Dry-on-Vine Raisin Grape Canopy Management for Open Gable Trellises

Matthew Fidelibus, UC Davis Viticulture & Enology, George Zhuang, UCCE Fresno County

Canopies distinctly separated into fruiting and renewal sections are a striking feature of overhead arbor dry-on-vine (DOV) vineyards (Figure 1). With such systems, canopy separation provides good sunlight exposure to renewal shoots, which should help optimize bud fruitfulness and minimize bud necrosis. Canopy separation also facilitates pruning and harvesting, and might provide a better microenvironment for raisin drying compared to a non-divided canopy. The success of the overhead arbor system has inspired attempts to adapt canopy separation to other trellis systems in hopes that it would help overcome certain inherent limitations or management problems.

A well-known example of the use of canopy separation on other trellising systems is the within-row-alternate-bearing (WRAB) for ‘Thompson Seedless’ on traditional trellises. Vines subjected to the WRAB system were pruned in such a way that fruiting canes and renewal shoots were left on opposite sides of vine trunks such that each were adjacent to similar structures of neighboring vines, resulting in divided canopies with alternating fruiting and renewal sections between vine trunks. A complete description and illustration of the WRAB system can be found in a California Agriculture article online: http://ucanr.edu/datastoreFiles/391-325.pdf. Several years of testing showed that the WRAB method per se did not affect yield compared to vines with non-divided canopies whose fruit were dried on trays (see the California Agriculture article cited above for details). Raisin grades were higher from WRAB DOV vines compared to tray dried raisins, but this was likely an effect of drying method rather than canopy separation, as similar results were found in a different study comparing the effect of drying method on raisin quality (http://ow.ly/tBSs300kJZC). A subsequent study that specifically tested the effect of canopy separation on ‘Thompson Seedless’ DOV performance did not show any consistent benefit of canopy separation on traditional trellises (http://www.pubhort.org/aps/61/v61_n2_a10.htm) and the use of WRAB techniques on traditional trellis is now uncommon.

Even though no clear benefit of the WRAB DOV system was shown for traditional trellis systems, many growers have experimented with it on open gable systems, in an attempt to improve fruitfulness and thereby yield. The effect that different canopy separation methods may have on raisin yield and quality on an open gable trellis was recently tested in a multi-year experiment at the Kearney Agricultural Center in Parlier. We subjected vines to one of three different canopy separation methods: 1) center-divided, with moveable rake wires to pull the renewal shoots away from the fruiting shoots on the trellis arms and toward the catch wire in the center of the trellis, 2) non-divided (Figure 2, A), or 3) WRAB. Vines assigned to the center-divided or non-divided treatments were cane pruned, leaving six to eight 15-node canes, and approximately 10 two-bud spurs per vine. Canes and spurs were left on all cordons; the only difference between those treatments was the use of rake wires and a center-mounted foliage catch wire to separate renewal shoots from fruiting shoots in the center-divided vines. Vines assigned to the WRAB treatment were pruned such that the cordons between any two adjacent vines were either entirely spurs or canes, creating fruiting or renewal zones that alternated between pairs of vine trunks (Figure 2, B).

Figure 1. An overhead arbor trellis system with canopies divided into alternating fruiting and renewal sections.

In general, the renewal shoots on vines with WRAB canopies had a higher proportion of external leaves, and fewer leaf layers, than vines subjected to other canopy management treatments. Light intensity in the renewal zone was also highest in vines with WRAB canopies in spring, but as the canopy developed, treatment effects on light intensity diminished. Despite somewhat improved canopy structure and better light exposure in spring, none of the canopy separation treatments have improved raisin yield or quality in any of the three study years. Thus, other factors, such as irrigation and nutrient management could be more important that canopy separation in regards to optimizing raisin grape yield on an open gable trellis.