Despite significantly increased understanding of the grapevine leafroll virus and its chief vector, the mealybug, this vine disease remains one of the most pressing problems facing the South African wine industry. Leafroll saps the strength and quality of vineyards in just a few years. Yet there is still a concerning lack of urgency to address it.
Vergelegen was an early pioneer in the fight against this silent killer in South Africa. The farm’s commitment to eradicating the virus has resulted in a decades-long collaboration with Professor Gerhard Pietersen of the Agricultural Research Council’s Plant Protection Research Institute at the University of Pretoria. Over the last 25 years, Professor Pietersen has demonstrated the scientific and commercial merits of managing, and even eradicating, grapevine leafroll virus.
Slow progress
Despite the fact that grapevine leafroll’s detrimental effects on vineyards and wines are well-documented, some farms have been slow or even unwilling to address the problem. The effect of this sluggishness is evident in the rows of red leaves among red wine cultivars throughout the Cape winelands. And that is only the tip of the iceberg. In most white cultivars, the infection remains hidden from sight, only manifesting later with lower yields and reduced quality.
The economic impact of its slow drag on the South African wine industry in terms of production, quality and prices, has not been quantified, but it’s certain to be significant. “It’s incredible how quickly the vines start to decline,” says Vinpro viticultural consultant Francois Viljoen. “Once a vineyard shows symptoms, the drop in yield and quality means it can lose profitability in just three to five years, and by then they should already have been ripped out.”
Francois is particularly concerned about the shorter life expectancy among affected vineyards. This creates a downward spiral where diseased vineyards impact profitability, which inhibits replanting and renewal programmes. “You’re lucky if they live 20 years,” Francois says. “Producers should be factoring in the advantage of having vineyards that will maintain good yields and higher quality for an additional five or ten years, and take care to manage leafroll in those vineyards that have been replanted.”
Best practices
Fortunately, producers have an ever-expanding toolbox of interventions at their disposal, thanks to research such Prof Pietersen’s at Vergelegen.
The mealybug-control programme at Vergelegen was implemented in three phases: planting new, virus-free vineyards; uprooting and replanting badly infected red wine cultivars; and testing and treating white wine cultivars where infection is invisible to the eye.
It all starts with clean plantings. Producers should be putting the clean plant material that’s being painstakingly prepared by nurseries such as Vititec to good use, Francois says. “The plant material available to South African producers through our plant certification scheme is truly world-class.”
That’s because infected plant material allows the disease to spread even without any mealybug infestation. “Both the scion and the rootstock can be carriers of the virus and are able to infect one another,” Prof Pietersen says. “You can’t easily pick up the leafroll virus in rootstocks, since they don’t show any symptoms, but we have seen clear evidence they are able to spread the disease.”
New research
Over a course of three years, Prof Pietersen’s team worked to determine why the type 3 leafroll virus (GLRaV3), a highly transmissible version of the leafroll virus, was so difficult to detect in rootstocks, using both protein-based ELISA and more sensitive RNA-based PCR tests. “In one case, which we called the ‘immune’ case, it seemed that the virus didn’t even multiply in the rootstock. In a second case, which we call the ‘tolerant’ case, the virus did multiply, but never reached high concentrations. And in the third case, the rootstocks were clearly susceptible.”
The initial tests all used Richter 99, which is easy to cultivate. The second phase of the project repeated the same tests on other rootstocks, including Ruggeri, 101-14, Richter 110, Paulsen, and Ramsay (which is also used by the table grape industry), to compare results. “We observed the same patterns in all of them – in every clone, some were immune, some tolerant, and some susceptible,” Prof Pietersen says.
Further research at Stellenbosch University will now seek to identify what makes some rootstocks immune. “It’s difficult work, but if we can identify the gene that prevents Type 3 leafroll from replicating in rootstocks, we can transfer that gene to vitis vinifera in the future, whether through conventional breeding or by genetic manipulation. “It’s high-risk, high-reward research that will create enormous opportunities if we succeed.”
Calling in air support
Integrated pest management comprises three pillars: chemical, technological and biological. Vergelegen has strengthened its technological and biological approach by calling in SkyBugs, a new South African venture that can deliver beneficial insects to specific hot spots in the vineyards. The company is a partnership between FieldBUGS, which specialises in biological solutions, and agritech company Aerobotics, which uses drones to survey and target problem areas.
“Beneficial insects are another tool in producers’ toolbox for a holistic and sustainable leafroll management strategy.” – FieldBUGS founder and general manager Braam Jonker.
According to FieldBUGS technical support specialist Ruan Erasmus, these specially equipped drones fly 30 metres above a vineyard block at a consistent speed of 10 m/s to release the insects onto the vines. “Each flight can cover up to 20 hectares, after which the drone is landed, and a new battery and cartridge of insects are inserted,” he explains.
An advantage over manual delivery methods is that the insects are uniformly dispersed and can be released closer to hatching time, making them more effective and efficient. SkyBugs also benefits from the Aerobotics’ AeroView platform, which offers precise GPS mapping through a mobile app. “This allows us to follow up after a release to see if the insects are actually working and to scout for parasitised mealybugs and catch infestations early.”
SkyBugs is currently trialling a device to release live insects or even larger predators the same way.
South African producers still on the fence should take a cue from other wine-producing countries such as Nieu Zealand and the United States, which consider leafroll to be a major threat to the long-term sustainability of the wine industry. With the local availability of innovative technologies such as SkyBugs and access to cutting-edge research by global experts such as Prof Pietersen at our doorstep, there is no excuse not to act decisively.