The length of canes and the positions of nodes on those canes appears to affect yields of dry-on-the vine raisin grapes varieties – DOVine, Fiesta and Selma Pete. That, in turn, has implications as to the best time to sample berries for determining when to start cutting the canes and initiating the grape drying process.

Those are the conclusions of Matthew Fidelibus, University of California Cooperative Extension specialist, from a two-year study he conducted comparing production of these three varieties, with the later-ripening Thompson Seedless variety, long the mainstay of California’s raisin industry.

Based at the UC Kearney Agricultural Center, Parlier, Calif., he describes this research in the Spring, 2017, issue of VitTips, a UCCE newsletter that focuses on viticulture in the San Joaquin Valley.

Dry-on-the vine systems, which feature the use of overhead arbor has become increasingly popular over the past two decades, because it facilitates machine harvesting of the raisins. In this system, the canes are cut and the grapes are left on the vines for six weeks or longer to dry before the clusters are harvested mechanically. This approach offers lower production costs and less risk of damage from rain during the drying process than conventional trellis systems where the clusters are harvested by hand and laid on trays on the ground to dry three to four weeks, depending on the weather.

Basal buds with low-fertility are desirable in cane-pruned DOV varieties because canes are generally severed at or above the third or fourth node. Consequently, fruit on shoots emerging from the first several nodes will not dry on the vine.

Differences in the proportion of clusters produced in the head of the vine—on shoots from spurs and the basal nodes of canes—suggest the fruitfulness of nodes at different positions may vary from one cultivar to another, Fidelibus notes.

That’s important because canes are usually cut at or above the third or fourth node. Consequently, any fruit on shoots emerging from the first several nodes won’t dry on the vine. If these fresh grapes are harvested along with the dried clusters, they could increase moisture levels in the raisins and contribute to spoilage.

Many DOV growers with overhead trellises strip the flower clusters, or even the shoots, from nodes between the base of the cane and the first foliage catch wire, the outer edge of the machine-harvestable area. The number of nodes that are stripped varies with the internode length of the canes and the training system and trellis design in the vineyard.

Because the arms on vines trained to a single head are generally located close to the trunk (near the center of the vine row) canes must span a relatively long distance to reach the first foliage catch wire. However, since canes from cordons or split heads are generally established closer to the catch wires, they may not need to grow far before reaching the wires.

Consequently, on overhead arbor systems, the first five to twelve nodes commonly are located between the vine and the first trellis wire where they may not contribute to raisin yield. That’s why growers with overhead arbor trellises typically leave extra-long canes having 20 or more primary nodes, with or without lateral shoots, Fidelibus explains.

Nevertheless, up to this point, researchers had not studied the relationships between cane length, node position and productivity for the varieties of raisin grape varieties now being used for DOV or how the long canes used on modern DOV systems affect yields.

Fidelibus’s study was designed to address that information gap. Among his findings:

“In general, shorter canes had slightly more uniform bud break, especially from nodes near the tip of the canes, he says.

‘Fiesta’ had the most fruitful nodes, followed by ‘Selma Pete.’ Thompson Seedless and DOVine produced the fewest clusters per node.

However, 15-node canes generally had heavier clusters than 20-node canes at most node positions, except for Selma Pete and Thompson Seedless.

Berries from clusters on short canes had similar or greater soluble solids than berries from clusters on long canes at a given node position. Regardless of cane length, soluble solids generally decreased with node position in both years.”

“The strong effects of node position on soluble solids and cluster weights are important factors to consider when taking berry samples to determine when to initiate drying, as clusters on shoots from the apical portion of canes are bearing a heavier crop of fruit with potentially lower ripeness, than clusters from shoots arising from the basal part of canes,” Fidelibus reports

Because percent bud break, fruitfulness, and cluster weights all increased with node position, particularly for Selma Pete, Fidelibus conducted a second study. He wanted to see if distributing the same number of nodes over longer canes might result in more nodes having higher productivity than canes with the same number of nodes distributed over a shorter length. Fidelibus compared the productivity of Selma Pete grapevines on an open gable trellis having 120 nodes distributed on either eight, 15-node canes, or six, 20-node canes.

He found cane length did not significantly affect overall soluble solids in either year of the two years of the study. However, in one year, vines having fewer longer canes were more productive than vines having more shorter canes.

“The raisin data suggests that leaving fewer, longer canes was as or more productive than leaving more, shorter canes,” Fidelibus says. “But other factors, such as cane selection, can profoundly affect productivity. For example, canes that grew in full sun tend to have better bud break and higher fruitfulness than canes which developed in the shade. As a result the need to retain a large number of canes to meet a desired bud count might require selecting some less desirable canes.”