PIERCE'S DISEASE is a vine infection by the bacterium Xyella fastidiosa. It is introduced by an insect, the glassy-winged sharpshooter, that feeds on vine sap by puncturing vine stems and shoots. The bacteria multiply inside the plant and eventually block the flow of water and nutrients, debilitating the vine's metabolic functions, reducing the crop and killing the vine in the worst cases within three to five years of infection. The insects move from vine to vine, spreading the disease throughout vineyards and regions and causing millions of dollars in damage.
The disease is native to the Southeast of North America and thrives especially in warm, humid areas and along streams. Native Muscadinia grapes are resistant, but no vinifera types have been found that are; Chardonnay and Pinot Noir are particularly susceptible.
A researcher first discovered it in 1892 and called it Anaheim disease; it also affects almonds and alfalfa and the name was later changed, reflecting the name of its discoverer, Pierce. Since there has been no cure, it is one of the principal reasons that cuttings of grapevines and other plants are quarantined in moving between countries.
Entire appellations have been ruined in the destructive path of the glassy-winged sharpshooter. Temecula in southern California was hit hard in the 1990s and the GWS has spread northward to other regions, including the Central Valley and North Coast.
In early 2003, Ximed Group, a chemical company based in the United Kingdom, claimed to have found a cure for the disease by applying terpene, a naturally-occurring botanical substance that apparently has no side effects. A test vineyard in South Carolina reported the disease completely eradicated in 86% of the treated vines.
A team from UC Davis announced in June, 2008, that it had identified a resistant native American species (Vitis Arizonica) and successfully crossbred with several generations of V. vinifera that resulted in vines with both resistance and good wine production characteristics. Currently undergoing field testing at several sites, once proven to be effective, genetic implants will replace cross-breeding to rapidly propogate additional varieties.