Grapevine red blotch associated virus: A newly identified disease in vineyards
Grapevine red blotch associated virus ("red blotch"), a newly identified viral disease of grapevines, has received considerable media attention in the past few months. Grape varieties affected include not only reds, such as Merlot, Zinfandel, Mourvedre, Petite Syrah, Cabernet Franc, Cabernet sauvignon, Malbec, Pinot Noir and Petit Verdot, but also white varieties such as Chardonnay and Riesling as well as some table grape varieties and some rootstocks. The virus may be present in more varieties but the symptom expression may have more to do with the susceptibility of the particular grapevine variety.
Red blotch has been identified in many states and, in the past year, some vineyards in Ontario have been confirmed to be infected. These vineyards had historic difficulty maturing fruit. Researchers speculate that this disease has been "around" for a long time but has not been identified because the standard tests conducted for diagnoses weren't specific to this type of virus. The only way to reliably test for this disease is through DNA sequencing specific to this virus.
Both leafroll and red blotch cause basal leaves in red varieties to turn red in late August through September. A diagnostic symptom of leafroll is that the veins stay green while the rest of the leaf turns red and roll downward. Red blotch does not cause leaves to roll, the red discolouration is blotchy and irregular and the smaller veins turn red rather than staying green. The symptoms in white varieties are much less distinctive. Infected vines have fruit with low Brix accumulation. In California, reductions of up to 5ºBrix have been reported in infected vines. Since it is relatively new, the impacts on Brix in cooler climates are not known at this time. In a Washington study, shoots on symptomatic vines of Merlot and Cabernet franc were 23 and 18% shorter than healthy vines at veraison and yield was 22 and 37% less, respectively. We also do not have information regarding the effect of red blotch on yield or vine cold hardiness or longevity under our climatic conditions.
The research community is still learning about this disease. We do know that the virus can be transmitted by grafting. California researchers indicate that in some commercial plantings, the virus has shown up over time in young, healthy vineyards next to old infected vineyards. There is some speculation that an insect vector could help spread the disease but there are no definitive answers at this time. Recent research in Washington has demonstrated that Virginia creeper leafhopper (also present in Ontario) is able to transmit red blotch virus from infected to healthy vines in greenhouse studies. Drs. Marc Fuchs and Greg Loeb, from Cornell University, are investigating potential vectors for this disease in eastern vineyards. From what we know about red blotch virus, it is highly unlikely that it can be mechanically transmitted by pruning, hedging, thinning and harvesting.
We don't know at this point what threat red blotch poses to our industry. Since it has likely been here for a long time, identifying it is more likely to be an exercise in diagnosing vineyards with a history of low Brix to get an explanation for this problem rather than eliminating a source of inoculum. There are probably strains of the red blotch virus with different degrees of virulence. That means that even though a vineyard may test positive for the virus, it doesn't mean the fruit yield or quality or vine health will be affected.
Removal of an infected vineyard is an economic decision - if the virus is present and significantly reducing Brix, a grower may decide to pull the block. CFIA's position at this point is not to recommend removal of affected vineyards but rather to prohibit movement of propagation material from the vineyard. The removed vines should be burned on the site of the vineyard rather than being moved elsewhere.
The GGO and OMAF/MRA are conducting a preliminary survey of grapevine red blotch of 20 vineyards in Ontario. If your vineyard has persistent, inexplicable low or declining Brix levels and typical foliar symptoms of red blotch as described, please contact Wendy McFadden-Smith at firstname.lastname@example.org.For more information about red blotch, please go to the following web site: http://ucanr.edu/sites/NCPNGrapes/files/161782.pdf.
Fig 1: Potassium deficiency: Mid-shoot leaves lighten in colour, then turn purple and eventually brown spots appear along the margins of young blades. Leaf margins dry up and roll up or down, and blades become distorted and ruffled.
Fig 2: Grapevine leafroll virus: In late summer to early fall, diseased leaves turn reddish in red varieties or yellowish in white varieties, thicken and become more brittle. Leaves roll downwards starting at the base of the shoots. The leaf blade may be bright yellow or red, but the main veins remain green.
Fig 3: Grapevine red blotch virus: Blotches of pink or red veins on green leaves in the fall, when grape leaves would normally be turning a uniform gold color. Grapes are slow to develop sugar levels sufficient for winemaking, with some grapes never fully maturing. Infected vines tend to be smaller than uninfected ones.
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