Tobacco ringspot virus and grapevine leafroll virus can cause serious losses in vineyards.
Annemiek Schilder, associate professor in the Center for Integrated Plant Systems at Michigan State University, along with her colleagues, has been studying these two viruses, as well as vine mealybug and its relationship to leafroll disease.
“We were called to a particular vineyard, and there was a lot of leafroll there, and it apparently had spread quite fast,” Schilder says. “The one main factor that was thought to be involved is grapevine mealybug.”
Last year, Schilder’s colleague Rufus Isaacs, professor in the Center for Integrated Plant Systems at MSU, monitored for mealybug in the affected vineyard, while Schilder evaluated the two viruses. They looked at how many of the vines were infected and how severe the symptoms were. They also looked at yield loss and fruit quality.
“It seemed like when the vines had either mild symptoms or moderate symptoms, there wasn’t a lot of measurable yield loss,” Schilder says. “But when you have severe and very severe symptoms, that’s when we started to see a big reduction in yield, and if the vines were jointly infected with both viruses, then we saw something like an 80% reduction in yield.”
The harsh winter last year had an impact on the vines, too, particularly those that were infected, which sustained more winter injury than healthy vines, according to Schilder.
Understanding The Viruses
Grapevine leafroll virus is one of the most common viruses of grapevines, and it’s usually brought into the vineyards with planting material, Schilder says. If grapevine mealybug is also present, it can spread the virus quite quickly.
Tobacco ringspot virus is soilborne, affecting weeds such as dandelions, and vectored by nematodes. These nematodes feed on the roots of infected plants and transmit virus particles to healthy vines.
Overall, tobacco ringspot is the more lethal of the two.
“You see the vines getting smaller and weaker, and in the case of tobacco ringspot, it would actually kill vines,” Schilder says. “Leafroll is more chronic and doesn’t really kill the vines. It might weaken them, lower the yield and lower the Brix, but it doesn’t usually kill the vines.”
How To Minimize Losses
As with all viruses, once a plant is infected with tobacco ringspot or grapevine leafroll virus, it’s infected for life. You can’t treat it or cure it, so vine removal is generally the only option.
Growers can, however, minimize the risk of planting infected vines by purchasing virus-tested planting material. Granted, not all cultivars are available, Schilder notes, but virus-tested material is worth the investment.
If the virus does show up in a new planting, Schilder recommends removing those vines as quickly as possible before any spread can take place.
“If you have leafroll virus, you should monitor for mealybugs and apply insecticides,” Schilder says. “They’re notoriously difficult to control, because they are usually under the bark where insecticides would have a hard time penetrating.”
In this case, Schilder says dormant oils can be effective.
Tobacco ringspot virus can be more difficult to control, because even if your plants are clean, the pathogen could be in the soil. “If there was a fruit crop before, like raspberries, apples or even blueberries, it could be infected with the virus,” Schilder notes. “And if there’s a resident population of dagger nematodes that are virus-carrying, even if you pulled out a planting and replanted with grapes, there’s a risk from those nematodes.”
The nematodes can live at least one to two years and even longer in some cases. If the virus and nematodes are present, Schilder suggests either fumigating the soil (if you want to plant right away), or growing a cover crop such as grass for two years with very strict broadleaf weed control. “You’re not going to get rid of the nematodes, but you’re going to get rid of the virus,” she says. “They [nematodes] might still be there in the soil, but they won’t be able to move the virus into the host.”
There’s also the option to grow with virus- or nematode-resistant rootstocks, which Schilder says stop the nematodes from feeding on or infecting vines. The University of California-Davis has a number of nematode-resistant rootstocks.
If you don’t know what has been planted in your ground before, prior to planting a new vineyard, you might want to test the weeds such as dandelions to determine if they have the virus. “If you have tobacco or tomato ringspot in areas like Michigan or New York or some other states where the virus is endemic, you can look for it before you even plant by looking at the virus infection status of the weeds,” Schilder says.