Major Maladies of the Vine

Farming is a challenge, to say the least. Not only do farmers have to contend with the vagaries of the increasingly vagarious seasons, they also have to face an evolving roster of pestilences.

Arguably, vineyard managers have it easier than conventional produce farmers. Vines are effectively weeds; they subsist on relatively little and are reasonably hard to kill. Plus, wine grapes are not subject to the same aesthetic standards as other fruits and vegetables. Apples and peaches practically have to place in a swimsuit competition to grace our tables, but wine grapes are sublimated from solid to liquid form prior to consumption, which renders their blemishes inconsequential.

Even still, so-called “winegrowers” have their work cut out for them. New pests and diseases emerge or cross national borders at a rate that even science has a hard time keeping up with, and formerly vanquished foes adapt or pop up in new and unexpected places.

Keeping a vineyard healthy and happy is a war that requires constant vigilance, and there are four main enemies that farmers face: viral diseases, fungal diseases, bacterial diseases, and pests. These malady families contain many different members, each with its own set of symptoms. Their severity is often conditional, based on the physical and climatic qualities of the vineyard, its varietal composition, and the farming philosophy employed by its caretaker. In short, each of the world’s vineyards has a specific set of problems as well as its own customized response protocols.

For this global, if incomplete, survey, I interviewed Pete Richmond and Stan Zervas of Silverado Farming Company, which tends 600 acres of Napa and Sonoma; Matt Stafford of Craggy Range in New Zealand; Jean-Baptiste Lécaillon of Roederer, which farms 600 acres of Champagne (300 of which are biodynamic); Ignacio Casali, viticulturalist for Amayna, which sustainably farms 470 acres in Chile’s Leyda Valley, and Fred Merwarth of Hermann J. Wiemer Vineyard in the Finger Lakes. In addition to farming 135 acres, the Wiemer estate runs a medium-sized nursery that grafts between 350,000 and 400,000 plants a year.

Pests

Cutworms, nematodes, beetles, borers, and spiders. If that doesn’t make your skin crawl, then perhaps mites, flies, moths, thrips, and aphids will. These two lists are but creepy cross-sections of the pests that plague viticulturalists around the world. Of course, the most notorious insect to ever attack grapevines is undoubtedly phylloxera, which came very close to wiping out the world’s wine industry in the late 19th century. Phylloxera, an aphid, proved impossible to eradicate, but vignerons found a solution in grafting vinifera vines onto native American rootstocks, some of which are resistant to phylloxera’s deadly attention.

A similar strategy has been assumed in the fight against nematodes, which, like phylloxera, live below ground and feast on roots. Nematodes are microscopic roundworms, and there are many different species. Some are benevolent, preying on cutworms and other vineyard pests, while others attack the roots, effectively strangling the plant. Some strains don’t quite kill the vine but nibble quietly, spreading viruses that eventually do the job for them. In almost all their guises, nematodes are significant adversaries of the vine. “Nematodes are a very big issue in Oakville, Rutherford, Alexander Valley, and Sonoma Valley,” Pete Richmond explains. “There, our only real option is to use resistant rootstock, such as 039-16. This is a very vigorous stock, so you might assume that the wine quality wouldn’t be that good, but some of the most famous vineyards in Napa Valley are planted on it.”

Vines stunted by root-knot nematodes (Photo credit: Michael V. McKenry; used with permission from the University of California Statewide IPM Program)

The more obvious but less utilized option is to leave the vineyard unplanted for a number of years. With no vines to feed them, nematodes will eventually starve and die. Dominus is famous for leaving the ground bare prior to a replant, but this is one of only a handful of wineries that use this strategy. As fewer vines means less wine, fallow fields are an expensive proposition, especially in pricey Napa Valley. Pete would argue that using resistant rootstock has the same starvation effect, and that after one lifespan of the vine on such material, the bugs will be gone, and vineyard owners can replant with the rootstock of their choosing.

Chile is one of the rare pockets of the world where phylloxera has not penetrated; as such, most of the nation’s vines are planted on their own roots. Nematodes, on the other hand, are increasingly an issue, even in the remote Leyda Valley. Ignacio Casali has been forced to graft his vines to resistant roots where the pest pressure is most severe but finds that in moderately infected areas, the nematode population can be controlled through careful farming. “Nematodes like water, so if you manage the irrigation in the correct way, you can keep their population low. Also, if you create high fertility in your soil, the vines will develop more roots and will live better [and] longer.”

