Leafroll disease is “the No. 1 hot button issue for Napa Valley wine grape growers,” says Randy Heinzen, Napa Valley vineyards manager for Beckstoffer Vineyard at Rutherford, Calif.
Considered one of the most severe grape diseases in the world, leafroll accounts for about 60 percent of grapevine losses caused by viruses. There is no cure and vines remain infected for life. The disease can slash the normal expected productive life of a vineyard by 50 percent or more.
Leafroll disease has been showing up in an increasing number of California vineyards over the past decade — particularly in the Napa Valley and the Central Coast. The disease is caused by various strains of grapevine leafroll associated viruses (GLRaVs). The major threat to California vineyards is GLRaV-3, for which mealybugs are the disease vector.
The vine mealybug is the newest and increasingly important vector of this virus in California; however, it can also be transmitted by the other mealybugs such as grape, obscure, long-tailed, citrus and Gill’s mealybug. as well as soft scale.
The most successful approach to controlling leafroll disease has been the use of virus-free grapevine nursery stock — root or scion — produced through the California Grapevine Registration and Certification Program, a program administered by the California Department of Food and Agriculture.
Heinzen helped organize a seminar to update growers on the nature of the disease and what growers can do to control the spread of leafroll. Sponsored by the University of California Cooperative Extension, Napa Valley Grapegrowers and the Napa Valley Vineyard Technical group, the meeting was held earlier this spring at Yountville, Calif.
Heinzen, who participated in the seminar along with University of California researchers, described some of the problems growers face in controlling leafroll disease. “The interconnectedness of our vineyards and the high value of the crop in Napa Valley, combined with a vector that’s particularly adept at spreading the disease, makes it a great concern to us,” he says. “Nearly undetectable low levels of mealybug can spread the disease from an infected block to a neighboring virus-free block.”
In the past, the grape mealybug has been the main mealybug vector for Napa Valley growers. Natural predators tend to prevent massive outbreaks of grape mealybug and populations usually don’t reach economically-damaging levels requiring control.
But, it’s a different matter with vine mealybug. This invasive species from the Mediterranean region was introduced into the Napa Valley about a decade ago, where it has gained a foothold, particularly in the southern part. It’s a prolific pest that produces many more generations in a year than grape mealybug.
“We don’t know which of the mealybugs that feed on grapevines are the most effective at spreading the leafroll virus,” says Kent Daane, a University of California Cooperative Extension entomologist, who also spoke at the seminar. “However, based on sheer numbers, as the vine mealybug population continues to grow, it has the potential to be a much greater threat for spreading the pathogen.”
Heinzen is controlling the spread of leafroll disease in new blocks by planting only nursery stock that is virus-free. In heritage vineyards, where the value of the vines outweighs the cost to replant the entire block, he rogues out vines suspected of having the disease, as well as those adjacent, and replaces them with new, clean stock.
“It’s costly in the short-term,” he says, “but given the economic value of our crop, it makes sense financially to rouge out leafroll virus as we find it.
Heinzen says more research is needed on how to control the virus.
“There’s much we don’t know about the disease and possible vectors,” he says. “But, a lot of bright minds are looking for answers. Growers can do their part by continuing to support the agencies and scientists who are spearheading this research.”