What California Can Teach South Australia About Pierce’s Disease and Vineyard Biosecurity
27 May, 2026 - Samantha Scarratt
Through an Outbound Knowledge Exchange bursary, supported by Great Wine Capitals and Wine Australia, I recently travelled from Adelaide to San Francisco and Napa Valley to better understand how California manages Pierce’s disease of grapevines (caused by Xylella fastidiosa) and its insect vectors. California has lived with this disease for more than a century. Australia has not. That difference alone makes California an invaluable place to learn.
Pierce’s disease and the glassy‑winged sharpshooter (GWSS), its most damaging vector, remain a major and ongoing problem in Southern California. There, GWSS has established large, mobile populations, spreading the bacterium rapidly across vineyards and landscapes and driving substantial vineyard losses. In contrast, Napa Valley and other northern Californian regions have experienced lower disease impact in recent years. This difference is not because Pierce’s disease has disappeared in Napa, but because vector pressure is low, disease vines are being removed and GWSS has largely been kept out.
In Napa Valley, disease transmission is dominated by native sharpshooters, particularly the blue‑green sharpshooter. While these insects are highly efficient at transmitting the bacterium vine‑to‑vine, they tend to occur at lower densities and are closely tied to riparian vegetation. This ecological constraint means Pierce’s disease is more manageable at a regional scale than in areas where GWSS is present.
Keeping GWSS out of Napa has not happened by accident. It reflects decades of sustained investment in surveillance, quarantine and vector suppression led by the California Department of Food and Agriculture (CDFA), an agency with a role broadly comparable to biosecurity at PIRSA in South Australia. Early in the GWSS incursion, state officials enacted strict quarantine measures, including mandatory insecticide treatment for plants shipped out of Southern California. These controls, combined with ongoing monitoring and biological control programs, have been central to protecting premium northern wine regions.
As a result, while Pierce’s disease remains biologically present and actively researched, it is not currently the dominant day‑to‑day concern in Napa. Instead, vineyard managers are now more focused on diseases causing greater current economic impact, most notably grapevine red blotch virus. Red blotch affects fruit quality and yield, has limited treatment options, and is spread by vectors such as the three‑cornered alfalfa hopper. Vineyards are responding with intensive surveillance using yellow sticky traps, targeted insecticide programs and rapid removal of symptomatic vines. The industry response reflects hard‑earned lessons from decades of managing Pierce’s disease: early detection and decisive action matter.
One of the strongest technical messages from California is that Pierce’s disease is cyclical, not static. Major outbreaks occurred in the 1990s and again in mid 2010s. Periods of lower apparent impact, such as those currently seen in Napa, are shaped by climate, vector populations and management systems, not eradication. Researchers are now particularly focused on climate change, noting that warmer winter minimums and rising night‑time temperatures are reducing vine recovery and allowing the disease to persist and move farther north. Winter temperature currently limits the disease’s range, but climate projections suggest this boundary is shifting.
Another important insight is that not all grapevine varieties respond the same way. Some varieties, such as Chardonnay, recover poorly once infected, while others, including Cabernet Sauvignon and Merlot, tend to tolerate infection better. Research on varietal susceptibility is ongoing, with new findings expected to be published soon. This variability has clear implications for vineyard planning, risk assessment and future breeding efforts in Australia.
California is also investing heavily in long‑term solutions. Five Pierce’s disease‑tolerant grapevine varieties have already been released commercially, and some have been recently imported into Australia, and around 500 advanced selections are currently being tested across different climates and environments as the possible next selection of Pierce’s disease and powdery mildew tolerant grapevines. These are not seen as a silver bullet, and recent findings have shown that some commercially available varieties are still exhibiting Pierce’s disease symptoms and Xylella fastidiosa infection under high disease pressure, such as in Texas. Uptake has therefore been cautious, particularly in premium wine regions. However, large corporate vineyards are increasingly using tolerant grapevines strategically, especially along waterways and in blocks historically affected by Pierce’s disease, as part of broader risk‑management strategies.
Perhaps the most powerful example of integrated management came from Napa’s large‑scale river and floodplain restoration projects. The Napa River restoration project restored more than 14 miles of the Napa River at a cost of around US$16 million. This project reduced sediment entering waterways, expanded riparian zones, improved habitat for native fish, and deliberately managed vegetation that supports Pierce’s disease vectors. Importantly, they delivered wins for growers, the environment and the community. Disease control, flood management and ecological restoration were treated as aligned objectives, not competing ones, a model highly relevant to South Australia’s own catchment‑based governance systems.
Another structural advantage in California is Foundation Plant Services (FPS), a centralised program that cleans, tests and certifies grapevine material before it enters commercial nurseries. FPS provides a level of consistency that is not widely replicated elsewhere, limiting the spread of viruses and bacterial diseases through the propagation chain. Its role highlights the importance of clean plant systems as a foundation of vineyard biosecurity.
A recurring recommendation throughout the visit was the importance of building capability before crisis. One proposal was embedding Australian postdoctoral researchers in Californian Pierce’s disease and vector programs, allowing them to learn directly from more than 100 years of experience and bring that knowledge home.
For South Australia, the message from California is clear. While we remain free of Pierce’s disease and GWSS, preparedness is our greatest advantage. Strong border controls, active surveillance during quiet periods, clean plant pathways, landscape‑scale thinking and investment in people and research will determine whether Pierce’s disease becomes a future crisis—or a well‑managed risk we never have to live with.
Acknowledgement
Vinehealth Australia would like to sincerely thank Great Wine Capitals and Wine Australia for their support through the Outbound Knowledge Exchange Bursary. The insights, relationships and practical learnings gained through this visit could not have been achieved through reading journal articles alone or virtual engagement. The opportunity to meet directly with researchers, practitioners and regulators, and to observe the environment in which they operate, has been invaluable. The focus now is on effectively communicating these findings to industry and translating them into practical actions that strengthen preparedness for Pierce’s disease and other biosecurity threats facing the South Australian and Australian vineyards.
Related links
Xylella fastidiosa and Pierce's Disease – Vinehealth Australia
Grapevine Red Blotch Virus Outbreak – Vinehealth Australia
Samantha Scarratt was supported by the Great Wine Capitals Outbound Knowledge Exchange bursary program, and Wine Australia.
Photos by Samantha Scarratt.