California Ag TODAY: California Ag News SJV Grape Symposium Focuses on Raisins

"The primary difference between vineyard sites with or without raisin moth damage appeared to be well-timed and effective insecticide sprays," Daane said. "One problem for organic sites may be the availability of insecticide materials that have long enough residual activity to control the larvae of adult moths entering the vineyard, and once the larvae are deep inside the grape cluster they are difficult to control."

In addition to Daane's report, the San Joaquin Valley Grape Symposium included the following research updates:

• Rootstocks for raisin production by Sonet Von Zyl, Fresno State University 

• Raisin production canopy management by Matthew Fidelibus, UC Cooperative Extension specialist in the Department of Viticulture and Enology at UC Davis, based at the UC Kearney Ag REC in Parlier 

• Raisin grape breeding program by Craig Ledbetter, USDA Agricultural Research Service, based in Parlier 

• Economics of producing raisins, by Annette Levi, Fresno State University 

• Grapevine trunk diseases and grower survey

California Ag Today Editors have included the following presentations in more detail.

Von Zyl spoke about her work in evaluating nematode resistant rootstocks for use with early ripening raisin varieties grown for dried on the vine raisin production

“DOV raisin production relies on two essential components: 1) early maturing varieties and 2) new trellis systems developed specifically for DOV production,” Von Zyl said. Currently two varieties are used for DOV production on high capacity systems namely Fiesta and Selma Pete. New vineyards for raisin production are being planted to one of these varieties due to their comparable production to Thompson Seedless. Both ripen earlier than Thompson Seedless and have potential for mechanical harvesting when DOV farmed. Fiesta has increased slowly since its release because it was thought to have a large seed trace.

This has since been proven incorrect and plantings stand at 12,685 bearing and non-bearing acres (California Agricultural Statistics Service, 2010). Selma Pete acreage stands at 3,143 total (bearing and non-bearing) but interest is high for this variety specifically for DOV production using the open gable trellis system. In 2010, 6,716 acres of Fiesta and 1,245 acres of Selma Pete were produced using an overhead trellis system. It is conceivable that these two varieties may become the standards for DOV raisin production as older Thompson Seedless vineyards get removed.

Unlike traditional raisin production, trellis systems are vital to the success of DOV raisin production. Currently, two trellis systems are most prevalent among DOV growers, the open gable and the overhead trellis systems. The basic principle of the open gable trellis (syn: Y-trellis) maintains the fruit on both sides of the trellis after cane severing. In traditional raisin production vineyards, vines are head-trained, but for DOV production it is more desirable to train vines as bilateral, quadrilateral cordons or a split head to facilitate cane severance and pruning.

Freedom and 1103 Paulsen are rootstocks which are commercially available that have some resistance to nematodes and are considered high vigor stocks. The use of rootstocks that impart vigor and have nematode resistance will be important in DOV vineyard establishment.

Establishing a DOV vineyard is costly. Setbacks due to weak vines, which do not fill the trellis system, cost growers time and money. It is conceivable that rootstocks will play an important role in new DOV vineyards. Some of the rootstocks previously mentioned, are currently being evaluated under DOV raisin production using a south-side trellis system.

Matthew Fidelibus, Extension Specialist, Department of Viticulture and Enology UC, Davis spoke about the Impact of canopy management practices on the fruitfulness, yield, and quality of dry-on- vine raisin grapes on open gable trellis systems

Most modern dry-on-vine (DOV) raisin vineyards in California have an overhead arbor or an open gable trellis system. The overhead arbor has a slightly greater yield potential than the open gable, but is more costly to install and requires more specialized farming and harvest equipment than the open gable. Approximately 1/3 of Selma Pete vineyards are on an overhead trellis system (USDA NASS 2012); most of the rest are on an open gable.

The original open gable DOV trellis is comprised of steel posts topped at 4.5 ft with 6 ft-wide V- shaped steel cross arm assemblies supporting six fruiting wires, three on each side). The bottom two wires on each cross arm support fruiting canes. Cordon support wires are affixed immediately below the base of the cross arm assemblies, and a vertical one foot tall post extension is mounted in the center of the cross arm assembly to support a foliage catch wire. In spring, moveable rake wires are pulled toward the center of the trellis, thus gathering the renewal shoots emerging from spurs and guiding them toward the center of the trellis where they can be supported by the center-mounted foliage catch wire.

Fruiting canes of vines subjected to the center-divided canopy management system are tied to both cross-arms creating a balanced crop load on the trellis, and helping to provide a physical separation of the renewal shoots from the fruiting shoots on canes. Canopy management practices such as the separation of renewal and fruiting zones may increase productivity if they promote the exposure of renewal shoots to sunlight (Shaulis and May, 1971). Christensen (1979) showed that Thompson Seedless canes originating from renewal shoots that grew under sun-exposed conditions, “sun canes”, had better bud break than shade canes, and the shoots from sun canes were more vigorous and productive than shoots from shade canes.

