Symbiosis Control

The number of people and organizations currently working on Pierce’s disease is high, too high to list. Instead, the following background is provided as introduction and further contacts can be found in the links below. 

PD Background

Pierce’s disease (PD) is a scorch disease of grapevine caused by one specific strain of the bacterium, Xylella fastidiosa. The pathogen is carried by xylem-feeding leafhopper insects, sharpshooters, attached to the cibarial cavity of the foregut and is transmitted physically. Newly molted young stages of sharpshooters are pathogen-free and are only infective upon re-acquiring the pathogen in subsequent feeding from infected plant xylem fluids. Once acquired, the insects can infect subsequent plants without delay, which reinforces the notion that transmission is physical.

The PD pathogen started causing disease in grapevines circa 1843, which suggests it was brought in around that time. Inability to grow grapevines in Orange County lead to the disease being named “Orange County disease” until the true pathogen was identified years later.

Native leafhoppers in California are efficient at transmitting the pathogen causing PD but do not preferentially visit vineyards, preferring instead riparian habitats at water courses. Therefore, PD has been an occasional problem in California (with the single exception of Orange County in southern California). This all changed with the arrival of the glassy-winged sharpshooter (GWSS), Homalodisca coagulata, probably sometime in the 1980’s. GWSS has a much wider host range than native sharpshooter leafhoppers, and although it is less efficient as a vector than native blue-green sharpshooters, the sheer force of numbers is enough to overwhelm and infect vineyards in the infested area.

Moreover, GWSS is now present in such large numbers in southern California that various other strains of Xylella fastidiosa are showing up as pathogens. This has lead to the appearance of oleander leaf scorch, almond leaf scorch, phoney peach disease and olive leaf scorch, plus scorch diseases in crepe myrtle, jacaranda and liquid amber trees.

PD first appeared in Temecula, CA, which is about 30 miles south of Riverside in a regional wine and vineyard area. By the late 1990’s, hundreds of acres of grapevines, especially Chardonnays, were lost. Local, state, and federal elements generated a large amount of funding to address the immediate threat and find a lasting solution. State and federal agencies mounted successful area-wide insecticide treatments and biological control releases that have suppressed GWSS numbers. These measures offer temporary relief at a high cost met by emergency funds, which likely cannot be sustained in the long term. A new technique that offers a more permanent solution is badly needed, and symbiotic control is one of these.

Links for More Information

PD has been a problem in Florida and Texas for a very long time. The PD problem and crisis in California is new and has generated a number of web-based resources. These can be accessed by typing GWSS or Pierce’s disease into a search engine or by visiting the following list of sites:

• Alexander Purcell Xylella web page, introduction to PD. http://www.cnr.berkeley.edu/xylella/
• Alexander Purcell web page, introduction to PD. http://www.cnr.berkeley.edu/xylella/page2.html
• State of California, Pierce’s disease control program. http://www.cdfa.ca.gov/phpps/pdcp/
• University of California, New and Information Outreach (online media kit), GWSS and PD. http://news.ucanr.org/mediakits/gwsskit/gwsskit.shtml
• The Wine Institute, San Francisco, CA, PD Update revised 2002. http://www.wineinstitute.org/communications/pierces_disease/pierces_disease_update.htm
• American Association of Wineries, Position Paper, GWSS/PD Program. http://www.americanwineries.org/publicpolicy/piercesdisease.htm
• Jim Kamas, Texas Cooperative Extension. http://winegrapes.tamu.edu/grow/diseases/piercesdisease.shtml
• University of California, Report of the Pierce’s disease research and emergency response task force. http://news.ucanr.org/speeches/executivesummary.html
• Dan Bryant, Western Farm Press, 24 Oct 2003, GWSS-Pierce’s disease project progresses. http://westernfarmpress.com/news/farming_gwsspierces_disease_project/
• Kern and Tulare County GWSS Update Newsletter. http://www.kernag.com/kpp/newsletter.htm