In the not-too-distant past, more extreme measures were commonplace. Pete recalls, “In the old days, you would just drape a tarp across the vineyard and pump in methyl bromide. That would kill the nematodes, but it also killed all the soil fungi, so you essentially had sterile soils. And even then, its effectiveness against nematodes wasn’t complete.” Methyl bromide, a toxic fumigant, was identified in 1987’s Montreal Protocol as a contributing factor to ozone layer depletion and was subject to a slow phaseout that ended as recently as January 2005. “But even if we could still use methyl bromide, we wouldn’t. Our employees today wouldn’t touch the stuff. There’s been a generational shift away from hard chemicals.”

Indeed, in speaking to many grapegrowers, both in this survey and beyond, there seems to be an increasing emphasis on approaching pests and diseases in a more holistic way. That is to say, by working to create healthy and self-balancing habitats, utilizing natural predators, discouraging pest reproduction through the application of pheromones (known colloquially as “sexual confusion”), and removing known vectors (organisms that transmit disease) or their preferred habitats such as certain grasses or plants. Ignacio elaborates, “Today, we work less on trying to heal affected vineyards. Instead, we have a stronger focus on prevention, using better sources of nursery material and better cultural practices in adult vineyards.” Jean Baptiste concurs. “We aim to have a resilient ecosystem where the insects are regulated by natural predators,” he explains. This type of thinking is both environmentally sound and forward-looking, as UC Davis scientist Dr. Andy Walker thinks it’s just a matter of time before certain chemical fixes will be outlawed in the developed world. However, as most fine wine regions are farmed as monocultures, “balance” will always be a relative term and can be easily disrupted, demanding close attention and quick thinking on the part of vineyard managers.

Mites are another nearly universal concern for grapegrowers, and a good example of how more natural approaches can prevail. Mites are the diminutive cousins of spiders, and they feed on the chlorophyll of the plant, which can disrupt photosynthesis. “We try to manage, not eradicate,” explains Stan. “We monitor the populations closely. If by the first week of August, we are seeing 12 mites or less on the back of each of the leaves, I know we can make it through the season without treatment. If we see a flareup, we’ll add a miticide to our fungal spray regimen.” Though organic miticides are available, many producers prefer to use oils, which smother the mites, or release a predatory species. “Red mites used to be a problem,” Jean-Baptiste says, “but we are now using natural enemies like Typhlodroms [another type of mite] to regulate them.”

Damage to Pinot Noir foliage caused by Willamette spider mite (Photo credit: Jack Kelly Clark; used with permission from the University of California Statewide IPM Program)

Bat and bird boxes are another effective method of battling pests. When Spring Mountain Vineyards in Napa Valley suffered a particularly nasty bout of Pierce’s disease, they installed over 1,000 bluebird boxes in the hopes that the birds could consume the sharpshooters responsible, which did help alleviate the outbreak. But microclimates being what they are, not all “cures” are universal, and one man’s predator can be another man’s pest. Birds are a good example. Matt Stafford claims that birds are the number one nuisance he faces on the North Island of New Zealand. “For Craggy Range’s Gimblett Gravels vineyard alone, we have 2,236 individual nets, a number of gas bangers positioned around the vineyard, and two full-time bird scare-ers who race around the vineyard from sunrise to sunset shooing away flocks.” They give their Martinborough vineyard the same treatment, a necessary but expensive measure. “The nets, the application and removal costs, and staff costs are significant,” he admits.

