Test Identifies Virus Already Present
By Laurie Greene, Associate Editor
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
Cooper said, “The real difference is
that before we merely saw symptoms; now we can identify the virus with a test.”
blotch-associated virus (GRBaV) is the latest addition to the list of more than
75 graft-transmissible agents in grapevines. The National Clean Plant Network
says 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.
Service reports that in
2008, these grapevines exhibited symptoms resembling leafroll disease, and
produced clusters with reduced sugar content causing delayed harvests.
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. |
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, much
before the onset of symptoms.
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.
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.
Cooper summed up, “There will be no quarantine. This is not an actionable pest;
we just do not know enough about it.”
what are growers to do?
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.
Finally, Napa Valley Grapegrowers is
stressing the importance of encouraging the release of funding by the California PD/GWSS Board
for further research and understanding.
Labels: GRBaV grapevine red blotch-associated virus, New Red Blotch PCR test, New Test Identifies Virus Already Present, Red Blotch on Grapevines not new