The specific benefit that center-divided canopies, or other possible canopy division practices, may have on the exposure of renewal shoots to sunlight, or on bud fruitfulness, has not been determined for Fiesta or Selma Pete on open gable DOV trellises. Such information is needed to help growers understand which elements of the original open gable design concept are critical to ensuring high yields.

Since the open gable trellis was commercialized, growers and trellis companies have significantly modified or omitted several of the original trellis design features and canopy management practices for various reasons including: to reduce the cost of trellis materials, facilitate pruning, more distinctly separate the fruiting and renewal zones, and to enable the vines to be harvested with smaller, less expensive, and more widely available, machines. Vineyard trellis systems with a narrower cross arm span have also been modified with a narrower cross arm angle, with many also having a substantially lowered cordon wire. These changes were mounted foliage catch wire and rake wires, thus reducing the cost of trellis materials and installation. Without catch wires, the renewal shoots cannot be guided into the center of the trellis, but the canopy can still be separated into renewal and fruiting zones by pruning in such a

way that canes and spurs are on separate cordons, a pruning style known as within-row-alternate- bearing (WRAB) or, more commonly, as the ‘Peacock’ method, after the person who invented this pruning style (Peacock and Swanson, Fidelibus et al. 2007; Figure 3).

Clearly, the move to DOV has inspired creative canopy management practices, but reports of disappointing or declining yields may indicate that some of the practices being implemented may be undesirable. Poor performance may be related to some of the trellis design and vine training changes described above, but the fact that multiple changes were often implemented at once makes it difficult to determine which factor or factors may be most important. Therefore, we have begun work to determine how different trellis designs and canopy management practices may affect canopy structure, light environment, and productivity of ‘Selma Pete’ and ‘Fiesta’ grapevines on open gable trellises.

Kearney canopy separation trial. Canopy separation method affected the number of leaf layers and the proportion of the leaves on the exterior of the vine canopies (Table 1). The canopies of all vines had similar numbers of horizontal leaf layers when measured in July, but vines with non-divided and center-divided canopies had fewer exterior leaves, horizontally and vertically, and more leaf layers, vertically, than vines with WRAB canopies (Table 1). Fewer leaf layers should improve light levels in the canopy, an effect that could promote bud fruitfulness and budbreak, and having more leaves on the exterior of the canopy is also desirable as exterior leaves have the greatest photosynthetic capacity due to their better sunlight exposure.

Canopy separation practices, especially WRAB, increased light intensity in the renewal zone early in the season, but as the canopy developed over time, differences between treatments, with respect to light intensity, diminished. (Figure 4). Renewal shoots growing under better light exposure could form dormant buds with greater fruitfulness than those from renewal shoots grown under lower light intensities, and shoots exposed to high light levels generally have lower rates of bud necrosis over winter than shoots exposed to low light conditions. Increased bud fruitfulness and improved budbreak could potentially increase vine yields.

Vines with center-divided, or non-divided, canopies may retain fruiting canes and spurs on any cordon (A). Vines with canopies separated in the Within-Row-Alternate-Bearing (WRAB) style are pruned so that all the canes are on the cordons between two adjacent vines, with spurs on the other cordons, and fruiting and renewal sections thus alternating down the vine rows (B).

Larry Williams, Department of Viticulture and Enology UC Davis discussed a brief review of mineral nutrition of grapevines and fertilization guidelines for California Vineyards.

Determination of N fertilizer amounts

Once the decision has been made to fertilize the vineyard, the appropriate amount of fertilizer should be applied. Mineral nutrient budgets (i.e. the amount of nutrients the vine needs for proper growth and development) have been established in various studies around the world. It was determined that Thompson Seedless grapevines needed approximately 39 kg N ha-1 (~ 35 lbs N acre-1) for the leaves, 11 kg N ha-1 (10.7 lbs N acre-1) for the stems (main axis of the shoot) and 34 kg N ha-1 (~ 30 lbs N acre-1) for the fruit (Williams, 1987). The vineyard density in that study was 1120 vines per hectare (454 vines per acre; 12’ rows x 8’ vine spacings) and the trellis system was a 0.45 m crossarm. The total N (found in the fruit at harvest, leaves as they fell from the vine and pruning wood) in wine grape vineyards using a VSP trellis system varied from 24 to 65 kg N ha-1 (21 - 58 lbs N acre-1) over a three year period (L.E. Williams, unpublished data). The differences in N per hectare (acre) in that study were primarily due to differences in row spacing and final yield.