In the Finger Lakes, Fred Merwarth contends mostly with flying insects. The grape berry moth is a long-standing foe. This troublesome pest lays its eggs inside the grapes, where its offspring hatch, consume the pulp, and fly away to infect more berries. If the moths attack early in the season, the affected berries typically shrivel and drop off, resulting in a measurable, but marginal, crop loss. More severe damage comes later in the growing season, when the grapes are juicier. Then, affected berries don’t shrivel but drip, and the result is sour rot that can spread to the entire cluster. In warmer years, grape berry moths can go through as many as five different generational cycles, or “flights,” which can add up to monumental damage. At Wiemer, where they try to avoid the use of pesticides, they are having a difficult time fighting the moth. “What we’d like to do is get to where Europe is with the moth, and employ pheromones to disrupt mating, but that’s not available in the US yet.” In the meantime, Fred is experimenting with plant-based organic insecticides like neem oil, but isn’t yet convinced of their efficacy. “I suspect the chemical companies aren’t producing this pheromone because they are making too much money selling the other treatments,” he postulates, “but even if these pheromones were available, they would only be successful if every vineyard used them. A handful of vineyards in isolation won’t be able to affect reproduction in a significant way.”

Last year, the Finger Lakes added a new name to its enemy list: the spotted wing Drosophila. “Drosophila” is just another name for fruit fly—a ubiquitous companion to grapegrowing and wine production that is irritating but typically harmless. The rare spotted wing version, however, is considerably more nefarious. Most fruit flies are drawn to damaged grapes, but this fly does the damage itself, piercing the skin and initiating rot. Worst of all, it prefers to feed on the cooler, shadier backside of the cluster, so the problem tends to remain invisible until severe. This fly originated in Southeast Asia and has only recently made landfall in Europe and the United States. Fred’s current strategy is to pull leaves, which eliminates the fly’s shady safe haven. “I have colleagues in Germany who have had good results with this tactic.” 

The arrival of a brand-new species is understandably alarming, but Pete and Stan take the long view. They’ve been farming for decades (32 and 25 years, respectively) and have successfully battled enough bugs to remain optimistic. “There’s always a new imbalance in the universe,” says Pete, who farms a third of his vineyards organically and employs sustainable practices whenever possible. “Fifteen years ago, we had a big blow-up of white flies because the parasite that feeds on them was out of whack. It was bad for three years, and then things normalized. Before that, it was mealybugs. Most recently, the European grapevine moth, which we successfully eradicated by working together as a community.”

Stan interjects, “One of my first jobs out of school was working in citrus in Ventura County, right when the glassy-winged sharpshooter first showed up. Everyone was saying it was going to be the end of the industry, so I got nervous and started looking around for another job. The old timers said to me, ‘Relax. Every 10 years, there’s a new pest that spells the end of the industry, and after a few ugly years, we always figure it out.’” He breaks from his reverie. “Believe me,” he states plainly, “vine pests and diseases aren’t going to kill California viticulture. What’s going to end us is water and urban sprawl.”

Fungal

Viticultural textbooks are thick with references to fungal diseases, and almost all end with a single word: rot. This is no poetic flourish on the part of agricultural scientists; most fungal infections cause decay. Decay of the fruit, decay of the leaves, decay of the wood—fungi are the necromancers of the viticultural sphere.

Two of the most pernicious fungi are oidium (or powdery mildew) and peronospora (downy mildew), which spread from America to Europe and the world during the mid-to-late 1800s. These two mildews are almost opposite in their manifestation. Downy mildew is the more traditional, propagating in cool, moist conditions. Powdery mildew, on the other hand, prefers it dry and somewhat hot, germinating most rapidly between 70 and 85 degrees Fahrenheit. This makes it particularly troublesome in California and other places with Mediterranean climates where rains are scarce during the growing season. The fight against fungi, which also includes black rot and botrytis, can be quite time-consuming. “Mildew control takes up the most tractor hours,” explains Fred. “It’s not the most labor intensive, because one person can sit on a tractor and do it. But because mildew can wipe out your crop pretty quickly, treating mildews is probably the most important aspect of viticulture.” 

Powdery mildew (Photo credit: Jack Kelly Clark; used with permission from the University of California Statewide IPM Program)

Powdery mildew will not kill the vine, but it can have a dramatically negative effect on vine health and fruit quality. As it cannot be destroyed, it must be managed, and getting a jump on the season is essential. If powdery mildew, or really any mildew, is allowed to infect a vine during flowering or fruit set, it can be impossible to remove from the cluster. Late season infections are generally less problematic but can be disastrous if not properly contained. “Late infections can affect how your buds are set for the following year, which can give you problems with fruit set and budbreak. Plus, you might see ‘dieback’ on the end of your canes, where the wood turns black and traces down the trellis to the base of the plants.” This is the point at which the issue becomes serious. Because of this, most mildews are treated multiple times (six is standard, though more is required for organic viticulture) during the growing season, whether or not a vineyard is symptomatic.

