New DNA sequencing technology has been used to discover the grapevine red blotch-associated virus (GRBaV) in grapevines affected by “red blotch,” a disease that discolors grape leaves in early fall and reduces sugar levels in grapes.
Nursery managers received the new information during an annual industry update at the University of California, Davis, Feb. 20, 2013. 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,” says Deborah Golino, director of the University of California, Davis-based Foundation Plant Services and a cooperative extension plant disease specialist. “Due to the distribution of the virus in many parts of the United States and evidence that it can be transmitted by grafting, we suspect that red blotch disease is widespread wherever grapes are grown.
“Red blotch disease has been identified among red and white vinifera
cultivars both young (first leaf) and mature (5- to 20-years-old), grape vineyards in California, New York, Virginia, Maryland, Pennsylvania, Texas and Washington. A virus almost genetically identical to red blotch also was found in Canada.”
GRBaV has been detected in nine red cultivars: Cabernet franc, Cabernet Sauvignon, Malbec, Merlot, Mourvédre, Petite Sirah, Petit Verdot, Pinot Noir and Zinfandel.” Detection in Chardonnay and Riesling vines occurred during harvest 2012, and the description of the symptoms will be available after harvest 2013.
Red blotch disease was first recognized in 2008 in a Napa Valley vineyard by Jim Wolpert, a UC Davis-based cooperative extension viticulturist, and Mike Anderson, a viticulture researcher and manager of UC Davis’ Oakville Experiment Station.
Investigations into what appeared to be a new disease began in 2009. GRBaV was reported in independent studies in California and New York in 2012. There is a very good correlation between the presence of GRBaV and red blotch symptoms.
Two virologists—Mysore Sudarshana, a USDA-ARS researcher at UC Davis and Maher Al Rwahnih, a researcher at Foundation Plant Services (UC Davis)—teamed up in an effort to identify the virus causing red blotch disease. Al Rwahnih is an early user of next-generation, or high-throughput DNA sequencing technology since 2007.
Sudarshana reports that one block of Cabernet Sauvignon on 101-14 rootstock planted in 2001 at the Oakville Station Experimental Vineyard has GRBaV in 46% of the vines.
Presence of the virus in a vine can be confirmed using an extremely sensitive laboratory test known as polymerase chain reaction, or PCR, to detect small amounts of genetic material. This analytical process uses an amplification technique that multiplies the existing DNA and similar genetic material to identify the virus. Analysis of the genomic nucleotide sequence indicates a new circular, monopartite DNA virus that is tentatively assigned to the family Geminiviridae
Several labs offer a test for GRBaV. In October 2012, Agri-Analysis VINEGUARD testing service, which screens for up to 19 viral and bacterial pathogens, added GRBaV. AL&L Crop Solutions began offering testing for GRFaV in November 2012. CSP Labs Inc. offers a virus Multi-Scan PCR test that detects 23 viruses including GRBaV. Eurofins STA Labs implemented a proprietary test for red blotch to their HealthCheck portfolio in November 2012.
The Foundation Plant Services’ Classic Foundation Vineyard block, used to produce disease-free scion and rootstock vines, has been partially tested and appears to have a very low-level incidence of red blotch virus, reports Golino. Only three of 1,600 vines tested to date were found to have the virus; one each of the following varieties and selections: Chardonnay 68—a private clonal selection no longer listed as available from FPS; Ruby Cabernet 02—a cross of Carignane x Cabernet Sauvignon developed by UCD professor Harold Olmo; Thomcord 02—a black table grape selection, a cross of Thompson Seedless x Concord.
Testing of all vines in the Classic Foundation Vineyard, the source of the majority of grapevine nursery stock in California, will be completed in 2013.
“If a vine 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,” Golino adds. “This is always a complex decision, and there is no one- case-fits-all answer.”
Golino encourages vineyard owners and managers to evaluate their vineyards for red blotch disease as they would for any other viruses. Symptoms include blotches of pink or red veins on the leaf undersides in early fall, when grape leaves would normally be turning a uniform gold color. Growers also might notice that the grapes are slow to develop sugar levels sufficient for winemaking, with some grapes never fully maturing.
“If there are visual signs of red blotch and poor sugar development, growers should test their vines for both red blotch and leafroll virus because the symptoms of the two viruses are so similar,” Golino notes.
While the virus likely can be found in all types of grapes, including rootstock, and table and raisin grapes, it was first detected in wine grapes because they are carefully monitored for sugar content to determine harvest date.
Based on the wide host and geographic distribution of GRBaV, and the fact that the virus is transmitted by grafting, it is likely that spread primarily occurs through propagation material. Also, an increased incidence of GRBaV over time in young, healthy vineyards that are adjacent to old, infected vineyards suggests the possibility of a vector. Like other viruses, once it is present in a vineyard there is no cure.
However, evidence suggests that GRBaV can be eliminated using microshoot tip culture, the same method used to eliminate other viruses, to establish clean Foundation vines.
“The good news is that our new Russell Ranch Foundation Vineyard block has been tested, and there is no sign of red blotch virus in any of those vines.” This vineyard is propagated exclusively with vines that have been through tissue culture therapy to exclude red blotch and other viruses. Test records from the Foundation Plant Services vineyards are available at: fps.ucdavis.edu. < br />
Golino cautions that many more new viruses and other microbes are likely to be found in grapevines in the next few years to add to the list of more than 75 graft-transmissible agents that have been identified in grapevines, thanks to powerful new DNA sequencing technology.
“Some of those will be disease agents, some beneficial, and some neutral,” she explains. “We will have our work cut out for us in understanding the role of these microbes, but the ultimate result will be increased ability to create superior grapevine material.”
“The appearance of red blotch virus,” says John Aguirre, president of California Association of Wine grape Growers, “underscores the vital importance of the Foundation Plant Services and adequate funding for the U.S. Department of Agriculture’s (USDA) National Clean Plant Network. Growers need clean plant material to meet winery demands for increased wine grape production and improved quality.”
Progress on identifying and characterizing red blotch has been a collaborative effort between many USDA-ARS and university-based researchers over several years, notes Golino.
Golino announced that California rootstock nurserymen acted in February to assist the California viticulture industry with research addressing GRBaV. The California Grape Rootstock Research Foundation, which funds research to enhance California viticulture and the grape nursery industry, has agreed to provide seed money to jumpstart research on the virus.
More information on red blotch disease is available on the University of California Integrated Viticulture website at: iv.ucdavis.edu/Viticultural_Information/?uid=284&ds=351.