Photo: Gerhard Pietersen
Despite huge awareness campaigns, many South African grape farmers are still not taking grapevine leafroll disease, caused by a virus, seriously enough.
Vinpro consultant François Viljoen said during a leafroll information session held at Vergelegen wine estate near Somerset West, Western Cape, that he simply could not understand why: “By now it is general knowledge that leafroll virus negatively affects farm income, as it results in lower production volumes and grape quality.
“It also significantly reduces the longevity of the vineyards, meaning vineyards should be replaced over shorter periods. You will be lucky if a leafroll-infected vine lasts longer than 15 years.”
Prof Gerhard Pietersen, a plant virologist at Patho Solutions, said the virus is severely underestimated in the table grape industry: “We tested 145 vines from 25 vineyards in five major table grape-producing regions between 2017 and 2020. The virus was found in every vineyard we sampled, and over 110 of the samples tested positive.”
Despite this, he did not know of any table grape farmer who followed a leafroll management strategy like the one used in the wine grape industry.
“Farmers in the table grape industry seem to be less aware of the negative impact of leafroll virus on production. This is probably because the virus takes a few years to manifest symptoms on vigorously growing vines, and table grape vineyards are replaced over a much shorter time than wine grape vineyards, in response to changing market trends and genetic improvements,” he said.
Nevertheless, the virus is taking a toll on table grape income. Pietersen pointed out that Patho had identified leafroll virus as the cause of poor colouration on Crimson seedless grapes in a study done between 2019 and 2021. Research on wine grapes had also revealed that leafroll led to uneven ripening and berry sizes.
He said table grape farmers can significantly improve their income by paying more attention to leafroll virus, especially when producing their tried-and-tested varieties that are not influenced by fluctuating market demand.
“Replacing table grape vineyards is extremely expensive, so the longer you can keep a vineyard, the better.”
Virus-free material
Current leafroll virus prevention and management strategies in the wine grape industry are based on the use of virus-free plant material; the identification and removal of infected plant material before the virus has time to spread to other vines; the management of the mealybug, which is the main vector involved in the spreading of the virus; and vineyard practices aimed at preventing humans and farm equipment from spreading the virus.
Using certified vines, specifically tested for grapevine leafroll virus, has greatly lowered
the risk of new plant material introducing the virus into vineyards. Unfortunately, grapevine leafroll-associated virus 3 (GLRaV-3), which predominantly occurs in South Africa, is poorly detected on non-Vitis vinifera (common grapevine) rootstocks.
Pietersen explained that rootstocks susceptible to leafroll tested positive for GLRaV-3, but the virus was unevenly distributed and present in concentrations that were too low to diagnose in rootstocks that are tolerant to the disease.
Therefore, the tolerant rootstocks, as is the case with the susceptible rootstocks, will transmit the disease to vines during the grafting process.
Some rootstocks have been found to be immune to the virus; in other words, the disease is unable to replicate in the rootstock. For this reason, Patho recently initiated research aimed at identifying immune rootstocks from 101-14 Mgt, Paulsen 1103, Richter 99, Richter 110, Ruggeri 140, and US 8-7 rootstocks.
“The rootstocks will be tested and artificially inoculated with buds that carry the disease and then later budded with susceptible virus-free indicator vines to ensure that no virus replication can occur in the selected rootstocks. Over time, the immune material will be distributed to plant improvement organisations within the South African wine industry,” said Gerhard.
It will take a few years before all the rootstocks can be replaced by immune plant material, but once accomplished, they will no longer present a route through which healthy scion planting material can be infected.
Research is also under way at Stellenbosch University, under the direction of Pietersen and Dr Justin Lashbrooke, aimed at identifying the genetic basis for immunity on Ramsey rootstocks to understand the mechanism of immunity and possibly, in future, transfer it to important wine cultivars.
International trends
Gert Pietersen, Gerhard’s son and the managing director of Patho, gave an overview of what was happening in other countries in the fight against leafroll virus.
The US and EU are subsidising the replacement of vineyards at certain infection rates that believed, if replicated here, could move South Africa from a “management to an eradication” programme.
