Science

Red Blotch Rising

A new virus that started spreading across California vineyards—and the nation—in the 2000s is under investigation

red blotch afflicting leaves
Photo credit: Whiteway / iStock.

They thought it was leaf roll disease.

Since at least the early 2000s, a virus had been creeping through California vineyards. Certain vines started to stand out, their leaves a flame in a sea of green. Their fruit was late to ripen, berry sizes were small, and the sugars from those plants were muted.

The disease may have looked a lot like leaf roll, but growers and winemakers were in for a surprise.

In 2008, when symptoms resembling those of leaf roll began appearing in an eight-year-old Cabernet Sauvignon vineyard at the experimental research station of the Department of Viticulture and Enology at the University of California, Oakville, researchers decided to test the plants—suspecting that they would find a new strain of the virus. Yet no leaf roll virus was present. By 2011, the researchers had drilled down on the DNA and discovered a whole new virus, which they called red blotch.

Meanwhile, across the country in a Cabernet Franc vineyard in New York, researchers were DNA-testing a vine that also had symptoms like those of leaf roll—and they found nothing related to leaf roll. What they did find in the DNA, they named “grapevine cabernet franc–associated virus.” Then, in the spring of 2011, researchers at Cornell sent the DNA to virologists at UC Davis who discovered a match: Red blotch had arrived in New York too. In the fall of 2012, researchers from New York and California presented their findings jointly to the International Council for the Study of Viruses and Virus-like Diseases of the Grapevine; together, the researchers decided to scrap the New York moniker and officially name the disease grapevine red blotch–associated virus—GRBaV to scientists, red blotch to the rest of us.

Red Leaf, Pale Grapes

All the while, in every infected vine, yields were falling, sugars were tanking, and grape colors were paling.

Leaf roll and red blotch may be two distinct diseases, but their effect on grapes and wine is somewhat similar. In grapevines, leaf roll is typically recognized by its trademark reddening of leaves along with the cupping or curling downward of the leaves’ edges. In red-fruited vines, while leaves turn red, their veins usually stay green, starting from the root; white cultivars typically exhibit leaf rolling and a yellowish color by late in the season. Diseased vines tend to decline over time, and berry clusters lack vibrant color, are late to mature, and have lower-than-normal sugar levels. In a place like Washington State, this can be very worrisome as it increases the risk of damage from frost and rain. In California, with its higher heat and prolonged growing season, leaf roll is less of a concern, says Melissa Hansen, the research program manager for Washington State Wine.

“In California,” she says, “they have leaf roll and they don’t even worry about it. They can hang their fruit a bit longer and get it riper. [With] red blotch, they’re pulling their vineyards out.”

While red blotch looks remarkably similar to leaf roll, it’s not accompanied by the telltale leaf curling. Diagnosing red blotch usually begins with recognizing the symptoms—red spots or blotches that occur on a leaf early in the season. The discoloration then spreads until, by late season, most of the leaf is red; the leaf may also drop early. But the only way to be certain the problem is red blotch is to test a sample in the lab.

Like leaf roll, red blotch causes a reduction in sugars, delays fruit from ripening, and affects berry color. It differs from leaf roll in that it also results in smaller berry size, lowers the pH, raises titratable acidity, and reduces yield. Because there’s no cure for red blotch, the best medicine for an infected plant is to remove the entire vine.

In general, white varieties with red blotch give a slightly lower yield, but otherwise the impact of the virus is relatively small, says Anita Oberholster, extension specialist in enology at UC Davis. For reds, however there’s an impact on sugar accumulation and color development that results in a wine with slightly less color, an altered aroma profile, and a slightly altered mouthfeel. In her experience, when diseased and undiseased vines were harvested at the same time, grapes affected with red blotch would typically come in three Brix lower.

In a recent study, the juice from one diseased Cabernet Sauvignon vine was 2.65 Bx lower than a healthy counterpart, and the juice from a diseased Chardonnay vine was 1.0 Bx lower, as reported by Mysore R. Sudarshana, Keith L. Perry, and Marc F. Fuchs in their 2015 paper “Grapevine Red Blotch-Associated Virus, an Emerging Threat to the Grapevine Industry,” published in the journal Phytopathology.

Some of these effects can be mitigated, says Oberholster, by letting fruit on red blotch vines hang, which isn’t always practical. For those working with diseased vines that resulted from propagation—say, an entire block was planted with infected source material—that might not be too difficult. But for those dealing with random interspersion of diseased plants in the vineyard, it can be harder, especially when machine harvesting.

But can consumers really taste the difference? Actually, yes, says Oberholster. Recently she cofermented healthy fruit and fruit from vines with red blotch that had been harvested simultaneously and created wines with 5, 15, and 25 percent red blotch fruit included, then subjected submitted those wines to a panel. All the tasters knew was that they were tasting Cabernet Sauvignon—not that some of the wines had red blotch fruit incorporated. In the samples that were more than 15 percent diseased, says Oberholster, “you could see a clear impact on the resulting ethanol content of the wines and color impact.”

Hang Time

Typically, red blotch causes delayed ripening—an additional one to three weeks on average, says Oberholster, although she’s seen that vary from site to site and by season. It also means reduced sugar levels. Where Oberholster is located, near Sacramento, such delays can often be overcome by letting the fruit hang longer.

Kevin Sass, the winemaker for Halter Ranch, says that in Paso Robles, though, adding hang time may not solve the problem. Rather, what he sees in Paso fruit that has red blotch and is left to hang is a tendency toward dehydration, not added sugar, which makes him wonder if that’s how Paso Robles earned a reputation for Zin-like Cabs. Now that Halter and others in the area have begun weeding out the red blotch problem, he says that those Cabernets don’t taste so much like Zinfandel anymore.