Historically, the most common treatment against powdery mildew was sulfur dust, as sulfur particles kill germinating spores, but its use is waning in certain parts of the world. “We don’t use much sulfur dust anymore,” Pete explains, “both because we tend to spray near homes and for the health of our employees. If you’ve ever walked through a vineyard right after it’s been dusted and you have even a little bit of asthma, it can really knock you out.” As an alternative, Pete’s company uses alternating application of Stylet Oil and wetable sulfur early in the season and then drops out the sulfur at bloom. They cut the sulfur at that time because of the perceived risk of H2S in the finished wine. “Emphasis on perceived,” Stan adds. “Our younger winemakers worry about that the most. The established [wineries] have no problem with us using sulfur all year long.” In addition to spraying, they also strip leaves and open the canopies where mildew pressure is the highest. This allows for better ventilation and more effective drying in the fruit zone.

In high-pressure regions, fighting mildew can be the biggest hurdle to organic viticulture. The Hermann J. Wiemer winery aspires to convert entirely to organic farming, but Fred is not sure that is possible in the Finger Lakes, at least not in every vineyard. “Right now—and I hate this—our fungicide program is organic late in the season but only sustainable early in the season,” he explains. Fred is forced to juggle multiple farming philosophies to ensure that there is actually fruit on the vine come harvest. “Though there are new essential oil-based fungicides and pesticides on the market, there isn’t always a lot of research besides trial and error. And that’s what we did last year—trial and error.” For the 2017 growing season, Fred really pushed organic farming on the three main Wiemer vineyards. The Magdalena and Josef Vineyards experienced significant crop loss, largely due to mildew, but the more isolated HJW Vineyard near the winery performed better. “The problem was partly that we have neighbors that are on different spray cycles. That said, it was amazing that we were able to run fully organic and biodynamic on the 33-acre HJW Vineyard.” 

In New Zealand, Matt Stafford recently learned that insects aren’t the only organisms that can evolve—fungi can mutate, too. “One hundred and forty-four years after powdery mildew was first identified in New Zealand, the sexual stage was identified in 2014.” This new form of powdery mildew is more aggressive and can overwinter in the vine’s bark, as opposed to inside the dormant buds. The La Niña weather patterns of recent years have exacerbated the situation. As the combination of high humidity (with low rainfall) and high heat is ideal for powdery mildew propagation, growers have had to increase the frequency of their sprays.

Downy mildew is less of a burden in California, but it is rampant on the US East Coast and in the cooler, wetter corners of the world. Its effects are devastating. Within a relatively short span of time, an unchecked downy mildew infection can destroy all the living tissue of a vine, starting with the leaves. At Roederer, it is the disease the viticultural team spends the most time fighting. The typical treatment is known as “Bordeaux mixture,” a combination of copper sulfate and calcium oxide. As Roederer farms biodynamically, they are restricted in both what they can use and the quantities they can apply to the vineyard. While they rely on a copper-based, natural “biocontrol” spray, they also focus on improving overall vine health as a mechanism for resistance. “We fight it by having balanced and energetic vines with proper fertilization and using biodynamic preparations.”

Botrytis is the third most internationally significant form of rot, but unlike the first two, it has a bright side. When it infects the proper grape, such as Riesling, at the proper time (i.e., late in the season), it can have a complicating and concentrating effect on the finished wine. Because of this, it enjoys the nickname “noble rot,” an oxymoronic moniker that underscores its dual nature.

Of course, not all botrytis is benevolent. If it infects even a Riesling cluster too soon in the year, it can lead to sour rot. And its presence on red grapes is almost always a disaster. In Chile, it is the primary disease Ignacio faces every year. As with Roederer’s approach to downy mildew, Ignacio’s strategy is twofold, focusing both within the vine and without. “We work to improve the ventilation of the cluster area, achieve the correct distribution of the fruit, and utilize efficient irrigation. We also use phytosanitary applications with copper, and curatives with citrus or tea extracts.”

