Pierce's disease infection begins as a drying or "scorching" of leaves. The leaves become slightly yellowed along the margins before drying, or the outer edge of a leaf may dry suddenly while still green. The woody portions of diseased canes are generally dry. Some or all of the fruit clusters may shrivel and dry up at any time following fruit set. Diseased vines become increasingly stunted over time and have fewer and shorter canes that produce dwarfed leaves and little usable fruit.
Pierce's disease is usually evident by the scorching and reddening of leaves. Symptoms of several other disorders of grapes can be confused with Pierce's disease. Any disorder that weakens or causes deterioration in vine growth in late summer or fall may lead to delayed or stunted growth the following spring. Eutypa dieback symptoms in spring include stunted shoots and small, chlorotic, distorted leaves, which resemble those of Pierce's disease. Pruning wound cankers, however, are not associated with Pierce's disease as they are with Eutypa dieback. Phylloxera causes a decline of vines; look for phylloxera insects on roots. Oak root fungus can cause wilting, along with discoloration and drying of fruit and foliage, but fungal fans will be evident beneath bark in crown area.
Pierce's disease is a lethal disease of grapevines and is caused by the bacterium Xylella fastidiosa. It is spread by certain kinds of leafhoppers known as sharpshooters. The green sharpshooter, Draeculacephala minerva, and the red-headed sharpshooter, Carneocephala fulgida, are the primary vectors in California's Central Valley. The blue-green sharpshooter, Graphocephala atropunctata, is the most prevalent in coastal regions such as the Napa Valley. A new large sharpshooter, the glassy-winged sharpshooter was introduced into California in the 1990's. It is now prevalent in southern California and parts of the San Joaquin Valley. It spreads Pierce's disease more effectively than the other sharpshooters.
Sharpshooters pick up the bacterium by feeding on symptomless plant hosts as well as infected grapes and transfer it to healthy grapes by feeding. Leaves become scorched and dwarfed, grapes may raisin, and overall vines become stunted and late in growth.
Pierce's disease is spread to vines by sharpshooters from many alternate hosts for the disease including bermudagrass, blackberry, and willow. There is little spread from grape to grape. Control of sharpshooters is not effective, although removal of alternate hosts may help. Grape varieties vary in susceptibility. 'Sylvaner', 'Thompson Seedless', and 'Ruby Cabernet' are less susceptible than other varieties. Remove vines as they become unproductive and replant with less susceptible cultivars.
For more information, see the Pest Notes: Glassy-Winged Sharpshooter.
Wilting of fruit cluster
Early leaf symptoms
Interveinal chlorosis on leaves