However, he wondered for how much longer the US would continue to subsidise replacements, given the huge industrial reform that was taking place in the wine industry because of shrinking global demand. “California is planning to remove about 25 000ha of vineyards to balance supply and demand once again.”
Gert added that South African farmers can strategically use contracted services by Patho to get rid of virus-infected vineyards. After that, they need to stick to the proven leafroll virus management strategy by planting virus-free plant material in virus-free soil. They also need to defend these gains through good vineyard and mealybug control practices.
Gert said that in New Zealand, field trials had been done over the past three years to determine the effectiveness of a sensing tool called VinEye to accurately identify leafroll virus on specific red varieties.
The tool, which consists of a self-driving robot that uses machine learning (ML) to identify the disease based on photographs of grapevines, has so far been tested on 60ha of land, reportedly with promising results.
Gert said that articles he had read about the technology claim it is more cost effective than using people to identify the disease: “Perhaps this might be true in New Zealand, but it will definitely not be cheaper than using humans in South Africa.”
He explained that the current practice is to mark infected vines once they had been identified and remove them in winter when there are no leaves to indicate a problem.
“Perhaps in future the robot might be modified to have an arm and spray can to mark the vines, but until then farmers are dependent on a geolocation and if that is not perfect or clear enough, people might accidently remove a healthy vine, with the sick one then infecting other vines.”
Local solutions
In related research under way at Stellenbosch University under the leadership of Prof Lizel Mostert in collaboration with Patho, an MLg algorithm is being developed to recognise symptoms of grapevine leafroll. Gert said:
“The more appropriate use of such an algorithm, in my opinion, would be to integrate it into a scouting app, such as our Vine Cloud. A scout will then be able to use the app to confirm an infection if he or she is uncertain.”
Patho only uses Vine Cloud internally, but Gert plans to roll it out to local clients before the start of the next wine season.
He also touched on the importance of ELISA (enzyme-linked immunosorbent assay) testing, which was developed by Patho between 2017 and 2019, to identify GLRaV-3: “It is a robust test that can be implemented in a rather rudimentary laboratory. The kits are in a workable format, allowing farmers to easily test varieties for the disease.”
The test is primarily used on white wine varieties because they don’t show the characteristic red veins that form on the leaves of red varieties.
The symptoms usually start with a few uneven reddish spots during veraison (ripening), which then spread to cover most of the leaf, except for the green area bordering the main leaf veins. The leaves of some of the varieties may also curl downwards.
The leaves of infected white varieties might turn yellow, as is the case with Chardonnay, Cape Riesling and Sémillon, while those of Chardonnay may also curl downwards.
Each test can be used for 10 pooled vines, generally a single bay. “If the test is negative, you know that the bay is clean, but if it is positive, you will have to test the individual vines in that bay to identify the infected vines,” Gert explained.
Biological control
Between 2003 and 2009, Vergelegen replanted all its vineyards with virus-free vines. This, in combination with good disease monitoring, frequent testing, good sanitation and mealybug management practices, has allowed the estate to keep leafroll incidents below 0,05% and 0,3% in red and white varieties, respectively.
In November the estate started working with SkyBugs to release biological agents via drones to fight mealybugs. SkyBugs is a collaborative venture with FieldBugs, which supplies biological control agents, and agritech company Aerobotics, which collaborates with a network of drone pilots to disperse the insects accurately.
The biological agents consist of the parasitic wasps Anagyrus vladimiri and Coccidoxenoides perminutus, which are attracted to the pheromones released by female mealybugs and lay their eggs in the mealybug females. Mealybug ladybird beetles (Cryptolaemus Montrouzieri), which can eat between 100 and 200 mealybugs a day, are still released by hand.
Fieldbugs technical support specialist Ruan Erasmus said the first stage of the programme involved scouting and data assessment to identify blocks where the biological agents needed to be released. The first releases at Vergelegen were done in November, with follow-up flights in December, January and February.
Traditionally, the beneficial insects are dispersed by hanging compostable tubes of insects in vines. But Erasmus said using a drone was better as there was wider dispersal, which is important as predatory wasps fly no more than 90m daily and beneficial insects can be released onto high trees near vineyards where mealybugs normally escape detection.
Email Prof Gerhard Pietersen at [email protected] or Ruan Erasmus at [email protected].