The Hawke’s Bay region is also prone to botrytis, as harvest rains are a regular occurrence. As Craggy Range’s Gimblett Gravels vineyard is predominately dedicated to red grapes, the major concern is slipskin, wherein the botrytis infection literally causes the grape skins to slough off, leaving only the pulp of the berry on the vine. “Our Syrah and Bordeaux bunches are open hanging, but since our soils have almost no water holding capacity, the vines can rapidly draw in water when we have rain events near harvest,” Matt explains. “This can create some small cracking at the peduncle of the berry. From here, we see the grapes start to slip and are constantly walking through the vines kicking and shaking them to see the first signs of slipskin.”

Trunk diseases are numerous and are often caused by fungi. Pete Richmond considers these the single greatest threat to viticulture. “Wood rot diseases are probably the most severe that the world is facing right now. If it weren’t for trunk diseases, vineyards could live forever.” The most commonly seen are Eutypa and Botryosphaeria (also known as “dead arm”), though Yesca (Esca) gives Ignacio a good amount of trouble in the Leyda Valley. Though each disease has its own particular pathology, they all generally work the same way, killing the vine by cutting off the sap flow, which causes the wood to slowly die.

Darkened cankers developed in vascular tissue due to Botryosphaeria (Photo credit: William J. Moller; used with permission from the University of California Statewide IPM Program)

As with so many of the major vine ailments, wood fungi can’t be killed, so avoidance is the main objective. The primary points of entry are pruning wounds, and as fungal spores tend to propagate in the rain, many vineyard managers minimize risk through dry pruning, widely considered a best practice—though it is not always possible in places like Oregon or the Finger Lakes. Another tactic is to reduce the size of the cuts, which can be achieved by switching from cordon to cane pruning, for example. Painting the wounds immediately after they are cut is also common. These “paints” range in composition; many of the traditional fungicidal paints are quite toxic to the environment, so more natural-leaning farmers will often create their own paints using clay, oils, whey, and/or compost. While fungicides serve to kill the fungus, these mixtures aim to encourage the development of competitive fungi. Other growers simply use latex paint to form a physical barrier between the wound and the open air. But the presence of trunk disease does not always mean certain death for a vine. If symptoms are spotted, the affected areas can be cut away and the vine retrained, which helps prolong its lifespan.

Different varieties have different susceptibilities. Zinfandel and Carignan are notoriously resistant and can easily survive 100 years or more. New Zealand and its Sauvignon Blanc, however, might be in big trouble. As Matt explains, “Eutypa and Botryosphaeria dieback are significant issues for New Zealand viticulture given our short history and condensed vineyard planting period (most vines being 10 to 30 years old).” As Sauvignon Blanc is so vigorous, many cuts are needed to tame it, which is an open invitation for trunk disease. “As symptoms take 3 to 8 years to be identified, we are seeing high incidence in many Sauvignon Blanc vineyards aged 15 years and above.” He continues, “With the demand for Sauvignon Blanc so high, many people are replacing individual vines, which creates issues for block consistency. Ultimately, producers will need to work through the staged replanting of vineyards, but it will take some shift in mindset over the next 10 or so years.”

Viral

Viral disease is perhaps the least understood class of malady to plague farmers around the world. Part of the confusion stems from the fact that viruses rarely work alone; an affected vine typically houses several viruses, sometimes even multiple strains of the same type. And, as very few viruses actually kill vines, there hasn’t been the same pressure to research them. As Pete admits, “The interesting thing about viruses is that there [are] a lot of them sitting out there that we haven’t identified yet.” Recent advances, however, have shed some new light. 

For example, the hot topic of the day is red blotch, only discovered within the last decade. For generations, viticulturalists confused it for leafroll virus, which presents similar symptoms. “We had blocks that we were sure had leafroll,” Pete explains, “but they kept testing negative for it. Around eight years ago, people figured out it was a whole different virus: red blotch.” Scientists at UC Davis then tested some dried grape leaves they had in their archives and realized that red blotch has been present in California since at least the 1940s.

