Grape leafroll virus is becoming an increasing concern across California's grape producing areas. Once considered a minor problem and perhaps even a factor that ultimately improved wine quality, researchers are quickly becoming concerned that the virus is much more insidious than originally believed.
“We're seeing it spread in a way we've never seen it before,” says Deborah Golino, plant pathologist and director of the University of California, Davis' Foundation Plant Materials Service. Golino was addressing attendees at the first stop of UC's Central Coast Research Roadshow in Paso Robles, Calif. “What you get (with leafroll virus) is delayed fruit maturity, poor color and reduced yields. A vine with leafroll may be around in your field for many, many years, but you're getting a constant effect year after year. Your costs of managing that vineyard are going to be the same. However, your product is going to be affected and there's going to be less of it.”
Symptoms of leafroll virus vary among red and white varieties. It's easier to detect visually in red varieties which tend to display symptoms of the virus in the spring as well as the fall. During the spring, the most telltale fruit symptom of leafroll in reds is poor color or greenish berries. In the fall, leafroll symptoms in reds tend to show up as red leaves and rolled under leaves. Infected vines may also show reduced vigor.
“In severe cases you can sometimes see leaf curling, chlorosis and reduced vigor,” Golino says. “In white-fruited varieties, the symptoms are much harder to see.”
To make matters even more complicated, not all leafroll strains will always cause these symptoms or even be expressed with the same severity. Additionally, symptoms often differ among rootstocks, varieties and clones. The complexity of the disease itself adds to the difficulty in recognizing the symptoms.
“Leafroll disease is not caused by one single virus or pathogen,” Golino says. “It's caused by a related group of a family of viruses. They're not only complex, they're diverse, and they vary in the symptoms they cause, even within the groups. It's a little bit like human influenza viruses.” Researchers so far have described eight different leafroll viruses. “It's a big family of viruses and we do expect to find more of them,” Golino says. “They have different co-proteins. They have different genomes. Even within leafroll Type 2 there are strains that will cause symptoms that vary from no visual indications to complete kill of certain rootstocks.”
It's very difficult to predict where the problem is going to occur unless replicated field trials have been conducted to sort out the variables.
“In Vitis vinefera on their own roots, the leafroll viruses are not as severe as they are on some of the rootstocks we are using today,” Golino says. “AXR and St. George - the rootstocks that were used by the majority of growers 20 years ago - are actually pretty tolerant of these viruses. The rootstocks we're using today are much more susceptible to these viruses.”
Understanding transmission of the disease is one of the keys to managing the problem. Leafroll is transmitted by mealybugs, and there are at least six species of mealybugs currently present in California. Of those species, vine mealybug is most concerning, according to Golino.
“In laboratory analysis we have found that all of the grapevine mealybug species present in California can transmit Type 3 leafroll virus,” Golino says. “We have not been able to show transmission of Type 2 in the laboratory and we only have a small amount of data about the transmission of Type 1 at this point.”
The long-tailed mealybug, which is common on the Central Coast, can transmit Type 5 and Type 9. Obscure and vine mealybug can transmit Type 3.
“We have plenty of mealybugs in the state now that are quite good at transmitting these viruses,” Golino warns. “With vine mealybug spreading I think we are going to see even more spread of this disease just because of the biology of that particular mealybug.”
Research by UC Entomologist Kent Daane in the Carneros region of Napa County indicates that soft scales are also likely capable of transmitting leafroll. Aphids may be yet another vector.
While symptoms are appearing more frequently throughout the state, no one is certain why. It could be that there are simply more vectors than there have been in the past.
“We do have a soft scale now that we didn't used to have,” Golino says. “Vine mealybug is a growing problem.”
Another factor could be a shift in rootstock preference. “Maybe leafroll was always spread to this degree in California, but we didn't see evidence of the disease because of the tolerance of AXR and St. George to leafroll virus,” she says.
“I know from my experience over 20 years that AXR and St. George won't show symptoms of leafroll, but we can take it back to the lab and find that it does indeed have leafroll.”
The noted increase could also potentially be attributed to new strains of leafroll that are more readily transmitted. “The RNA plant viruses are very mutable,” Golino says. “They change very readily. Even within one given plant we see populations with multiple genetic mutations.”
The situation underscores the importance of going through legitimate channels when bringing plant material into the state.
As an example, researchers believe that so-called “suitcase” varieties from Israel are likely responsible for originally introducing vine mealybug into the Coachella Valley which is now contributing to the spread of leafroll virus.
“Don't do something that could endanger our entire industry,” Golino says.