Ask scientist Alan Wei about the grapevine disease known as red blotch and he won't mince his words: "Aside from drought, it's the most serious concern to confront grapegrowers in the past 10 years." Striking words from the owner and manager of Agri-Analysis, an agricultural diagnostics lab in Davis, Calif., especially considering the virus was only recently identified.
Red blotch can severely hinder the grapegrowing process, to the point that infected vines are best destroyed. Vines with blotch produce smaller grapes that fail to reach full physiological maturity and ripeness, yielding lower sugar contents and poorer quality wines. The telltale symptom of infection is red-spotted leaves that appear during the summer months, but vintners have also reported poor crops from healthy-looking vineyards which were later determined to be infected through DNA testing.
How the virus spreads has been a mystery, but researchers at the University of California at Davis and the USDA Agricultural Research Service may have made a breakthrough on that front. After running experiments with various insects found in infected vineyards, they theorize that the three-cornered alfalfa treehopper is a probable vector for the disease. Greenhouse-raised, disease-free control vines were found to have contracted red blotch after exposure to treehoppers from infected vineyards.
Red blotch went unrecognized in California's vineyards for years, largely because of the similarity of its symptoms to those of leaf roll, a virus which also turns leaves red but causes their edges to curl as well. Just how long red blotch has been in California is anyone's guess, but Deborah Golino, director of U.C. Davis' Foundation Plant Services, found evidence of red blotch in dried leaves from an Italian Swiss Colony vineyard in Sonoma County collected in the 1940s.
It wasn't until 2008 that Davis researchers Jim Wolpert and Mike Anderson identified red blotch as a distinct virus. Since then, it has been identified in numerous California regions, as well as 11 other states, including all major U.S wine states. The disease inflicts an estimated economic loss ranging from $3,500 to $27,700 per vineyard acre, according to Dr. Marc Fuchs of the School of Integrative Plant Science at Cornell University.
There is no known cure for red blotch, and no treatment that can restore grape quality. Fuchs recommends that infected vines be tagged in the fall and pulled up during the dormant season when insect activity is at its lowest—if greater than 25 percent of a vineyard is diseased, the entire vineyard should be replaced.
How does the recent discovery of a probable vector change the picture? The treehopper mostly feeds on plants near vineyards—fruit trees and grasses. It sometimes feeds on leaves, but was considered a minor pest. Now growers can be more vigilant.
"Management strategies so far have relied exclusively on careful selection of clean planting materials from virus-tested stocks," said Fuchs. "So the identification of the three-cornered alfalfa treehopper as a vector is putting researchers in a position to develop more effective management strategies."