DNA sequencing technology has been used to identify the grape virus that causes red blotch, a disease that discolours grape leaves in autumn and lowers sugar levels in grapes. Researchers at the University of California, Davis, say while the virus was first discovered in the US, scientists in other countries have also started testing for it and are likely to find it wherever grapes are grown.
The virus is of particular concern to wine grape growers, whose grapes must reach a certain sugar content level before they are suitable for winemaking. “The most urgent research need now is to determine how the virus spreads,” explains disease specialist Deborah Golino, director of the UC Davis-based Foundation Plant Services. She adds that vineyard owners and managers should evaluate their vineyards for red blotch disease as they would for any other virus.
Symptoms include blotches of pink or red veins on green leaves in autumn, when they would normally turn a uniform gold colour. Growers might also notice that their grapes are slow to develop sugar levels sufficient for winemaking, with some grapes never fully maturing. According to Golino, red blotch disease has been identified among both young and mature grape vineyards in seven states. A virus with virtually identical genetics was also found in Canada.
While the virus, also called grapevine red blotch-associated virus or GRBaV, is likely to be found in all types of grapes, it was first detected in wine grapes because they are monitored with care. The presence of the virus in a vine can be confirmed using a laboratory test known as polymerase chain reaction, or PCR, to detect small amounts of genetic material. This multiplies the existing DNA to identify the virus.
“If the vineyard tests positive for red blotch, the grower needs to decide, from an economic standpoint, when is the best time to remove the diseased vines and replant the vineyard,” Golino says. Many more viruses and other microbes could be found in grapevines over the next few years, thanks to powerful new DNA sequencing technology.
“Some of these will be disease agents, some beneficial, and some neutral,” she explains. “It will require much research to understand the role of these microbes, but the result will be worth it – growers will be better able to create superior vineyards.” Red blotch disease was first recognised in 2008 in a Napa Valley vineyard.