Are viruses to blame for the colouring problem in the Crimson Seedless table grape cultivar? By Jorisna Bonthuys
New research provides strong evidence that the colouring problem of the Crimson Seedless table grape cultivar is due to an infectious agent – or a combination of agents – affecting the vines, with grapevine leafroll-associated virus 3 (GLRaV-3) potentially being the main culprit. This particular virus is also the cause of grapevine leafroll disease.
Gerhard Pietersen, a former professor in Stellenbosch University’s Department of Genetics and now at Patho Solutions, was involved in the research project that yielded these findings. The project, funded by the South African Table Grape Industry (SATI), has been ongoing since 2019.
Crimson Seedless is one of the country’s most widely planted table grape cultivars and enjoys high consumer demand, here and across the globe. The cultivar has one big problem, however: the older the vines get, the slower the berries ripen and colour, often resulting in ripe berries without the deep red hue that their name promises.
Although Crimson Seedless – one of the top-six cultivars exported from SA – is the most studied cultivar globally, much remains to be understood, including the cause of the lack of colour formation in berries, Pietersen says.Read More
Genetic variability and mutations, environmental conditions, and infection by one or more viruses are factors that may be possible causes of what has become known as the “Crimson Seedless bunch disease”.
Pietersen has focused his investigation on the likelihood of it having a viral cause, as a common symptom induced by many plant viruses is delayed ripening and a lack of complete colouring in fruit. He says vine viruses are regarded as one of a grape producer’s biggest enemies. They can drastically reduce the lifespan of a vineyard and seriously hamper the quality of the grapes.
Pietersen focused his research on vines in seven vineyards in the Hex River Valley and the Boland area to find vines that consistently displayed Crimson Seedless bunch disease. He also investigated vines not showing berry colouring problems.
He monitored these vines during harvest for three years and plotted the position of infected vines within the vineyards.
He then selected 12 vines for collection and further analysis. Of these, nine displayed extreme and consistent symptoms of Crimson Seedless bunch disease. The remaining samples included vines showing signs of the disease surrounded by essentially healthy vines.
The samples were tested for all known and unknown viruses, using next-generation sequencing – large-scale DNA sequencing that allows for querying the entire genetic code found in that sample.
The technique also provides an effective way to study and identify new virus strains and other pathogens. “Next-generation sequencing provides a much better understanding of the diversity of viruses and viroids present in local table grape vines,” Pietersen explains. (Viroids are a class of infectious agents even smaller than viruses consisting of tightly folded RNA and lacking the protein coat that viruses have.)
Because the pattern of spread of this disease is similar to that of leafroll disease caused by GLRaV-3, and the ubiquitous presence of GLRaV-3 in these vines, Pietersen suspected that this particular virus could be playing a major role in this disease, possibly with the help of the other viruses or viroids present.
For this reason, he also chose to monitor leafroll symptoms in autumn on the leaves of vineyards he had monitored for the bunch disease in this project.
In two selected vineyards, he also tested the vines using an enzyme-linked immunosorbent assay called the “ELISA test” for GLRaV-3. Enzyme-linked immunosorbent assay (ELISA) is a test that can detect viruses, sometimes even in the absence of any symptoms of plant disease.
Pietersen found a high correlation between vines showing the Crimson Seedless bunch disease on the bunches at harvest in February and March, the presence of leafroll symptoms in June, and the presence of GLRaV-3 in these vines.
He says the fact that scientists now know that GLRaV-3 is involved in this disease and that the disease spreads slowly – mainly down vine rows – means that table grape producers can manage this problem by roguing (removing) infected vines, as is done for wine grapes infected with GLRaV-3. Producers can also focus on controlling mealybugs, the main vector of GLRaV-3, through an integrated pest management approach.
Plant material is also important. Crimson Seedless clones that are now available have undergone virus elimination processes. His advice to producers is to use these certified virus-free plant materials when establishing new vines or replanting their blocks.
“Using virus-free plant material is, however, only the first step,” he emphasises. “Producers must also actively manage infected old vineyards while phasing them out and planting new material.”
It is a well-established fact that GLRaV-3 is the primary reason why so many wine vineyards are replaced early, at 15 – 20 years of age, says Pietersen. This virus reduces production volume, causes uneven ripening and, critically for the wine industry, the build-up of sugars. The virus may be introduced into a block via infected cuttings or mealybugs. In most cases, the symptoms are only visible to the producer during autumn or when the vines are under stress.
According to Pietersen, there are many valuable lessons for the growers of Crimson Seedless to learn from the local wine industry when it comes to managing grapevine leafroll disease and grapevine mealybug problems. Farms like Vergelegen, for instance, provide a model for the industry of what can be achieved with integrated pest management and best practices.
Mealybug most commonly transmits the disease to the vines closest to the infected one, leading to many new infections. This is especially common if equipment and clothing are not sanitised in between farming activities.
“Monitor for grapevine mealybug continuously throughout the grape-growing season using visual inspection of vines,” Pietersen emphasises.
“In the presence of Crimson Seedless bunch disease, no mealybugs can be tolerated in the vines,” he adds. “If producers are to really get a grip on this (colouring) disease, they will have to take GLRaV-3 and its vector seriously.”
Pietersen says that more work is needed to understand the interaction of the other viruses and viroids detected on vines affected by Crimson Seedless bunch disease. This can be done with polymerase chain reaction (PCR) tests, followed by transmission trials in which healthy vines are inoculated with combinations of the relevant viruses and viroids. The PCR test could detect virus fragments even after the vine infection had passed.
He concludes: “While we have made a good start on this problem, we simply do not know enough. More research is needed to understand the synergistic role of viruses and viroids in the table grape industry to improve the international competitiveness of the table grape sector.”
*Contact Pietersen at email@example.com for information.