Leafroll and red blotch share certain characteristics but also diverge in significant ways. With leafroll, the grape leaves turn red and curl in on themselves. Yield is unaffected, but the ripening will be slightly curtailed. With red blotch, the leaves don’t curl, but they do turn red. Red blotch also limits ripening, but unlike leafroll, it can affect the flavor of the fruit, and therefore the resulting wine. Recently, several research efforts have been initiated to try to determine the taste threshold of red blotch-affected wines, which some experts put at around 5%. Still, not everyone is unilaterally against these two viruses. Many lovers of Napa Valley Cabernet from the 1960s and 70s credit the era’s relatively low alcohol levels to the rampant presence of virus, and very few of these wines taste “off.” In fact, the presence of virus in regions such as California and Barolo today is seen by some as a weapon against overripeness in an era of global warming. Some producers even consider viruses a part of their terroir, or at least the specific signature of their vineyards, while others find the idea of virus in their vineyards abhorrent and go to great lengths to eradicate it.

Damage due to leafroll, pictured at left, versus red blotch, right (Photo credit: Stan Zervas)

Viruses are spread either by pests or through the use of unclean nursery material. Red blotch is thought to be spread via the alfalfa hopper, and viticulturalists are now studying that bug in earnest, trying to determine its preferred foods and vegetative nesting areas. Fanleaf, which affects fruit set, is spread by nematodes, and leafroll is spread via the soft scale mealybug. According to Fred, “There are differences between the East Coast and the West as to how leafroll is transferred. In the east, it can only be spread via vine cuttings. It cannot jump from vine to vine, as we do not have that mealybug out here.”

One way of combatting virus is to attack the vector. But this is not always a simple proposition. In 2008, Craggy Range began to see a massive uptick in the number of leafroll-affected vines at Gimblett Gravels. To fight this, they invested considerable time and money to contain the mealybug population. “The natural predator [of the mealybug] is a parasitic wasp which is attracted to environments with flowering plants. Unfortunately, Gimblett Gravels has very poor soils, so establishing inter-row or under-vine flowering plants is a challenge.” As they did not want to turn to powerful insecticides, they sought other, more natural methods, such as pheromone traps and planting red clover in the more fertile sites. The hope there was that the bugs would turn to the clover as a food source instead of the vines.

In Napa and Sonoma, Pete and Stan will often plant Sauvignon Blanc in areas with a high repeated incidence of red blotch. This is a winning strategy for them in that the disease doesn’t seem to affect white varieties as much as red ones, both in terms of ripeness and flavor. Craggy Range, on the other hand, is not content to have virus in their white grapes. They noticed that leafroll tended to spread downwind, as the wind would affect the trajectory of the mealybugs. When they noticed high levels of infection downwind from their Chardonnay blocks, they tested for the virus, even though no physical symptoms had manifested. They found that a remarkable number of plants tested positive and removed 70% of the block after the 2011 vintage.

Removing vines sounds like a fairly straightforward solution, but as with everything else in viticulture, the reality is more complicated. Per Stan, “The question is, if I have a red vine among the green, how many adjacent vines do I have to remove in order to stop the spread? We recently had a vineyard where there were green vines in the fall that tested positive for red blotch, and didn’t turn red until year three.” This problem is compounded by the fact that the vineyards they tend are spread across two counties, and are often surrounded by virus-infected neighbors. As Pete bemoans, “We have vineyards in Oakville and Rutherford that I’m trying to keep clean, but they are surrounded by red, and insects don’t respect property boundaries.” 

“In the vineyard setting, every grower has to make their own decision as to whether a virus is making enough of an economic or qualitative impact to justify the expense of pulling and replanting. It’s very personal,” Fred admits. “But in the nursery, we are more draconian.” As the single most effective way to fight the spread of virus is to use clean nursery material, Fred has an especially important role to play. In Wiemer’s nursery blocks, they immediately rip out any symptomatic vines, as well as the surrounding vines. How many surrounding vines they remove depends on the virulence of the infection as well as the spacing of the vineyard. Beyond that, they test, test, and test again.

“We are working with New York State and Cornell on a certification program,” Fred explains. “Once a nursery block is declared clean, we do 25% testing every year. That means that within a four-year span of time, every vine will have been tested once.” The problem with these tests, however, is that they don’t test for viral families but rather each specific strain. “There are around 14 different strains of leafroll and a half a dozen strains of red blotch that we know of, and they all vary in terms of virulence. The most problematic in terms of viticulture are leafrolls 1 through 4.” In addition, a virus might not show up on every part of the plant, as it doesn’t always spread evenly through the vine. Fred’s current strategy is to sample six leaves taken from different parts of the same vine, focusing on older tissue because younger material might not yet show the infection. “We only understand a part of the spectrum, how each strain affects the plant. We still have a long way to go.” 

