In 2008, Cabernet Sauvignon grapevines on rootstock 101-14 in research plots at the University of California-Davis’ Department of Enology & Viticulture, Oakville Station, exhibited symptoms resembling leafroll disease. These grapevines produced clusters with reduced sugar content, causing delayed harvests. Among the grapevine pests and diseases, only leafroll diseases exhibit similar canopy symptoms and most importantly cause reduced sugar accumulation in the berries. Occasionally, the color development was also poor in some clusters and the clusters also exhibited increased acidity as well.
Laboratory tests conducted at private and public grapevine testing services centers failed to detect any of the leafroll and rugose wood viruses in these samples. Commercial vineyards planted with Cabernet Franc and Cabernet Sauvignon in Napa County also exhibited similar disease symptoms as at the Oakville Station. Finally, an ARS research biologist, M.R. Sudarshana, found the actual virus, now called “Red Blotch” for obvious reasons, in 2011. “We didn’t know anything about it before,” he said recently. “Nobody knew of its existence.”
Red blotch has now been found in many of the state’s major winegrape growing regions. In addition to Napa, Sudarshana says it has been found in Sonoma, San Luis Obispo, and Fresno counties. It has been observed in vineyards planted with red grape varieties such as Cabernet Franc, Cabernet Sauvignon, Merlot, Petite Sirah, Petit Verdot, and Zinfandel. However, Sudarshana found red blotch late in 2012 in some obscure white varieties at the Oakville Station but said they didn’t get a chance to do many tests.
If your grapevines, red or white, are producing fruit with Brix values lower than expected and are not showing classic leafroll-like symptoms, Sudarshana recommends that you contact your local viticulture farm advisor.
The symptoms generally start appearing in late August through September as irregular blotches on leaf blades on basal portions of shoots. The secondary and tertiary veins turn partly or fully red. Occasionally, the reddening of leaf blade in the interveinal zones between secondary veins resembles those of leafroll diseases, but the leaf margins do not roll downward. Also, in leafroll-affected red varieties, the secondary and tertiary veins remain green. It is not known if the disease has any effect on fruit yield, or if it can cause vine decline. The most significant impact of the disease appears to be on the Brix units of the berries. Brix of grapes in vines showing red blotch symptoms has been found to be four to five units lower than those with green canopies and this difference is higher than those normally seen in leafroll-affected grapevines.
The disease symptoms do not appear to be caused by nutritional deficiencies, nor by stress, nor by bacteria, fungi, and/or nematodes. A unique DNA virus was found associated with diseased grapevines from vineyards planted with Cabernet Franc, Cabernet Sauvignon, and Zinfandel, and this new virus has been proposed to be called as “Grapevine red blotch-associated virus” (GRBaV). In PCR assays, this new virus has been found consistently associated with symptomatic grapevines. A virus originally called “Grapevine Cabernet Franc-associated virus,” similar to GRBaV, has been found in vineyards in New York and Pennsylvania by researchers at Cornell University. Also, a genetically identical virus has been found in grapevines in Canada.
Grapevine red blotch does not appear to be of recent origin. This disease escaped attention of vineyard owners and managers because of leafroll-like symptoms. This also means that diagnosis based on the leaf symptoms can be challenging. A molecular assay, DNA-based PCR, is currently available and the virus can be detected in the petioles of basal leaves, much before the onset of symptoms, and also in dormant canes. Several private laboratories have started providing diagnosis services to detect the virus by molecular assays.