Red blotch disease symptom. (Photo by Frank Zalom)

The cover features a photo of feeding injury caused by the three-cornered alfalfa hopper, Spissistilus festinus. Last year the Zalom team hypothesized that the three-cornered alfalfa hopper could transmit the Grapevine red blotch-associated virus, GRBaV, based in part on phylogeneic analysis of coat protein sequences of 23 geminiviruses that revealed that GRBaV-CP was most similar to that of another geminivirus that was transmitted by another treehopper. Their research, published in the journal, confirmed that the alfalfa hopper “was able to both acquire the virus from a grapevine infested with GRBaV and transmit the virus to healthy grapevines in the laboratory.”

“In commercial vineyards, lateral shoots of grapevines girdled due to feed injury by the adult three-cornered alfalfa hopper also tested positive for the virus using digital PCR,” the scientists noted in their abstract. “These findings represent an important step in understanding the biology of GRBaV and develop management guidelines.”

The disease, first noticed in 2008 and attributed to a newly identified virus in 2012, is present in many major grape production regions of the United States and Canada. It can reduce fruit quality and ripening.

The research team consisted of Zalom, distinguished professor, UC Davis Department of Entomology and Nematology;  Sudarshana, USDA/ARS research biologist based at the UC Davis Department of Plant Pathology; Brian Bahder, then a postdoctoral researcher in the Zalom lab and now an assistant professor and insect vector ecologist with the University of Florida; and Maya Jayanth, then a student in the Sudarshana lab.

Zalom, a past president of the 7000-member Entomological Society of America, Sudarshana, and UC Cooperative Extension farm advisors Rhonda Smith and Lynn Wunderlich published a National Pest Alert on the Grapevine Red Blotch-associated virus earlier this year.

In the Pest Alert, Zalom noted that “red leaf symptoms that differed from other known red leaf diseases affecting grape foliage” were first noticed in red wine grape cultivars in Napa County, and subsequently in many other California grape growing counties as well.

“Leaf symptoms first appear approximately mid-summer; however, timing of symptom expression differs among grapevine cultivars and year,” Zalom wrote. “In red-fruited cultivars, common symptoms include red blotches originating from the leaf margin or within the leaf blade and primary and secondary veins that often turn red. In white fruit cultivars, symptoms appear as pale green to pale yellow patches.”

Zalom noted that symptoms usually start on basal leaves and progress up the shoot. In some cultivars, such as Chardonnay and Zinfandel, “marginal burning may occur similar to severe potassium deficiency. In some red-fruited cultivars such as Malbec and Mourvèdre, the entire blade may turn red by harvest.” Grapes produced on infected vines are characterizes by reduced brix and other changes that can seriously impact wine quality.

“Foliar symptoms are generally distinct from those of grapevine leafroll disease (GLD) early in the season, but leaf blade coloration may resemble those of GLD by late fall,” Zalom pointed out. “At this time, red blotch disease is not known to kill grapevines.” However, the effect of the virus infections on yield and fruit quality varies and no cure exists at this time.

For more information on the Grapevine Red Blotch-associated virus, access https://www.ncipmc.org/action/alerts/redblotch.pdf.

Related Links:
Three-cornered alfalfa hopper, Spissistilus festinus (Viticultural Information, UC Davis)

Phylogeny of Geminivirus Coat Protein Sequences and Digital PCR Aid in Identifying Spissistilus festinus as a Vector of Grapevine Red Blotch-Associated Virus. (Phytopathology journal)
See here also in PubMed U.S. National Library of Medicine, National Institutes of Health