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Introduction

Pierce's disease is a threat to grapes in California and in southern states from Florida to Texas. Disease symptoms vary with species and cultivar, but are typified by marginal browning of leaves and death of vines. This disease is favored by the hot weather found in the southeastern U.S.

Symptoms of Pierce's disease

Symptoms vary with the different species and cultivars. Symptoms in spring and early summer include delayed shoot growth, leaf mottling, and dwarfing of new shoots. Late summer and fall symptoms are more dramatic and include burning, scorching, or drying of leaves (Figures 1, 2); wilting or premature coloring of fruit; and uneven cane maturity. Scorching begins near the margin of the leaf blade where tissues become completely desiccated and die. As summer progresses into fall, scorching progressively spreads inward in concentric zones until the entire leaf blade is affected.
Figure 1. Marginal grape leaf scorch caused by Pierce's disease.	Figure 2. Late season marginal grape leaf scorch caused by Pierce's disease.
Leaf blades often fall from the vine at the point of attachment to the petiole, leaving the petiole still attached to the shoot (Figure 3). Healthy grapes also sometimes leave occasional petioles behind in the fall, but not to the same extent as diseased grapes.
Figure 3. Grape leaf petioles still attached to infected grape vine after leaf blade drop.
The disease progresses along the grape vine with symptoms developing in adjacent leaves along the shoot both above and below the point of initial infection. Flower clusters on infected vines usually dry up. Late in the season, wood on affected canes fails to mature normally, leaving green "islands" of tissue which persist into the dormant season and can be seen on canes throughout the winter (Figure 4, 5). Tips of shoots often die the first year the vine is infected. Initially, only one or a few canes on a vine show foliar and wood symptoms. Symptoms are more pronounced in vines that are stressed by high temperatures and drought conditions.
Figure 4. Uneven maturation of grape vines with Pierce's disease.	Figure 5. Uneven grape vine maturation and retention of leaf petioles due to Pierce's disease.

Grape susceptibility and disease spread

For many years, trees, especially oaks, in Kentucky landscapes have suffered from bacterial leaf scorch disease, also caused by X. fastidiosa, but a different strain from the one that causes Pierce's disease. Leaf scorching symptoms associated with this disease annually appear in late summer. Symptoms are quite striking on pin and red oaks with individual leaves turning 1/3 to 2/3 brown on the leaf ends and margins (Figures 6, 7). The causal agent of bacterial leaf scorch is also vectored by leafhoppers or other xylem-feeding insects. As far as is known, the grape pathogen is the same as, but not identical to the tree leaf scorch pathogen. Thus, the disease would not be spread from trees to grapes.

Figure 6. Bacterial leaf scorch of pin oak.

Figure 7. Bacterial leaf scorch of red oak.

Materials and Methods

Grape leaves showing symptoms of bacterial leaf scorch were collected from a vineyard in Hancock County and delivered to the U.K. Plant Disease Diagnostic Laboratory. Petioles from affected leaves were crushed with a mortar and pestle so that the extract could be tested for presence of the pathogen using a special laboratory test, an enzyme-linked immunosorbent assay (ELISA) developed for X. fastidiosa ("Pathoscreen-Xf", Agdia, Inc., Elkhart, IN). Color reactions for the ELISA test were evaluated visually and by using an ELISA plate reader. To overcome doubts due the possibility that the ELISA test might give a false positive reaction, specimens were sent to a laboratory in California which specializes in testing for Pierce's disease using a rapid-cycling real-time polymerase chain reaction (PCR) assay for presence of bacterial DNA. (California Seed & Plant Lab., Inc., Elverta, CA).

Results and Discussion

In our laboratory, the ELISA test reacted strongly positive for X. fastidiosa, the Pierce's disease pathogen for some of the samples. The PCR assay done by the laboratory in California corroborated our ELISA test so the specimens again were positive. Thus, Pierce's disease of grapes caused by X. fastidiosa was discovered for the first time in western Kentucky (1). This disease can be devastating to grape production - much more Kentucky research is needed.

In other regions, X. fastidiosa is distributed in a wide range of monocot and dicot native plants, infected, but not showing symptoms. We do not know if the bacterium has become established in the wild, and if so, on which plants. We have preliminary evidence that X. fastidiosa can live in some grasses, weeds, and woody plants here in Kentucky. These plants do not show scorch symptoms but could be reservoirs of the bacteria. We do not know if these plants harbor the Pierce's disease strain, however. Pierce's disease could be carried from infected vegetation to grapes or from diseased grapes to healthy grapes by insect vectors but we know little about which vectors would be involved here in Kentucky. Where the disease is isolated, removal of infected vines should keep further spread to a minimum.

With an emerging grape industry developing in Kentucky, it is important that growers and County Extension Agents be on the lookout for this disease. Personnel in the U.K. Plant Disease Diagnostic Laboratory can run specialized tests to determine the presence of the Pierce's disease bacterium. Growers suspecting Pierce's disease in their vineyards are urged to contact their County Extension Office.

Literature Cited

• Bachi, P.R., J.W. Beale, J.R. Hartman, D.E. Hershman, W.C. Nesmith, and P.C. Vincelli. 2002. Plant Diseases in Kentucky - Plant Disease Diagnostic Laboratory Summary, 2001. U.K. Department of Plant Pathology (in press).

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	Original document: 11 November 2001 Last updated: 11 November 2001