In the late 1990s, Temecula Valley was riding a wave of strong sales and even stronger optimism. But a persistent and then unknown disease would bring the region, along with most of California’s winegrowing industry, to a halt.
J.D. Harkey, the director of operations at Drake Enterprises, recalls the scene: “These vines looked like they were torched—stunted growth, yellowing of the leaves, burned edges of the leaves—and nobody had any idea what it was. Large blocks of Barbera, Chardonnay, and Riesling were just dying.”
Over time, university researchers, alongside the USDA, identified the cause: Xylella fastidiosa, a bacterium that causes Pierce’s disease carried, in this case, by the glassy-winged sharpshooter.
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In the years that followed, the disease would reshape Southern California vineyards, costing growers more than $48 million a year. More significantly, it would offer an ongoing lesson in how to fight a pathogen that seemed untouchable.
Understanding Pierce’s Disease
Xylella fastidiosa is a bacterium that attacks many plant species and can cause different diseases depending on the host. In grapevines, certain strains can cause Pierce’s disease by blocking their water transport, causing them to die within a matter of years. Vines appear torched or burned; it’s because the plants can no longer absorb water, causing them to die of dehydration.
This particular disease, discovered by Newton B. Pierce, first surfaced near Anaheim, California, in 1892. Cold climates can suppress the bacterium, but hot climates only accelerate the damage. This temperature sensitivity explains why the disease has remained confined to warmer wine regions for over a century, and why a warming climate means new regions are now facing threats from Pierce’s disease.
Cases of Pierce’s disease in continental Europe are still considered low, particularly because of the average lower temperatures, but Pierce’s disease is actively being contained in Portugal, as well as on the Balearic island of Mallorca in Spain. With an estimated potential economic damage of €5.5 billion, Europe is keeping a watchful eye on the disease, and California winegrowers and researchers are sharing what they’ve learned to help Europe avoid the same mistakes.
The Current Landscape of Pierce’s Disease in Europe
Xylella fastidiosa was first detected in Europe on olive trees in Italy in 2013, and later, Pierce’s disease was found in grapevines on the island of Mallorca in 2016. But it wasn’t detected on grapevines in continental Europe until 2023, when a single infected grapevine was detected in Portugal. While swiftly managed, that case raised questions over the possibility of spreading to other parts of mainland Europe.
The European Union has since conducted annual surveys to monitor its spread. Since 2017, the European Food Safety Authority has hosted a nearly annual conference on the latest Xylella fastidiosa findings and to discuss recent scientific developments. Today, cases of Xylella fastidiosa persist in regulated zones across Spain (including Mallorca), Italy, Portugal, and France, affecting a range of host plants such as almond trees, olive trees, and grapevines.
Warming temperatures are considered a major driver of the increasing risk of Pierce’s disease in Europe, but findings from a 2025 study using high-resolution climate data offer new insight. Vineyard microclimates near river basins and valleys are especially vulnerable, as they connect areas that “would otherwise remain isolated,” according to the study’s authors. Historically, winters were too cold for the pathogen to survive, but with more regions exceeding the minimum threshold, these topographically complex areas may act as Pierce’s disease hotspots. Researchers estimate that 41.2 percent of European vineyards are at climate risk of Pierce’s disease, up from 21.8 percent in previous lower-resolution models.
Olakunle Olawole, Ph.D., an assistant professor in the Department of Microbiology and Plant Pathology at University of California, Riverside has watched the disease play out in Europe. “Portugal initially detected Xylella fastidiosa in 2019, but more recent studies have identified the same subspecies responsible for Pierce’s disease in California,” says Olawole. “California’s experience offers valuable lessons for Europe in surveillance, early detection, integrated vector management, and sustained investment in research, even though management strategies must ultimately be tailored to local conditions.”
While Pierce’s disease hasn’t tipped Europe into endemic territory quite yet, it’s a far cry from being harmless, especially as climate change reshapes the very conditions the bacterium thrives on. “In many parts of Europe, expansion is likely to be silent at first,” says Eduardo Moralejo, Ph.D., a leading plant pathologist and Xylella fastidiosa researcher at Institute for Cross-Disciplinary Physics and Complex Systems in Mallorca. “The bacterium may spread without immediately causing large epidemics, because current climatic conditions are not yet highly conducive to severe disease. That creates a false sense of security. By the time symptoms become obvious, the pathogen may already be established.
Early Reactions to Pierce’s Disease in Europe
Cati Ribot, a winemaker in Santa Margalida, Mallorca, is avoiding a fear-based approach. “Xylella fastidiosa is just another plant disease,” she says. “We need to learn to live with it without becoming obsessed. Our focus is on maintaining a healthy, balanced vineyard, while recognizing that every year, we will inevitably lose some vines—not only because of Xylella fastidiosa, but also due to other factors.”
