By Elina Coneva and Kassie Conner, Auburn University
Pierce’s disease (PD) on grapes is the major limiting factor for growing Vitis vinifera (European) grapes in Alabama and the southeastern U.S. Generally, PD infection causes vine decline, yield loss, and vine death typically occurs within two to three years of infection. Management efforts are focused on the development of grape selections resistant to this devastating bacterial disease. The UC Davis grape breeding program has developed predominantly V. vinifera progeny breeding lines with PD resistance and has recently released five new cultivars for commercial use. Information on production technology for the newly developed hybrids is lacking for Alabama and the S.E. region.
An experimental site was established at the Chilton Research and Extension Center near Clanton, Alabama in 2017 with the purpose to: 1) investigate the production potential of newly developed Pierce’s Disease (PD) resistant 94% Vitis vinifera grape ‘502-20’ in Alabama where the PD risk is high and to: 2) establish the optimal planting distance for sustainable production. Three planting distances (6’ X 12’, 7’ X 12’, and 8’ X 12’), where tested to evaluate the cropping potential, vegetative growth, and assess the fruit quality of ‘502-20’ grape. Vines are trained to the high-cordon divided canopy Watson training system utilized for improved vineyard management practices, improved canopy environment, and enhanced fruit quality.
During the early spring, vines were dormant pruned to 12 spurs per vine (6 spurs/cordon) with two buds per spur retained for a total number of 24 buds retained per vine. The dormant pruning weights were recorded for each individual plant and vine flowering progression was observed. Young shoots were trained and tied to the support wires as soon as they reached the proper length. Shoot thinning was conducted in early spring to maintain the desirable shoot number in the canopy. Petiole samples were submitted to the Plant Diagnostic Lab where Dr. Conner conducted a PCR test to establish the presence of Xf infection in the experimental vines.
The final stages of berry veraison and fruit maturity were documented during the summer. Fruit was harvested on August 7, 2020 and on August 13, 2021. Each season, the total yield per vine and the total cluster number per vine were measured and recorded. A five cluster per vine sample was collected to determine mean cluster weight. Other fruit quality attributes such as mean berry weight was determined on a sub-sample of 50 berries per vine. To record berry soluble solids content, the juice from 10 berries was extracted.
Our data on dormant pruning weight per vine suggests planting distance did not significantly affect the vigor of 502-20 vines, which was very uniform for all treatments in 2020, while in 2021 season the 7’ X 12’ planting distance resulted in slightly reduced vine vigor based on the results for plant dormant pruning weight (Figure 1).
Results for total yield per vine (Table 1) suggest similar cropping level (between 8.5 and 8.8 kg/vine) regardless of planting distances during 2020. During the next season, the 6’ X 12’ and 7’ X 12’ treatments yielded between 8.0 and 8.6 kg/vine respectively, while the 8’ X 12’ planting distance resulted in significantly higher crop of 12.6 kg/vine.
|Total Yield, kg|
|Total Yield, kg|
|6′ X 12′||8.5||8.0 b|
|7′ X 12′||8.8||8.6 b|
|8′ X 12′||8.8||12.6 a|
Average number of clusters/vine was not statistically different during the reported period, and varied between 27.7 and 31.6 during 2020 and between 35 and 54 in 2021 with the 8’ X 12’ treatment producing the highest number of clusters (31.6 and 54.0 respectively) in both seasons (Table 2). Mean cluster weight varied between 367.2 g for vines planted at 6’ X 12’ to 394.3 g for vines planted at 7’ X 12’ during the 2020 season, and was between 569.0 and 621.2 g in 2021 (Figure 2). The 7’ X 12’ planting distance resulted in the largest cluster size in both study years, although no statistical difference between the treatments was found.
|Planting density||Average No. of clusters/vine|
|Average No. of clusters/vine|
|Mean cluster weight, g|
|Mean cluster weight, g|
|6′ X 12′||28.0||35.0||367.2||569.0|
|7′ X 12′||27.7||35.0||394.3||621.2|
|8′ X 12′||31.6||54.0||377.0||597.9|
No statistical difference was found to affect the mean individual berry weight between planting distance treatments. In general, mean berry size was larger in 2021, likely due to the rainier season with above average rain events and rainwater accumulation (Table 3). Berry soluble solids content was similar for all planting distances in 2020 and was highest for the 8’ X 12’ distance in 2021, while the 7’ X 12’ planted grapes had the lowest sugar content of 16.3%.
|Planting distance||Mean berry weight, g|
|Mean berry weight, g|
|6′ X 12′||2.0||2.8||18.7||17.1 ab|
|7′ X 12′||2.1||2.9||18.5||16.3 b|
|8′ X 12′||2.2||2.8||18.4||18.1 a|
Petiole samples were collected from each individual vine after harvest in mid-October, 2021 to test for the presence of Xylella fastidiosa infection. The conducted PCR analysis returned negative results. No Pierce’s disease infected vines were found from the PD resistant predominantly V. vinifera selection 502-20 after five years of cultivation in the high PD risk zone of central Alabama, while the adjacent PD tolerant American and French-American hybrid bunch grapevines were showing 37% infected plants in 2021.
The study results are promising as yield and fruit quality of 502-20 grape suggest that cultivation of high-value PD resistant European grape progeny can provide an excellent opportunity for the development of value-added products and can contribute to sustain rural communities in the Southeast by creating conditions for economic growth and development, while boosting existing industries such as the local agritourism. Research will continue to more completely evaluate the cropping potential and determine the optimal cultivation technology for PD resistant predominantly V. vinifera grape.