Pierce's Disease

PIERCE'S DISEASE
Description
Symptoms
Disease Cycle
Control

Description:
Pierce's disease is a potentially devastating disease of grapevine in geographic areas where winters are mild. The disease is considered to be the principal limiting factor in the production of V. vinifera/French American hybrids and V. labrusca (American bunch grapes) in the Southeast. Most varieties of muscadine grapes are relatively resistant, although some (e.g. Carlos) are susceptible.

Symptoms:
By midsummer, margins of leaves infected by the Pierce’s disease bacterium develop a scorched appearance (Fig.1). Leaves may become yellow before scorching (Fig.2), while red varieties may show some red discoloration (Fig.3). The scorched leaf blades may eventually drop, leaving petioles attached to the cane (Fig.4). The bark on canes matures irregularly, leaving patches of green tissue (green islands) surrounded by mature brown tissue (Fig.5). Fruit clusters may ripen prematurely and become shriveled or "raisined" (Fig.1). These symptoms are more extreme in hot dry weather, and some varieties are more susceptible than others. Severely infected vines may die within 1 year of infection or, in the case of chronically infected vines, may live for 5 years or more. In these vines bud break is delayed and new shoot growth is stunted (Fig.6). Margins on leaves of affected muscadine vines also appear scorched (Fig.7). Although symptoms may be widespread on muscadine vines (Fig.8), they usually recover from the disease. Occasionally the tips of some shoots on heavily infected vines die back in the warmer growing regions of the Southeast.

Disease Cycle:
Pierce's disease is caused by Xylella fastidiosa, a gram-negative bacterium that survives and multiplies within the water conductive system (xylem) of its plant hosts. X. fastidiosa has a diverse natural host range with over 100 herbaceous and woody plant species. Many of these plant species are thought to be symptomless hosts, yet may serve as reservoirs of inoculum for later insect transmission. Many of the reservoir hosts in California (www.cnr.berkeley.edu/xylella/temp/hosts.htm) are indigenous to NC and the Southeast. Fifty of the plant host species reported in California occur in the Southeast, and different species in 43 additional genera reported also occur here. Sharpshooter leafhoppers (Cicadellidae) and spittle-bugs (Cercopidae) acquire and transmit the bacterium as they feed in the xylem of plants. In California, four sharpshooter species have been shown to transmit the bacterium: blue-green sharpshooter, green sharpshooter, red-headed sharpshooter, and the glassy-winged sharpshooter. The current epidemic in California is likely related to the introduction of the glassy-winged sharpshooter, which has a wider host range and can fly farther than other species of sharpshooters present. The major disease transmitting insects of X. fastidiosa have not yet been identified in North Carolina, but are currently under investigation.

Control:
Pierce's disease management in North Carolina should involve the use of multiple disease management practices:
-Site selection. The Pierce’s disease bacterium does not survive cold winter temperatures in grapevines. Consequently the risk of the disease is least in the mountainous regions of the Southeast and the risk increases from the Mountain region to the Piedmont to the Coastal Plain. Based on surveys for Pierce’s disease and historical data on the occurrence of minimum temperatures, a map has been constructed which identifies low, moderate and high risk areas in NC (Map 1). These areas are to serve as a guide for selecting potential vineyard sites. As more data are gathered the map may be modified. Vines in specific sites within moderate and high risk areas may escape Pierce’s disease because of unique ecological habitats.

Cultivar selection – All vinifera cultivars are susceptible to Pierce’s disease, yet some are more tolerant to the disease than others (Table 1, from UC/ANR Publication No. 21600, Pierce’s Disease), and young vines are more susceptible than mature ones.

Table 1 is based on observations of Pierce’s disease in California vineyards. Because the climate and growing conditions of the Southeast region differ from that of California, cultivar susceptibility to the disease may vary. There are a number of Pierce’s disease resistant cultivars, many of which were developed for production in Texas and Florida (Table 2). These cultivars have not been tested for suitability in the NC regions that have a high risk for Pierce’s disease.

Vegetation management - Since diseased vines are often found between 150 to 200 feet from the vector source, one management approach reduces the reservoir of inoculum by removing nearby reservoir hosts that are breeding sites for the various vectors and/or systemic hosts of X. fastidiosa. Preliminary surveys and ELISA testing have been conducted in our region for determining potential reservoir hosts for X. fastidiosa.

A large number of native plants harbor the X. fastidiosa bacterium in North Carolina, including oak (Fig. 9), sycamore (Fig.10), hickory, sweet gum, wild cherry, Bermuda grass, pokeweed, wild grape, blackberry, Virginia creeper, wild rose, and sumac. However, because the ELISA test is not strain-specific, and there are different strains of X. fastidiosa, it is unknown whether the bacteria infecting these native plants can also cause Pierce’s disease on grape.

Removal of infected vines. Infected vines should be removed as soon as they are detected, or should be flagged and observed for delayed budbreak (Fig.11) the following spring to reduce the possibility of vine to vine spread in the vineyard.

Pruning practices. When PD-symptoms are found only at the terminal end of canes, normal dormant pruning practices may remove infected wood. If infection is more severe, cutting the trunk off just above the graft union may generate symptom-free vines.

Insecticides - Before insecticides can be used effectively in a management program, studies are needed to determine exactly which are the major insects responsible for transmitting the Pierce’s disease bacterium in North Carolina. Attempts to control vectors of Pierce's disease with organophosphate insecticides in California have not been very effective.