Symptoms of Pierce's disease vary depending on variety and when vines were infected. Symptoms of those infected the previous growing season or earlier are more severe than vines infected in the current spring.

Advanced late-summer or fall symptoms of Pierce's disease on foliage of a white grape variety can include concentric rings of drying from the outer edge toward the center. Leaf margins of a red grape variety, on the other hand, turn red and then brown. Other symptoms:

• Fruit clusters shrivel or raisin
• Petioles of leaves remain attached to canes after leaf fall.
• Wood on new canes matures irregularly, producing patches of green, surrounded by mature brown bark

Not all of these symptoms are required to be present in vines infected with the pathogen.

Some vines recover from Pierce's disease, depending on the date of infection and temperatures in the winter following infection. Low winter temperatures favor recovery, while mild winters reduce it.

Recovery rates also depend on grape cultivar; recovery is higher in Chenin Blanc, Sylvaner, Ruby Cabernet, and White Riesling, compared to Barbera, Chardonnay, Mission, Fiesta and Pinot Noir. Thompson Seedless, Cabernet Sauvignon, Gray Riesling, Merlot, Napa Gamay, Petite Sirah and Sauvignon Blanc are intermediate in their susceptibility to this disease and in their probability of recovery.

In tolerant cultivars, the bacteria spread more slowly within the plant than in more susceptible cultivars. Once the vine has been infected for more than a year recovery is much less likely. In susceptible varieties, recovery is unlikely if disease symptoms are apparent in the growing season they became infected.

Young vines are more susceptible than mature vines. Rootstock species and hybrids vary greatly in susceptibility. Many rootstock species are resistant to Pierce's disease, but the rootstock does not confer resistance to susceptible Vinifera varieties grafted on to it.

UC IPM recommendations also include checking glassy-winged sharpshooter traps weekly and monitoring traps for glassy-winged sharpshooters throughout the season until daytime temperature remains below 65°F.. Growers should record the results. Traps should be replaced every 2 weeks or when they become excessively dirty or discolored and especially on edges adjoining other alternate glassy-winged sharpshooter hosts such as citrus.

More information on identifying and managing Pierce’s disease and the glassy-winged sharpshooter are available at www.ipm.ucdavis.edu.