While its symptoms were known, the cause of Pierce's disease remained a mystery. About 25 years ago, scientists at UC Davis linked Pierce's disease to an unidentified bacterium . UC researchers also discovered that the disease is spread by sharpshooters, an insect related to the common leafhopper.
Sharpshooters feed by piercing the xylem of plants and sucking the sap.
If the plant already carries X. fastidiosa , a sharpshooter can transmit the bacterium from diseased to healthy plants. Unlike phylloxera which causes a long, slow decline in production, a vine infected by X. fastidiosa and left untreated typically dies within a year or two. During this period, the vine produces no fruit. Pierce's disease has reemerged recently as a vineyard problem in both Northern and Southern California winegrape growing regions. An outbreak of this lethal disease of grapes began in Napa and Sonoma counties in the 1990s and PD has since been found in Mendocino and Lake counties.