In their fourth season of trials with dried-on-the-vine (DOV) plots, University of California researchers remain confident the practice can increase yields and cut costs significantly to help revive the state's depressed raisin industry.
Although other variations of DOV exist and trials with them continue, attention is now focused on what has become known as “the within-row, alternate bearing, DOV system.”
With it, just prior to bloom, six to eight shoots that will become fruiting canes are positioned on the wire to one side of each vine, within the row, and renewal spurs are left on the other side.
The fruiting canes are cut when the fruit reaches 19 to 20 degrees Brix, ideally in mid-August, to initiate drying. As the fruit dries, the vineyard shows a curious checkerboard pattern of green and dried foliage.
The raisins are harvested and those canes are removed at pruning time, leaving the spurs of vine's opposite side to become fruiting canes for the next crop.
Other DOV systems depend on new vines and expensive, overhead trellises ranging in cost from $2,000 to $4,000 per acre. This system, however, is easily adapted to existing, head-trained, Thompson Seedless vines having traditional, single-wire trellising.
Another major economy for the within-row configuration is use of standard, wine-grape mechanical harvesters. When the raisins are ready to be harvested, most wine grapes have been picked and there is plenty of equipment available.
As the crop dries on the vine, it is free from the hazards of fall rains on traditional trays. And drying is accomplished without coping with terracing of middles, sand in the fruit, dust that attracts mites, and disposal of trays after the raisins are picked up.
The Selma Pete variety, released in 2001, has performed well in the UC experiments. It has quality similar to Thompson but matures about two weeks earlier.
Some 100 growers came to the Kearney Research and Education Center at Parlier recently to hear the latest reports on the trials.
In describing the work, Bill Peacock, Tulare County farm advisor and one of the principal researchers, said, “When sugar reaches 19 to 20 Brix, you can cut fruiting canes and make a quality raisin. However, to successfully dry on the vine, fruiting canes should not be cut any later than the end of August or a dehydrator will be required to finish drying.”
Cutting canes during the week of Aug. 15, he added, has a probability of 90 percent for successful drying, although by the end of August, the chances are less than 50-50. Once dried to 14 percent moisture or less, the fruit is harvested.
Peacock said costs of the vine preparation and harvesting will vary with vineyard age and condition of the trellising. He figures average costs, including five steps, at $375 per acre, which is close to the costs for conventional, tray-dried production.
The per-acre costs for each step are: shoot selection and positioning in the spring, $75; cane severance in the summer, $100; hanging green clusters on the wire, $75; mechanical harvesting, $125; and trellis repair after mechanical harvesting, $50. Also calculated is a $50 credit against traditional winter pruning costs because of the cane severance during the summer.
If the production were two tons to the acre, the cost would drop to $187.50 per ton, and for a three-ton yield it would be $125 per ton, he said.
Peacock is collaborating with Steve Vasquez, Fresno County farm advisor; Matthew Fidelibus, UCCE viticulturist at KREC; and Fred Swanson, superintendent at KREC.
Enthused with the progress of the past four seasons, Peacock said the viticulture team is exploring new directions of research.
In a cross-arm project begun last year, they found that production was increased 8 percent for each foot of cross-arm width. “Raisins dried more uniformly with wide cross-arm systems and also more quickly. We estimate the cross-arm reduced drying time by about a week under normal circumstances,” he said.
They also grafted Selma Pete onto mature Thompsons at KREC this spring and got a 95 percent stand. Peacock reported the vines, trained to a split head, are strong and the first crop will be harvested this fall. Fruit will be evaluated against Thompson for yields and raisin quality.
With an interest in mechanization and high-density plantings, they also planted 2.5 acres to Selma Pete on Freedom this spring. The vines will be trained to a bilateral cordon trunk this year for the within-row system. Methods for mechanically severing canes at harvest will be developed.
“Our aim,” Peacock said, “is to produce four to five tons per acre of high-quality raisins using high-density. We hope to reduce the harvesting cost to less than $75 per ton and reduce the pruning cost to less than $100 per acre.”
Fidelibus, completing his first season as UCCE viticulturist at KREC, is monitoring the physiological effects of harvest pruning or HP, the basis of mechanical harvesting component of the within-row system.
Effects of HP
In a report compiled with David Smart, a UC, Davis plant physiologist specializing in roots, Fidelibus noted that the physiological effects of HP are mostly unknown.” The two will be investigating effects of HP on leaf area, fruit, and roots.
Considering the grower's point of view, Fidelibus said vineyards are best managed as a long-term investment and vine health and capacity need to be maintained for high yields.
“The fact that HP may reduce a vine's capacity on some training and trellis systems suggests that the capacity of vines subjected to HP could potentially be increased by management practices that limit negative effects of HP.”
The results of the study, he added, “may be used to help manage DOV systems, and vines subjected to HP may serve as a model system to determine the role of the canopy, postharvest, on the productivity of vines in warm climates.”