Studies to establish the feasibility of growing Pierce’s Disease (PD) resistant, predominantly European grape hybrids have been ongoing at Auburn University since 2010. This research can be reviewed more here.
Advanced selections for studies
We have evaluated advanced selections with early-, mid-, and late season ripening trained to the traditional vertical shoot positioning system (VSP) recommended for European grapes. This experiment is being conducted at the Chilton Research and Extension Center (CREC) near Clanton in central Alabama. It was found that all of the evaluated selections grew vigorously under humid southeastern conditions. The late season hybrid ‘U0501-12’ produced the highest dormant pruning weight. The early ripening ‘U0502-10’ was the most productive selection as measured by total yield per vine and also had the largest cluster size. None of the experimental vines exhibited symptoms nor did they test positive for PD. Under regional commercial management practices, no vine losses were observed from other pathogens. These results indicate PD resistant predominantly European (Vitis vinifera) hybrid grapevines can survive and consistently produce a good quality crop under humid conditions in central Alabama.
Watson trellis system
The outcomes of this initial study encouraged us to expand the experiment in 2017 and test another UC Davis developed advanced PD resistant European grape hybrid, named ‘U0502-20’. The primary goal is to evaluate the vine production potential when plants are trained to a highly efficient Watson trellis system. The Watson system is a relatively new trellising structure that continues to gain popularity in the southeastern viticulture. It features divided canopy training for better air movement and reduced risk of foliar disease development. Also, we aimed to determine the optimal planting distance for ‘U0502-20’ based on the vine vigor, productivity, and fruit quality evaluations when vines are planted at in-row distance of 6’, 7’, or 8’, and the between-row distance is 12’.
Adjusting growth to maintain desired fruit crop
Data is being collected to determine vine phenology, total yield, fruit quality and vigor of ‘U0502-20’ grape at each planting distance. Fruit cluster production was observed during the second growing season when clusters were removed before flowering in order to encourage root system establishment of the young vines. Annually, the experimental vines are dormant pruned to 12 spurs per vine (6 spurs/cordon) with two buds per spur retained for a total number of 24 buds per vine. Shoot thinning is conducted during the spring to maintain the desirable shoot number. Additionally, cluster thinning is applied to adjust the crop load to one cluster per shoot.
The ‘U0502-20’ vines produced the first commercial crop during the 2019 season. Current season results for total yield per vine (Fig. 1, 2) suggest similar cropping level regardless of planting distances with the 6’ in-row treatment producing 18.7 pounds per vine, and the 7’ and 8’ in-row distance treatments producing 19.4 pounds per vine. No statistical differences were found in the cumulative yield per vine during 2019-2020 when the plants produced between 36.2 and 36.8 pounds per vine. Mean cluster weight varied between 367.2 grams for vines planted at 6’ X 12’ to 394.3 grams for vines planted at 7’ X 12’ during the current season when the number of clusters harvested per vine ranged from 27.7 for plants at 7’ X 12’ to 31.6 for vines at 8’ X 12’. Mean berry size for all planting distances was slightly above 2.0 grams with soluble solids content of 18.4-18.7 percent.
Research will continue to more fully assess the vegetative and productive responses of PD resistant predominantly European grape ‘U0502-20’ and determine the optimal planting distance in Alabama conditions.