Bacterial

Of all the major disease families, the bacterial branch seems to be the sparest, boasting fewer members than its viral and fungal counterparts. That said, what it lacks in numbers it makes up for in impact; bacterial infections are often the deadliest maladies that a grapevine can suffer.

One of the more commonly seen bacterial infections is crown gall. Crown gall occurs around the world, but Fred claims it is particularly problematic on the East Coast of the United States. Crown gall affects not only grapevines, but virtually any plant with a trunk or stalk, a list that includes many bushes and trees. The “gall” is a discolored tumor that forms when Agrobacterium tumefaciens, a nearly universal inhabitant of soils, enters the woody part of the plant. The point of entry is typically an injury. In viticulture, this is often caused by mechanical weed removers, tractor contact, cold damage, or even improperly healed graft unions. The only known treatment for the formation of crown galls is vine removal. 

Crown gall at the base of a vine (Photo credit: William J. Moller; used with permission from the University of California Statewide IPM Program)

Flavescence dorée is perhaps the most terrifying bacterium at large in the wine world today. Though it has not yet arrived in Champagne, its proximity keeps Jean-Baptiste up at night. This is an especially contagious disease whose primary symptoms are yellowed leaves and withered fruit; infections often lead to total crop loss. So far, the disease has been mostly contained to the south of Europe, but its presence in parts of France has prompted government intervention. According to a statement released by the INRA in 2013 and updated in 2017, flavescence dorée “cannot be cured: it can only be treated by uprooting infected vines, management of grapevine nurseries, and insecticide control of the vector.” The vector for this particular affliction is the Scaphoideus titanus, or American grapevine leafhopper. As flavescence dorée is incurable, the current emphasis is on containment, and over half of France’s vineyards are now legally mandated to perform “compulsory control methods,” which include spraying with a non-organic pesticide. The backlash against this has been intense, especially among more natural-leaning producers. The most high-profile example of this resistance was the court case against the biodynamic Burgundy producer Emmanuel Giboulot, who refused to break with his preferred farming practices and spray the pesticide. A French court found Giboulot guilty and threatened him with jail time, though in the end he only had to pay a significant fine.

Pierce’s disease has an equally deadly grip on the US West Coast and southern states. According to Dr. Walker, Pierce’s disease is one of the great historical vine diseases of California. “Back in the 1840s and 50s, there were huge numbers of vineyards in the Los Angeles basin. A massive disease outbreak wiped them out in the 1850s and 60s, [and] Newton B. Pierce diagnosed the problem.” The area never really recovered, and since that time, Pierce’s disease has tortured California grapegrowers with episodic outbreaks that are impossible to predict, though Pete suspects warm winters are to blame. The disease is spread to vines via sharpshooter insects, primarily the glassy-winged sharpshooter in the south, and the blue-green sharpshooter in the north. These small insects tend to occupy riparian corridors but can travel small distances on their own and be carried a bit further by the wind.

Blue-green sharpshooter, pictured at left, and glassy-winged sharpshooter, right (Photo credit: Jack Kelly Clark; used with permission from the University of California Statewide IPM Program)

The disease works by cutting off the vascular system of the plant, which prevents it from taking up water. Clusters desiccate and the stems and shoots dehydrate and become mottled in appearance. In certain cases, vines can live with a small infection for some time, but in areas of intense pressure, an affected vine usually dies within a few years. Fortunately, Pierce’s is not a widespread phenomenon, but a chronic and intense local problem. That said, it affects an estimated 4 to 5% of North Coast vineyards, which represents a significant number of vines, and almost all of the less widely planted South Coast is either currently infected or at risk.

Fighting Pierce’s disease is a considerable challenge. Pete and Stan have seen the best results come from creek and river restoration. Explains Pete, “What the Rutherford Dust Society did was huge. They completely restored the river and creek beds of the region, getting rid of the blackberries and periwinkle that harbor the sharpshooter. The results were incredible.” But beyond that, the best answer has been simply to abandon the hotspots or turn to aggressive pesticides.

