New Test Identifies Virus Already Present

By Laurie Greene, Associate Editor


TODAY, in the midst of new Red Blotch discoveries, Monica Cooper, UCCE Viticulture Farm Advisor in Napa County told California Ag Today that Red Blotch is not new to California; it is a newly-discovered virus that affects grapevines. “We have seen it for awhile, but it has been confused with leaf roll disease that also causes leaf reddening, and it had not yet been genetically identified.”

Cooper said, “The real difference is that before we merely saw symptoms; now we can identify the virus with a test.”

Grapevine red blotch-associated virus (GRBaV) is the latest addition to the list of more than 75 graft-transmissible agents in grapevines. The National Clean Plant Networksays this recently reported virus is associated with the emerging red blotch disease first described on research plots and commercial vineyards of Cabernet Sauvignon in Napa Valley in 2008.

The USDA Agricultural ResearchService reports that in 2008, these grapevines exhibited symptoms resembling leafroll disease, and produced clusters with reduced sugar content causing delayed harvests.

Among the grapevine pests and diseases, only leafroll diseases exhibit similar canopy symptoms, cause reduced sugar accumulation in the berries and occasionally exhibit poor color development in some clusters and increased acidity as well. Potassium deficiency also has a similar appearance.

Red blotch leaf symptoms on a Cabernet Franc vine.  Photo by Marc Fuchs.

Laboratory tests in 2008 failed to detect any of the leafroll and rugose wood viruses in these samples. The disease symptoms were not caused by nutritional deficiencies, stress, bacteria, fungi and/or nematodes.

Red Blotch diagnosis based on the leaf symptoms can be challenging. A new molecular test based on genetic sequences was developed in 2012 by researchers at Foundation Plant Services at UC Davis, USDA and Cornell scientists.

This new PCR test is now commercially available, and the virus can be detected in the petioles of basal leaves, muchbefore the onset of symptoms.

Identification of the grapevine red blotch-associated virus (GRBaV) is the first step in a process to collect more information on the virus and the disease. Ongoing studies are investigating many aspects of virus and disease, for instance: researchers are investigating the exact role of GRBaV in red blotch disease, seeking more information on the genomic sequence of this DNA virus, and investigating the transmission of the virus and improved detection techniques.

The virus has been shown to be graft-transmissible and is likely responsible for the wide geographic distribution. Thus far, disease symptoms have been observed in vineyards planted with red grape varieties such as Cabernet Franc, Cabernet Sauvignon, Merlot, Petite Sirah, Petit Verdot and Zinfandel.

Cooper says that the disease is widespread throughout California. Similar viruses have been found in vineyards in New York, Pennsylvania and Canada.

Cooper says we don’t know enough about the disease. We don’t know the different varieties, the rootstocks of choice, how it moves (by insect or in plant materials only). It is also not known if the disease has any effect on fruit yield or if it causes vine decline.

As Cooper summed up, “There will be no quarantine. This is not an actionable pest; we just do not know enough about it.”

So, what are growers to do?

Cooper recommends use of planting material free of known pathogens, given that once viruses are present in the vineyard there is no known cure.

The symptoms generally start appearing in late August through September as irregular blotches on leaf blades on basal portions of shoots. The grapevines with red blotch stop accumulating soluble solids and end the season at four to six degrees brix lower than uninfected vines.  
If grapevines, red or white, are producing fruit with Brix values lower than expected and are not showing classic leafroll-like symptoms, contact your local ViticultureFarm Advisor.

Test your vines.

The Napa Valley Grapegrowersoffers the following information and links:

Finally, Napa Valley Grapegrowers is stressing the importance of encouraging the release of funding by the California PD/GWSS Boardfor further research and understanding.