Symptoms and Signs
Symptoms of Pierce's disease vary depending when a vine became infected. Chronically diseased vines were infected the previous growing season (or in years prior) and symptoms are more severe as compared to vines infected in the current spring. The following four symptoms in mid- to late summer indicate the presence of Pierce's disease in chronically diseased grapevines: (1) leaves become slightly yellow or red along margins in white and red varieties, respectively, and eventually leaf margins dry or die in concentric zones; (2) fruit clusters shrivel or raisin; (3) dried leaves fall leaving the petiole (leaf stem) attached to the cane ("matchsticks"); and (4) wood on new canes matures irregularly, producing patches of green, surrounded by mature brown bark ("green islands"). Not all four symptoms are required to be present in vines infected with the pathogen.
In vines that are infected in spring, usually only one or two shoots will show Pierce's disease symptoms late in the first season of infection, and these may be difficult to notice in varieties other than Pinot noir Barbara or Chardonnay which are very susceptible to this disease. Symptoms gradually spread along the cane from the point of infection toward the shoot tip and more slowly towards the base. By mid-season some or all fruit clusters on the infected cane of susceptible varieties may wilt and dry. Tips of canes may die back; roots may also die back.
Leaf symptoms vary among grape varieties. Pinot noir and Cabernet Sauvignon have highly regular zones of progressive marginal discoloration and drying on blades. In Chardonnay, Thompson Seedless, Sylvaner, and Chenin blanc, the discoloration and scorching may occur in sectors of the leaf rather than along the margins. Vines of susceptible varieties deteriorate rapidly after appearance of symptoms, especially in young vines where symptoms may appear over the entire vines in a single year.
Shoot growth of infected plants becomes progressively weaker as symptoms become more pronounced. In chronically diseased vines, the woody portions of the vine are usually dry and portions of the vine (arms or a cordon) may be dead. The last part of the vine to die is often the crown near the soil line; rootstock or scion suckers at the base of the vine may be present for a year or two prior to vine death.
Climatic differences between regions can affect the timing and severity of symptoms, but not the type of symptoms. Hot climates accelerate symptom development because vine water stress is more severe even with adequate soil moisture.
A year after the vines are infected some canes or spurs may fail to bud out, and shoot growth is stunted. This may occur in vines that did not have obvious symptoms the preceding year. New leaves become chlorotic (yellow) between leaf veins, and scorching appears on older leaves. From late April through summer infected vines may grow at a normal rate, but the total new growth is less than that of healthy vines. In late summer leaf burning symptoms reappear.
Comments on the Disease
Xyella fastidiosa is a bacterium that lives in the water-conducting system (the xylem) of host plants and is spread from plant to plant by sap-feeding insects that feed on xylem fluid. Symptoms appear when significant blockage occurs within xylem vessels due to the growth of the bacteria. (This bacterium is also responsible for alfalfa dwarf disease and almond leaf scorch in California.) Insect vectors for Pierce's disease belong to the sharpshooter (Cicadellidae) and spittlebug (Cercopidae) families. The blue-green sharpshooter (Graphocephala atropunctata) is the most important vector in coastal areas. The green sharpshooter (Draeculacephala minerva) and the red-headed sharpshooter (Carneocephala fulgida) are also present in coastal areas but are more important as vectors of this disease in the Central Valley. Other sucking insects, such as grape leafhoppers, are not vectors.
The glassy-winged sharpshooter is a vector of Pierce's Disease that became established in Southern California in the early 1990's and is a serious threat to California vineyards because compared to other vectors it flies greater distances and occurs on a wider range of host plants in various habitats. Unless monitoring for glassy-winged sharpshooter and preventative measures are maintained, the insect is expected to spread north and eventually become established in more regions.
The principal breeding habitat for the blue-green sharpshooter is riparian (riverbank) vegetation, although ornamental landscape plants may also harbor breeding populations. As the season progresses, these insects shift their feeding preference, always preferring to feed on plants with succulent growth. In the Central Valley, irrigated pastures, hay fields, or grasses on ditch backs are the principal breeding and feeding habitats for the green and red-headed sharpshooters. These two grass-feeding sharpshooters also occur along ditches, streams, or roadsides where grasses and sedges provide suitable breeding habitat.
Glassy-winged sharpshooter feeds and reproduces on several genera of trees, woody ornamentals, and annuals in its region of origin, the southeastern United States. Crepe myrtle and sumac are especially preferred. It reproduces on Eucalyptus, coast live oaks, and a wide range of trees in southern California.
Some vines recover from Pierce's disease; the probability of recovery depends on the date of infection and temperatures in the winter following infection. Vines that become infected in June or later by blue-green, green and red-headed sharpshooter vectors have the greatest probability of recovery. Therefore preventing spring infections (bud break through May) is critical to prevent systemic infections that cause chronic disease. Recovery is promoted by low winter temperatures; mild winters result in fewer vine recoveries.
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 over a year (i.e., bacteria survive the first winter and symptoms occur the following spring) 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.
Management
Insecticide treatments aimed at controlling the vector in areas adjacent to the vineyard have reduced the incidence of Pierce's disease by reducing the numbers of sharpshooters immigrating into the vineyards in early spring. The degree of control, however, is not effective for very susceptible varieties such as Chardonnay and Pinot Noir or for vines less than 3 years old. If a vineyard is near an area with a history of Pierce's disease, plant varieties that are less susceptible to this disease. Monitor and treat for insect vectors as described in the section on SHARPSHOOTERS.
During the dormant season, remove vines that have had Pierce's symptoms for more than one year; they are unlikely to recover or produce a significant crop. Also, remove vines with extensive foliar symptoms on most canes and with tip dieback of canes even if it is the first year that symptoms have been evident. From summer through harvest, mark slightly symptomatic vines; reexamine for symptoms the following spring through late summer or fall and remove vines that have symptoms for a second year. Research has shown that severe pruning – cutting a few inches above the graft union – and training up a new trunk is not a viable management practice; Pierce's disease symptoms reappear the second year.
Late season (after May) and winter feeding by the glassy-winged sharpshooter results in infections that can survive the winter to cause chronic Pierce's disease. This enables vine-to-vine spread of Pierce's disease, which has previously not been the case in California. Removing diseased vines as soon as possible when Pierce's disease first appears in a vineyard when glassy-winged sharpshooter is the vector is critical to help reduce the infection rate. Insecticide treatments of adjacent breeding habitats, such as citrus groves, have been the most effective approach in Southern California.
Riparian vegetation management has proven to be effective in reducing the damaging spring populations of blue-green sharpshooters in the North Coast; however approval of this strategy must comply with federal, state, and local regulations. The unauthorized removal of vegetation is prohibited or restricted due to concerns for water quality and habitat for anadromous fish (fish born in fresh water, spends most of its life in the sea and return to fresh water to spawn). Contact the California Department of Fish and Wildlife for more information.