Dr. Walker believes there is another way. He has been working for the last 20 years to breed Pierce’s disease-resistant (PD-resistant) grape varieties through natural crossings. “Around 60 years ago, cutting of Vitis arizonica were brought up from Mexico that exhibited total resistance to Pierce’s disease.” He and his colleagues have spent countless hours crossing this vine to a range of vinifera varieties, with the goal of creating resistant varieties that also produce quality wine. They are approaching the realization of their goal, with the development of five entirely new varieties that they just released to nurseries. These grapes—three red and two white—range from 94 to 97% vinifera, and their wines have been extensively taste-tested by a variety of winemakers. The varieties, which were loosely modeled on Zinfandel, Cabernet Sauvignon, Petite Sirah, Chardonnay, and Sauvignon Blanc, currently do not have names, only proprietary codes.

The goal, according to Dr. Walker, was not to create standalone varieties, but rather resistant grapes that could be planted in hot spots and used in blends. That said, he can’t help but wonder if a broader commercial application is possible. “Winemakers that taste and like the wines always ask, how will I market these? How will we convince the public to drink them?” He suspects that hope, if it exists, lies with the new generation of wine drinkers. “It will be interesting to see if people will outright reject non-varietally labeled wines, or if the broader, more accepting Millennial view of wine and wine styles will win out.”

“The fascinating thing about viticulture is how backwards it is,” Dr. Walker muses, “at least in terms of using improved plant material. In other types of horticulture, there is an acceptance of new crops all the time. Sure, we talk about heirloom varieties, but there is an influx of new peaches, apples, and table grapes every year.” The active creation of new grape types has a long history that was only recently abandoned. “Consider Müller-Thurgau, Petite Sirah, Zweigelt—and not to mention the interest in hybrids following the phylloxera epidemic. Somehow, in the mid-1900s, that industry completely stopped. As a wine drinking society, we decided that a small group of varieties were the most important and ceased creating new ones.” 

Dr. Walker’s newly minted varieties may be controversial, but they are surely a more traditional option than creating resistance through gene editing. And they provide a natural form of resistance that precludes the use of pesticides and herbicides, which is also hard to argue. But whether or not they gain commercial traction, they are but one tool in the fight against vine disease. The more important weapon, in Dr. Walker’s opinion, is community. “Vine diseases are easy to solve if you work as a group, but that’s hard to do.” Whether attempting to manage a pest or eradicate a virus, the efforts of an individual grower, such as Pete Richmond or Fred Merwarth, can be easily undone if those around them are following a different protocol. “This is part of the reason we still have so much virus,” concludes Dr. Walker, “because people are tackling these things vineyard by vineyard, rather than on a regional level.”

Ask Dr. Walker
Q: Regarding your new hybrid varieties, how can a cross that is predominately red create a white grape, and vice versa?

A: There are two forms of each grape gene. In the case of berries, a red berry form and a white berry form. Most grape genes are heterozygous, which means they have two different forms of each gene. A red grape will usually be RW for berry color. Since the R form is dominant to the W form, the berries are red. A red grape could also be RR. A white grape will always be WW, because W is the recessive form of the gene. If you cross a red grape (RW) with another red grape (RW) the possible offspring are 25% RR and red; 50% RW and red; and 25% WW and white.

Because white berry color is recessive (WW), it is often linked to other recessive gene forms that might reduce the viability of offspring, so in fact, you often see fewer than 25% white berries. 

If the red-berried grape you used as a parent in the cross was RR instead of RW, all the progeny would be red. And if you crossed a RW berry with a WW parent,­­ 50% would be RW and red and 50% would be WW and white. This is berry color in its simplest case. 

Q: What about pink-skinned and gray-skinned grapes?

A: They are the interesting parts to the story and are often the result of mutations in the anthocyanin pathway. The most interesting are transposons—bits of DNA that replicate themselves and pop in and out of the genome, turning color genes on and off. Both good examples of why the story is always much more complicated!

Q: What determines the strength of the hue? Is a black grape RR and a lighter red grape RW?

A: Simplistically yes, but there are more genes involved, too, and mutations/changes throughout the anthocyanin pathway.

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