The Pierce’s Disease Control Program within the California Department of Food and Agriculture (CDFA) has been slowing the spread of the glassy-winged sharpshooter (GWSS) and minimizing the impact of Pierce’s disease (PD) in California vineyards for over two decades. The program is overseen by the PD/GWSS Board and is made up of industry leaders, including growers and wineries alike.
The core strategies of the program are to protect California winegrapes through the following means:
• Contain the Spread: Prevent the spread of GWSS to non-infested areas with nursery inspections, trapping, treatment, bulk citrus inspections, area-wide treatment programs and biological control.
• Statewide Survey and Detection: Find new GWSS infestations and confirm that non-infested, at-risk areas remain free of infestation.
• Rapid Response: Respond quickly to detections of GWSS in new areas with surveying and treatments.
• Outreach: Raise awareness about PD and its vectors.
• Research: Sponsor research and development for sustainable solutions to PD and its vectors.
By design, the program is subject to a referendum of growers once every five years. This referendum is to hold the program accountable and to make sure growers are satisfied with the work the program is doing on their behalf to protect vineyards. Therefore, the California Association of Winegrape Growers (CAWG) co-sponsored legislation in 2024 to create the referendum in spring 2025.
Last year, when Assemblywoman Dawn Addis authored AB 1861 to extend this crucial line of defense for the wine industry against an invasive disease, she said, “We have a track record of collaboration among State, local, federal government and the industry itself when it comes to battling Pierce’s disease and the glassy-winged sharpshooter. I’m proud to extend this collaboration and to be part of the on-going success of California’s wine regions.”
AB 1861 was approved unanimously by the legislature in 2024 and was signed into law by Governor Newsom. This means CDFA will conduct a vote of growers this spring to determine whether the program and the PD/GWSS Board will be extended from 2026 to 2031. The last PD/GWSS referendum, conducted in 2020, passed with 78% approval of California winegrape growers. CAWG urges a ‘YES’ vote this spring to continue the program.
Continuing to Address PD/GWSS
Growers know all too well one of the most pervasive pests to vineyards is the aggressive PD, carried between plants by GWSS. PD is caused by the bacterium Xylella fastidiosa. Since the 1990s, GWSS has been one of the most invasive and deadly pests for vineyards. When a vine develops PD, its ability to draw in moisture is hindered, and the plant will either die or become unproductive. PD has caused millions of dollars in damage throughout the state.
Xylella fastidiosa works by blocking the xylem in plants, which conducts water around the plant. Symptoms include chlorosis and scorching of leaves. Entire vines will die within one to five years. GWSS, Homalodisca vitripennis (formerly H. coagulata), is a far-flying leafhopper, which means it can easily spread the disease-causing bacterium from one plant to another. When GWSS feeds on a plant infected with Xylella fastidiosa, it acquires the bacterium and transfers it to another plant when it feeds.
Over the last 23 years, the program has been fundamental in addressing the challenges posed by PD and other pests and diseases. In the last two years alone, there was a Notice of Treatment and Proclamation of an Emergency Program in Fresno, Madera, Solano, Stanislaus and Tulare counties to knock down populations of GWSS.
California’s first indication of a severe threat posed by this disease occurred in Temecula in August 1999, when more than 300 acres of vineyards were infected with PD and had to be destroyed. In response, the Legislature enacted a legislative package that year, creating the advisory task force. In 2001, the program was created to fight the spread and find solutions for PD and GWSS.
Program Has Solid Track Record
The program, funded through a combination of federal and industry funds as well as grape grower assessments, has demonstrated success in controlling the spread of PD and GWSS. These assessment funds are used for research, outreach and related activities on PD, GWSS and other designated pests and diseases of winegrapes.
The research overseen by the PD/GWSS Board is critical to advancing knowledge, improving practices, and guaranteeing the longevity of the California winegrape industry. The focus of current research projects ranges from investigating pests and diseases to evaluating existing control methods to exploring new promising control strategies.
Since 2001, the PD/GWSS Board has invested $55 million to support about 300 research grants to protect vineyards, prevent the spread of pests and diseases, and deliver practical and sustainable solutions. Research is focused on PD and GWSS, and other designated pests and diseases of winegrapes, including:
• Brown marmorated stink bug
• European grapevine moth
• Grapevine fanleaf disease
• Grapevine leafroll disease
• Grapevine red blotch disease
• Mealybugs
• Spotted lanternfly (SLF)
The PD/GWSS Board is looking ahead regarding SLF. Native to Asia, SLF started showing up about 10 years ago in eastern states and has caused substantial damage to vineyards in Pennsylvania, New York and the region. Education is crucial to early detection and treatment as the pest continues to make its way west. Investing in research now will also aid in identifying a means of quick treatment and eradication of the pest when it does make its way to California.
For all the aforementioned reasons, in support of growers, CAWG is urging the passage of the referendum in 2025.
