Update on Red Blotch Disease in Grapevines
Cornell professor details the virus for Napa wine grape growers
Napa, Calif.—There was standing room only as growers and winemakers skipped the wine reception at the Nov. 13 Napa Wine + Grape Expo to hear the latest intelligence on red blotch virus from Dr. Marc Fuchs of the Department of Plant Pathology at Cornell University in Geneva, New York.
Fuchs preceded his talk by acknowledging that viticulturists don’t have all the answers yet; many questions remain about the newly identified virus, its transmission methods and ways to overcome it. Fuchs presented a similar presentation during an event hosted by the University of California, Davis' Foundation Plant Services. A video and slide show of his presentation can be found here.
Majority of growers have seen it
A show of hands of growers and vintners who see signs of red blotch in their vineyards was asked for. Most in the room raised their hands, a sign of the malady’s spread in Napa Valley and elsewhere. Fuchs said he’d get the same response among New York grapegrowers.
The symptoms of red blotch disease – more formally, grapevine red blotch associated virus (GRBaV) – were first described in 2008, and transmission by grafts was confirmed in 2012. The virus was also recognized that year, and an assay was developed to detect and confirm it. The first potential vector was identified this year. That short time line contrasts with 80 years to take those steps with leafroll virus and 120 years with fanleaf virus.
The symptoms of the virus are red blotches and spots on leaves, though a small percent (2.5%) of infected vines don’t show any symptoms likely because of the latency of disease onset. The red ranges from pink to crimson. (Editor’s note: further description of GRBaV, numerous photos by Dr. Fuchs and others, and a short list of labs that can test for the virus can be viewed here.) Other viruses like leafroll also color leaves, but GRBaV displays a different pattern of red than leafroll does.
“Red blotch can dramatically affect quality,” said Fuchs. Red blotch virus delays ripening. It can lead to 5 or 6 lower Brix in mature grape berries. It also affects the anthocyanins and can lead to reduced color in red grapes and their wine.
Fuchs mentioned the challenge of confirming that the phenomenon met the criteria for a disease, a problem partly because viruses won’t grow in petri dishes, but only in their host. Nevertheless, researchers have found that GRBaV causes red blotch.
‘Test, don’t guess’
Unfortunately, he notes, your eyes aren’t enough to identify the virus. Polymerase chain reaction (PCR) assays are available to identify the virus. Tests can identify presence of the virus in any tissue of the vine – leaf, petiole, dormant canes or clusters at any time of year. Because of the difficulty of confusing symptoms of the virus with other viruses, he said, “Test, don’t guess.”
He said that research suggests that there may be two variants of the virus. Red blotch virus can be found in both red and white wine grapes, in table grapes, raisin grapes and rootstocks. So far, it’s been detected in wine-growing regions in California, Oregon, Washington, BritishColumbia, Virginia, Pennsylvania, Maryland, New Jersey, New York and Ontario.
Red blotch virus is spread by propagation and grafting, and anecdotal evidence suggests it may spread by other means. The Virginia creeper leafhopper (Erythroneura ziczac) seems to be able to transmit red blotch virus from vine to vine in the greenhouse, but it’s not yet proven that the leafhopper is a vector in the field.Fuchs says that at this point, the best way to manage the disease is to select planting material carefully.
Micro-shoot tip culture, used to eliminate other viruses from vines, works with GRBaV but only in 24% of the time, so it is inefficient. The same technique works more than 80% of the time for fanleaf and leafroll viruses, Fuchs added. You can remove the individual infected vines (“roguing”) or replace the whole vineyard. He suggests replacement if more than 25% of vines are infected.
At this point, he doesn’t recommend using an insecticide as there’s no proof that any insect is a vector. One proposed remedy doesn’t work. “There’s no need to sterilize equipment. Viruses can’t be transmitted by mechanical means in vineyards like pruning equipment,” he said, though that can help with bacteria, insects and other pests.