In another study (Williams, 1991) it was determined that Thompsons Seedless leaves contained greater than 22 kg N ha-1 (~ 19 lbs N acre-1) after they fell from the vine and the canes at pruning contained approximately 17 kg N ha-1 (~ 15 lbs N acre-1). These values are comparable to other studies using Thompson Seedless. The results from both studies mentioned above (Williams, 1987; 1991) would indicate that there is a considerable amount of N in both the leaves and canes of a vine and that when both are incorporated into the soil after leaf fall and pruning would contribute to the soil’s organic matter and the availability of N in subsequent years. The author has found that N from both leaves that fell from the vine after harvest and prunings incorporated into the soil is taken up the following growing season (unpublished data). Another interesting aspect of those two studies would be the difference in N within the leaves of the vines at harvest (39 kg N ha-1) and leaves after they’ve fallen from the vine (22 kg N ha-1). The difference in the amount of N in the leaves between the two (~ 15 lbs N/acre) would theoretically be the amount of N remobilized out the leaves during senescence after harvest and put into the vine’s N storage pool (~20% of the seasonal total N demand by the vine) indicating the importance of leaves as a source of N for recycling within the vine. A study is currently underway by the author (funding provided by the American Vineyard Foundation, California Table Grape Commission and California Raisin Marketing Board) to provide better metrics for the remobilization of N out of the leaves after harvest and back into the permanent structures of the vine (N storage reserves). Data should be finalized by the Spring of 2014.

The amount of K needed for growth of grapevines also has been determined. In the same vineyard used above to develop a N budget for Thompson Seedless grapevines, a K budget was developed (Williams et al., 1987). Leaves, stems and fruit needed approximately 13, 29 and 50 kg K ha-1 (~ 11, 26 and 44 lbs K acre-1), respectively, during the growing season. The amount of K in the leaves and canes at the end of the season were equivalent to 9 and 12 kg K ha-1. The amount of K found in the fruit at harvest, leaves as they fell from the vine and canes at pruning for two wine grape cultivars, on different rootstocks and at different locations ranged from 25 to 67 kg K ha-1 (22 - 60 lbs K acre-1) over a three year period (L.E. Williams, unpublished data). Differences among K per unit land area were due to same factors as discussed in the preceding paragraph for N in that study.

The above information in this section illustrates that there can be significant variation in the requirements of N and K per vineyard. This is due to differences in row spacings, trellis types, yield and overall growth of individual vines. Much of the N and K in the leaves and canes are returned to the soil for possible future use. Therefore, a better way in determining the fertilizer demands of a vineyard would be to calculate the amount of that nutrient removed in the fruit at harvest

Kinds of fertilizers

The choice of N fertilizers for raisin vineyards in California can be based mostly upon cost (Christensen and Peacock, 2000). The same may apply for table grape and wine grape growers. The nitrate form of N allows the fertilizer to be available to the vines shortly after an application while the ammonium and urea forms require their transformation to nitrate in the soil profile. The liquid forms of N fertilizers are gaining

in popularity due to their ease of handling and application via drip irrigation (fertigation). Many raisin and table grape growers will use farm manure as a source of N, with its application occurring during the dormant portion of the growing season. Lastly, the acidification potential of N fertilizers should be considered in a management program particularly in acid soils. This characteristic of N fertilizers has been outlined (Christensen and Peacock, 2000).

It has been concluded that one form of K fertilizer offers no advantage over the other forms (Christensen and Peacock, 2000). Thus cost may play a major role in determining which kind to use in California and whether it is to be used in a fertigation program. For vineyards with Mg deficiencies the choice of a fertilizer would probably be magnesium sulfate. The two micronutrients mostly commonly needed in California vineyards are zinc and boron. Foliar and soil applications of the two fertilizers have been used in California (Christensen et al., 1982). Soil applications of Zn are more effective under drip than furrow irrigation. Research has shown that neutral- or basic-Zn products are the most effective Zn fertilizers (Christensen and Peacock, 2000).

Timing of fertilization events

Nitrogen and potassium are required by the grapevine throughout its growth cycle. It has been shown that the major sink (the organ that requires the most of a particular mineral nutrient) for N is the leaves while the fruit is the major sink for K (Williams, 1987; Williams et al., 1987; Williams and Biscay, 1991). Approximately, two-thirds of the vine’s annual requirement for N occurs between budbreak and several weeks after berry set. This is the period when the canopy is formed by the vine. The remaining third of the vine’s annual requirement of N goes to the fruit after berry set. It should be pointed out that a portion of the N requirements of a grapevine could be derived from N reserves in the roots and other permanent structures of the vine.