Update on Grapevine Red Blotch Associated Virus

By Judit Monis, Ph.D. Eurofins STA Laboratories

Grapevine red blotch associated virus (GRBaV) is different from most other known grapevine infecting viruses in that its genetic material is DNA, rather than RNA.  The virus is associated with the presence of red, blotchy leaves and reduced sugar content in red fruited grape varieties.  Hence, the names of the disease and the virus, Red Blotch and Grapevine red blotch associated virus, respectively.  Progress has been made in the genetic characterization, structure, and most importantly the completion of Koch’s postulates of GRBaV (Marc Fuchs, Cornell University).  Both the molecular and structural characterization places GRBaV in the Geminiviridae family.    The completion of Koch’s postulates is important because it provides a “cause and effect” for this virus being responsible for the symptoms displayed in infected vines.  The work at Cornell University demonstrated that GRBaV is capable of reproducing red blotch foliar symptoms in red fruited grapevine varieties.

Grapevine red blotch associated virus also affects white fruited grapevine varieties and in spite of its name, GRBaV does not always produce Red Blotch symptoms.  For example, in varieties such as Chardonnay, Pinot Gris, or Sauvignon Blanc, infection of GRBaV displays yellow blotches or discoloration in leaves without showing red blotch symptoms. While leaf symptoms are disconcerting, more importantly, GRBaV has been reported to affect sugar accumulation in grapevines resulting in reduced Brix values and delayed fruit maturity.

Presently there is no information on the natural transmission of GRBaV (other than it is graft transmissible and propagated with infected rootstock and scion cuttings).  While there are anecdotal reports that GRBaV is spreading rapidly throughout California vineyards, there is limited field data supporting this viewpoint. Within a laboratory setting it has been reported by Washington State researchers that the “Virginia Creeper” leafhopper is able to transmit the GRBaV.  While this is interesting, other researchers have not been able to reproduce the work to confirm transmission in the vineyard.  Presently, there is on-going research at UC Davis to determine if leafhoppers or other insects are capable of spreading the GRBaV from vine to vine.

In spite of the recent discovery of GRBaV, this virus was found in a UC Davis grapevine herbarium specimen (Dr. Deborah Golino personal communication) indicating that the virus has been present in vineyards since the 1940’s.  It is not clear why if present since the 1940’s only recently did it attract attention from growers.  It is likely that Red Blotch disease was confused with other grapevine disorders that manifest similar symptoms.  These disorders include infection by leafroll viruses or fungal pathogens, insect feeding damage, vine nutritional deficiencies, environmental stresses, soil conditions, physical damage, poor vineyard management, etc.  Not surprisingly, our laboratory continues to report that specific fungi are found in vines displaying similar symptoms.

Since the discovery of GRBaV our laboratory developed a reliable testing method (HealthCheck Panel RB) to detect GRBaV. During HealthCheck Panel RB qualification period, we tested a great number of vines that were infected with GRBaV. We determined that our method can reliably detect GRBaV from any portion of the vine.  Our laboratory has presented some of the data at the American Phytopathological Society (APS) meeting in 2014 and will present more detailed research at the International Conference for Viruses and Virus-Like Diseases of the Grapevine (ICVG) to be held in Ankara, Turkey this year.  We are confident that GRBaV can be accurately detected in all tissue types: leaf blades (both mature and new), petioles, young or lignified canes, as well as cordons and trunks. In addition, the virus is detectable throughout the different seasons. Consequently, we are able to offer HealthCheck Panel RB in combination with other popular HealthCheck Panels (A, B, or PD).

Because of the importance and effects of GRBaV in California grapevine production, the viticulture community has requested to include this virus in the California Registration and Certification (R&C) program. The process is on-going and likely the regulations will be revised to require the exclusion of this virus in foundation mother blocks, nursery increase and certified blocks.  Additionally, The USDA Farm Services Agency has included GRBaV  as an approved disease for financial relief when replanting an infected vineyard block.  We still recommend testing planting material for the presence of GRBaV and other known important viruses (e.g., Grapevine leafroll and Vitiviruses) that may have escaped detection in the R&C Program.

To educate growers we have developed a special internet page that compiles useful information about Red Blotch Disease including a links to photo gallery showing Red Blotch symptoms in grapevines of many different wine grape varieties.  We continue to update the industry with relevant webinar presentations with information on transmission studies. To view the most recent recording please click on the link: Update on Grapevine Red Blotch and other Red-Leaf Causing Viruses