Vergelegen wine estate in Somerset West, a world leader in combating grapevine leafroll-associated virus 3 (GLRaV-3), the main cause of leafroll disease, recently hosted an information session to share its lessons with wine producers and industry stakeholders.
Leafroll disease is a major issue for winemakers globally as it decreases grapevine vigour and fruit quality, leading to lower yields. The virus affects grape colour, sugar content and flavour, resulting in economic losses, and high costs when vineyards must be replaced.
Vergelegen embarked 20 years ago on a programme to combat the virus, which is spread by insects called mealybugs. The estate now has the oldest white wine vineyards in the world that are virtually virus-free.
In a Vergelegen population of several hundred thousand vines, It is thought that infected vines, which are promptly removed, are infected by mealybugs carried from a distance by wind.
Vergelegen viticulturist Rudolf Kriel said the programme was implemented in three phases: Planting new vineyards in former citrus orchards; uprooting and replacing badly infected red wine cultivar vineyards; and testing and treating white wine cultivars that, apart from Chardonnay and Semillon, do not readily show the effects of the virus.
Professor Gerhard Pietersen, lead researcher at Patho Solutions, has worked closely with Vergelegen since inception of the project. He attributed “astounding results” to the estate’s comprehensive approach, and said that only 3% of South African wine producers apply the programme as thoroughly. Smaller growers tend to replace vineyards “piecemeal”, largely due to cost constraints, and with little coordination between growers.
He gave an example of 30 wine producers in New Zealand – an entire appellation – who systematically replaced vineyards with new plantings. “It was quite remarkable how leafroll virus dropped in the whole area.”
Use of certified vine planting material, confirmed as virus-free, is voluntary in South Africa. Californian authorities, on the other hand, are considering making virus-free material mandatory, and may subsidise growers who replant vineyards.
As virus infection levels have been managed to extremely low levels at Vergelegen, the estate has now shifted to biological control of the mealybug vector. Kriel works with SkyBugs, which is a partnership between FieldBUGS, which supplies predatory indigenous insects, and agritech company Aerobotics, which collaborates with a network of drone pilots to disperse the insects accurately over the vineyards.
The first stage is distributing predatory wasps, which are attracted by a pheromone released by female mealybugs. This proactive measure is followed by the distribution of ladybugs, which eat the mealybugs.
Speakers included Vergelegen viticulturist Rudolf Kriel, viticulture consultant Francois Viljoen, director of Patho Solutions Gert Pietersen, lead researcher at Patho Solutions Prof. Gerhard Pietersen, Vergelegen laboratory researcher Maritza van Rensburg, FieldBUGS technical support specialist Ruan Erasmus, and FieldBUGS technical director Brahm Jonker.
Viljoen said grapevine leafroll virus was “a huge challenge” in South Africa, but if replanted vineyards could be in production for 30 years instead of 20 years, producing better quality grapes that achieved higher prices, this was worthwhile.