Here Today, More Coming Tomorrow?

Today red blotch seems to have spread extensively in California and is making a hard push into southern Oregon. In their 2015 paper, Sudarshana, Perry, and Fuchs acknowledge that surveys of vineyards indicate the presence of red blotch in Maryland, New York, New Jersey, Pennsylvania, Virginia, Arkansas, Idaho, and Texas too. Its appearance and dissemination is a study in contrasts. It was only in 2016—eight years after the disease was first recognized—that researchers began to understand how red blotch spreads. The virus isn’t transmitted by being in close proximity to another diseased plant, nor is it blown through the air. Rather, it’s spread by a vector—an organism, typically an insect, that transmits a disease or parasite from one plant to another. Red blotch’s vector comes in the form of a little green bug, a hopper, that looks like it belongs in anime.

Last February, UC Davis researchers Brian Bahder and Frank Zalom announced that the three-cornered alfalfa hopper was capable of spreading red blotch. With a combination of piercing mouthparts and sap-sucking instincts, plus a proclivity for legume ground-cover crops, the little green caped insect made the perfect host for transmission.

In the lab, Bahder let the hoppers feed on infected vines and then moved them, insect by insect, to individual healthy plants—each quarantined in a virus-free cage—where they fed anew. Four months later, red blotch virus could be detected in the once healthy vines.

Yet the three-cornered alfalfa hopper doesn’t live in every affected state, and it isn’t even abundant in all California AVAs. So how is it that vines in places like Washington and New York have been infected with red blotch? While there’s a possibility that another vector exists, the hard truth is more likely that it’s the result of dirty source material.

Red blotch can be disseminated into a vineyard or region simply by planting infected vines or by grafting infected rootstock. And while it typically isn’t transmitted from plant to plant in the same vineyard, if roots graft underground, there’s the possibility for transmission.

Despite growers’ best efforts to plant clean and use certified rootstock, until recently, most certified clean rootstock wasn’t even tested for the disease, which is probably how, outside the three-cornered alfalfa hopper’s home territory, red blotch ended up in vineyards from New York to British Columbia.

“We brought it up from California into Washington, and now it’s sort of invading Oregon, southern Oregon, and moving up into Willamette Valley,” says Washington grower Dick Boushey. “They have an active vector in this hopper bug in California that is really spreading it. We don’t have [the bug], so the only way we can spread it is if we take wood off the infected vines.”

“I suspect red blotch to have arrived in New York in the mid 2000s, likely via the planting material,” says Marc Fuchs, associate professor of Plant Pathology and Plant Microbe-Biology at Cornell University, and among those responsible for first identifying the disease. Fuchs isolated red blotch in 2011, but growers had begun calling his attention to issue back in ’07 or ’08.

While the three-cornered hopper is present in New York, it’s largely found in alfalfa fields here, and doesn’t over winter—which means the state gets a reset of sorts every year.

Propagation is also how Kevin Sass, of Halter Ranch, thinks red blotch arrived in Paso Robles, where he says the hopper isn’t found in abundance. The hopper is, however, in the San Joaquin Valley, home to a handful of nurseries that supply most of California’s grapevines.

“I believe they couldn’t test for [red blotch] at that point or they didn’t even know what it was,” says Sass, “and basically they were just propagating dirty wood in the San Joaquin Valley and sending it to all of us. [From] 2000 to 2010 there was a huge boom in planting [new vines] in California, and especially in the Paso Robles area, and I think we got infected that way. It’s unfortunate; it’s nobody’s fault.”

When in Doubt, Rip It Out

Fuchs encourages winemakers to take extra care with their planting material, and to replace infected material as needed. His economic model holds that when a vineyard or block is more than 30 percent infected, it’s better to rip it out. With less than 30 percent red blotch, he recommends that growers rogue—replace individual infected vines with healthy ones. The good news is that red blotch doesn’t stay in the soil, so roguing is effective.

Since discovering the virus in Halter’s vineyards several years back, Sass and Halter’s vineyard manager, Lucas Pope, have rooted out most of the vineyards’ infected vines. Their threshold for replacing blocks was lower than Fuchs’s recommendation]—if 20 percent was infected, it got ripped out. Today, of Halter’s 281 acres under vine, 100 acres have been redeveloped, with another 30 to 40 acres to go.

“I don’t want anyone to think we’ve got major problems,” says Sass. “But the wine quality from six, seven years ago is a night-and-day difference. And we know this from a phenolic standpoint.”

Sass also recognizes that all the testing and replacing may not be economically feasible for many smaller vineyards. Testing for red blotch costs $70 per sample. At roughly 900 plants per acre, it would cost about $12,000 to test every vine in an acre. Instead, he recommends that vintners look for diseased vines in the fall and test them along with healthy-looking vines in the same block—since they most likely came from the same source material—and then make a decision. If you have a mix of healthy and unhealthy, or if it appears that less than 20 percent of the vineyard is infected, go rogue. If vintners do replant, Sass urges them to be sure they get clean source material.

“Just make sure you get clean wood, for your kids and grandkids. Because you might be okay right now, but let me tell you that our red blotch–infected vines that are now 16 years old are yielding us less than one ton per acre in Cabernet,” says Sass. “And that’s not feasible for anyone.”

When she’s not writing about beverage, travel, or weird science, Julie H. Case can be found deep in America’s forests, foraging for mushrooms.

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