There are several microscopic maladies that can affect grapevines. Particularly fascinating and challenging is the dreaded leafroll virus, since conventional antimicrobial measures are not effective against it.
They say small things amuse small minds. But what about incredibly small things? Louis Pasteur, the father of microbiology, made humankind aware of the presence of miniscule living beings all around us and even in us. Bacteria, yeasts and viruses are ubiquitous and, depending on the specific microorganism and situation, can be either beneficial or detrimental.
The humble grapevine (Vitis vinifera), like all living things, is susceptible to attack by various plant pathogens. Erysiphe necator (a fungus), Agrobacterium vitis (a bacteria) and the grapevine leafroll-associated virus (GLaV or leafroll virus) are but a few examples of pathogens that come to mind. While bacteria and fungi can be controlled with chemicals, there’s no chemical control against the leafroll virus. A consolation prize is that chemical control of insect vectors of the viruses that cause leafroll disease can help to limit the spread of the virus in a vineyard.
Leafroll virus was discovered in South African vineyards in 1936. The disease, which is spread by mealybugs, affects both white and red wine cultivars. Leafroll virus diminishes not only grape quality, but also yield. Imagine a huge, highly favoured block that eventually delivers not only poor quality grapes, but also a very small harvest. In the end, the decline becomes so severe all the once-prized vines have to be uprooted and destroyed. This is a horrifying scenario which is avoidable.
Professor Gerhard Pietersen of the Agricultural Research Council (ARC) Plant Protection Research Institute says the leafroll virus is present worldwide in every area where vines are planted. “If active measures aren’t taken against the leafroll virus no block older than 10 years will be less than 50% infected and no block older than 20 years will be less than 100% infected.”
While the situation in South Africa is fairly serious, our country is leading the pack when it comes to fighting this disease, he says. “The work we did at Vergelegen Wine Estate demonstrated the scientific and commercial value of managing the virus problem well and even eradicating it. As a result, both wine quality and production have improved. Winetech, which sponsored my research at Vergelegen, has invested R20 million since 2000 and the excellent results made the rest of the world sit up and take note. We’ve shown that by being vigilant, the leafroll virus can be stopped and eradicated before it infects a whole vineyard.”
In New Zealand 30 producers joined forces and managed to bring infection levels in the Gimblett Gravels wine district down to less than 0.5%. Golan Heights Winery in Israel and Napa Valley in California are also making strides in the battle against leafroll virus.
The programme at Vergelegen was implemented in three phases. During phase one Vergelegen planned for the replacement of 25 ha of citrus with vineyards. The first clean vineyard was planted in 1999. The vines were regularly tested and infected vines were pulled out.
In phase two the farm was divided in two. One half had clean, young vineyards and the other infected, old vineyards. The application of pheromone bait and spraying of systemic insecticide with low toxicity to mammals was then initiated. Alien-clearing efforts saw the return of indigenous fynbos, which attracted birds and insects. The new abundance of ladybirds, a predator of mealybugs, meant the use of chemicals against mealybugs could be limited. It was also found that mealybugs were spread mainly because of human activity, specifically by people and tools, and not by birds and wind. Newly settled vineyards were located far away from infected vineyards and farm boundaries and workers and their tools were divided into two teams – no worker was allowed to move from an infected vineyard to a clean vineyard.
Phase three – Several of the older white varietal blocks still produce excellent wines and only when the quality and quantity of grapes decrease are the vines removed and replaced. White varietals don’t show symptoms of infection as clearly as red varietals and these vines must be checked for infection annually. Every block and vine is recorded and charted and adjoining sections are tested. If there’s a positive result, every vine in the section must be tested. During 2012, 16 000 tests were carried out on more than 127 000 vines at Vergelegen. And in 2015 only 35 of more than 500 000 vines showed signs of the leafroll virus.
Vititec plant pathologist Tobie Oosthuizen explains how the test for leafroll virus works. “The test is called Elisa, which is an acronym for enzyme-linked immunosorbent assay. Purified virus particles are injected into a test animal, which then produces specific antibodies to the virus. These antibodies are extracted from the animal’s blood and purified. The Elisa test is done on a plastic plate with 96 wells. The antibodies bind to the well, after which the vineyard extract is added. If the virus is present in the sample it binds to the antibodies and a colour reaction confirms the presence of the virus.”
The road ahead might not be easy, but the successes to date encourage further work. “Red wine varietals respond extremely well to control measures and widespread implementation of such measures, while challenging, are critically important,” Professor Pietersen says. “White wine varietals don’t show symptoms of infection as clearly as red wine varietals, which means every plant must be tested yearly. And a great deal of work still has to be done to find rootstock that shows resistance to the virus.”
But good research and various plant-improvement organisations are making a positive impact. “Certified material is of good phytosanitary quality, but it’s up to the producer to keep the material virus-free,” Tobie says. “Remember, clean material can be infected once it is planted on the farm. Nevertheless, our efforts are continuing and this year we’ll test 302 142 vines for the disease.”