control of leafroll in table grapes, and certainly in Crimson Seedless, is definitely warranted,” concludes Pietersen. “The improved evenness of ripening and colouring up make this a very valuable proposition for table-grape growers.”