This balancing act of staying cautious while learning what life with Pierce’s disease looks like across Europe. “Producers are scared about the possibility of a case of Xylella in our vineyards,” says Michele Brusaferri, an agronomist at Etna winery Tasca d’Almerita. “We know that the only solution is to destroy vineyards, and that, especially for us, represents a [threat to] great historical patrimony. Early measures can avoid big losses, so early detection is fundamental.”
But Olawole is confident that options for fighting Pierce’s disease extend beyond eradication alone. “Removing infected plants remains an important strategy when outbreaks are detected early, and eradication is still feasible,” he says. “However, once Xylella becomes widely established, management shifts from eradication to slowing spread and minimizing disease impact. This is one of the key lessons from California’s experience with Pierce’s disease and from European outbreaks affecting olive, almond, and now grapevine.”
Olawole points to the many tools available—including genomic surveillance and rapid diagnostics—that help detect the bacterium quicker.
What Europe Can Learn from California’s Devastation
California is in a different phase than Europe in the fight against Pierce’s disease, which is no more evident than in their philosophies toward fighting it. With no mandate to report an infection, and no requirement to destroy infected vines, California growers are largely held accountable for how they manage the disease. Decades of battling the bug have taught California growers the value of interdepartment collaboration; European countries are still ironing out the best ways to communicate at all.
Brusaferri points to Puglia as a prime example of this. “With the exception of Puglia, there have not been big communication strategies from regional bodies to growers,” he says.
A lack of communication strategy also introduces the possibility of concealment. If the only option is to pull up vines, and if there’s no official notice or regulations in place, growers may be tempted to hide an infection in the hopes of preserving what they can.
California’s willingness to self-report was driven partly by culture, but also by a tangible reason to act early. When the pesticide Admire was introduced to the market, it helped allay some of the fears surrounding Pierce’s disease. “All of our customers, all of our growers, all of our vineyards are getting an application in late April to early May, when the sharpshooters start coming over and looking for a host plant to feed on. We have to time it properly so that when they show up, they taste it and they leave,” says Harkey.
But as many European wineries continue to lean more toward environmentally friendly and organic viticulture, it’s an uphill battle to find a long-term solution to prevent winemakers from pulling up their vines. Many EU winemakers are left with fewer options, still relying on prevention as their main line of defense.
The Path Forward Against Pierce’s Disease
Resistant grape varieties are one promising step forward. At the Ojai Vineyard in Ojai, California, owner and winemaker Adam Tolmach’s vines—all Vitis vinifera like Sauvignon Blanc and Syrah, which were planted in 1982—were thriving until a wave of sharpshooters invaded in 1985. Ten years later, all of the vines were removed, and with the high risk of repeat infections, replanting was futile. When M Andrew Walker, Ph.D., a professor in the Department of Viticulture and Enology at UC Davis developed a new generation of hybrid grapes crossed between Vitis vinifera and Vitis arizonica, Tolmach knew he wanted to trial them. He planted Ambulo Blanc, Caminante Blanc, and Paseante Noir across 1.2 acres.
“The trade-off was the new varieties are unknown both viticulturally and oenologically, and of course they are unknown to the public,” Tolmach says. “I was happy to plant them because I knew they would be resistant to Pierce’s disease, and thus I could grow them successfully.”
By selling his hybrid wines directly to consumers, Tolmach has confirmation that his decision has paid off. “I have been very happy with the quality, and since we are selling them exclusively to our direct customers and are confident of their quality, they have been easy to sell,” he says. The hybrids also come with an added benefit: in addition to their resistance to Pierce’s disease, the grapes are also moderately fungus resistant.
Olawole sees Tolmach’s experiment as part of a broader shift already underway. “I believe we’re entering an exciting period where these technologies will increasingly move from the laboratory into practical tools that help growers protect vineyards and orchards,” he says. “Ultimately, California’s experience highlights the importance of investing in research and preparedness before outbreaks become widespread.”
For Harkey, who lives with the impact of Temecula’s worst years, the battle against Pierce’s disease taught a worthwhile lesson. “We were the guinea pigs,” he says. “In the early 2000s, we thought this might be the end of the wine industry in Temecula, but we’ve been able to control it. It can be done. And today, we have flourishing vineyards now in Temecula, even with sharpshooters present.”
Dispatch
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Lauren Johnson-Wünscher is a wine and food writer based in Berlin, Germany. She holds an MBA in International Wine Business and WSET Level 3 Certification.
