Among the stops growers made during the University of California Grape Day tour at the Kearney Agricultural Research and Extension Center in Parlier on Aug. 11 were test plots of Scarlet Royal and Autumn King table grapes planted in 2013.

There, Ashraf El-Kereamy, UC Cooperative Extension viticulture advisor for Kern County, described a planned three-year study examining the effect of two pruning systems on grape yield and quality of these two varieties. He’s collaborating with Kaan Kurtural, Department of Viticulture and Enology, California State University, Fresno.

Released in 2006, both varieties were developed at the Kearney facility by USDA Agricultural Research Service plant breeders. The two varieties are licensed exclusively to the California Table Grape Commission, which is funding this project.

Scarlet Royal, a mid-season red seedless table grape, which ripens in mid-to-late August, fills the harvest window between Flame Seedless and Crimson Seedless. The initial experimental trials indicate mature quadrilateral cordon, spur-pruned vines grown on a gable system will yield 1,100 to 1,300 boxes per acre.

The white-seedless Autumn King variety features a very large, natural berry size and late harvest. It ripens in mid-September to late October, about eight weeks after Thompson Seedless. Mature Autumn King can produce up to 1,500 boxes per acre for head-trained, cane-pruned vines grown on a gable system.

El-Kereamy is comparing the impact of two pruning systems used by California growers on vine performance, productivity and fruit quality of the two cultivars.

One is quadrilateral cordon, spur pruning. It’s used when the basal buds on a cane are fertile and capable of producing clusters of grapes. With this method, the cane is pruned in the winter to leave only two buds (spur) across the cordon.

The other approach – head cane pruning – is used when the basal buds on a cane are infertile. In this case and during the winter, pruning leaves much longer canes. Each may have as many as 12 to 15 buds and four renewal spurs of two buds each.

El-Kereamy’s research is designed to help growers decide which pruning method is most appropriate for achieving their production goals. Currently, growers are using either spur or cane pruning with both Scarlet Royal and Autumn King, he notes.

Generally, spur pruning is easier and does not require the level of skill needed with cane pruning. El-Kereamy says. However, cane pruned vines have the potential to yield more clusters.

“In table grape, a special attention should be given to the quality of grapes produced under the two systems to determine the most suitable pruning methods for you,” El-Kereamy says. “Your choice of pruning systems is critical for optimizing grape yield and quality. Depending on the pruning method you select, you could end up with a vigorously-growing canopy that shades the basal buds, causing necrosis of the buds and preventing them from growing the next season. On the other hand, if your pruning system doesn’t leave enough growing buds on the cane, the canopy growth may not be enough to support the clusters and prevent sunburn of the grapes and loss of color and quality.”

This season, El-Kereamy is measuring vegetative growth of the varieties under each pruning system. During the coming winter he will weigh the pruned wood from each plot as another indication of the impact of pruning system on vine vigor.

For each of the next two years, El-Kereamy will continue to assess vegetative growth, In addition, he will determine bud fruitfulness by counting the number of clusters produced and the number of buds left on the vine after pruning.  At harvest, he’ll compare the two pruning systems in terms of yield and fruit quality.

“After completing the study, we will be able to show growers the advantages and disadvantage of spur and cane pruning for each of these two varieties,” El-Kereamy says.