Raisin Industry Continues To Make Push Toward Maximizing Productivity, Minimizing Cost
The U.S. is the leading global producer of raisins, but the land area planted to raisin grapes in California peaked in the early 1980s and has steadily declined since then.
Poor prices and high labor costs initiated vineyard pullouts, which have continued even after prices improved. Raisin vineyards continue to be removed due in part to continuing labor shortages and to the potential for better returns from other commodities, especially nut crops.
These social and financial factors have pressured domestic raisin growers to adopt cultural practices that maximize productivity and minimize production cost. Such pressure will likely intensify in the future, continuing the profound changes that have already occurred in the raisin industry.
One of the most fundamental changes in the California industry has been the adoption of new grapevine varieties. For nearly a century, Thompson Seedless has been California’s dominant raisin variety, accounting for 90% of the raisin-type grapes in 2012, but the variety composition is changing fast.Dry-on-vine (DOV) mechanized raisin-making practices, which consists of severing grapevine canes bearing clusters of mature fruit, thus separating the clusters from the permanent vine structures and causing the fruit to DOV, requires varieties that ripen earlier than Thompson Seedless. Of the approximately 9,000 acres of Thompson-type raisin grapes planted between 2006 and 2012, Thompson Seedless only accounted for approximately 34%, with the newer, earlier-ripening varieties Fiesta and Selma Pete accounting for 25% and 40%, respectively.
Increasing Yields With New Growing Systems
Raisin grape trellis systems have also changed substantially. Two types of DOV trellis systems are widely used in California — overhead arbor and open gable.
The canopy of vines on overhead arbor systems are separated into fruiting and renewal sections by pruning the vines and tying their canes in such a way that trellis wires span the spaces between adjacent rows support either fruiting canes or renewal shoots. By mid-summer, vine canopies may entirely cover the vineyard surface area, enabling yields that are 2 to 2.5 times higher than that of conventional raisin vineyards.
The open gable system was developed in the 1990s by L.P. Christensen, at the University of California Kearney Agricultural Research and Extension Center. The open gable system consists of a large Y-shaped trellis, similar to that commonly used by table grape growers in California. Vines may be head- or cordon-trained, and cane-pruned. Fruiting canes are tied to the lower two of three foliage catch wires on either trellis cross arm and, in the spring, rake wires are used to pull renewal shoots towards a single foliage catch wire in the center of the trellis, thus separating the canopy into fruiting and renewal zones. Raisin yields are typically 1.75 to 2.25 times greater than those of a conventional raisin vineyard.
New Cultivars Also Leading The Future Of Production
The berries of certain grape cultivars and selections can spontaneously DOV, without the need for cane severance. Diamond Muscat and Selma Pete have this tendency (Figure 1), but in most years the spontaneous drying process begins too late and proceeds too slowly to result in adequate drying without cane severance.
However, USDA Agricultural Research Service scientists have been working diligently toward the development of a natural DOV variety which does not need cane severance to initiate drying. These efforts have lead to the imminent release of Sunpreme, a new natural DOV variety that will soon be available for commercial plantings. Sunpreme is said to be fruitful on basal nodes, a trait that should enable spur pruning, by hand or by machine. The combination of spur pruning and natural drying may favor new trellis systems, and work to evaluate different trellis systems has begun.