Study Finds T-Trellis Raises Sunpreme Raisin Yields by About 10%

A three-year field study in California’s San Joaquin Valley found that two vineyard design choices, trellis system and rootstock, can raise yields in Sunpreme, a raisin grape bred to dry naturally on the vine, while heavier nitrogen use can lower raisin quality.

The research was published Thursday in OENO One by Matthew Fidelibus and co-authors. The trial ran from 2021 through 2023 at the University of California Kearney Agricultural Research and Extension Center in Parlier, in Fresno County. Researchers tested two trellis systems, a two-wire T-trellis and a high single-wire trellis, three rootstock options, and two annual nitrogen rates to measure effects on productivity, fruit drop, raisin quality, and vine nutrition.

Sunpreme is a relatively new variety in California. It was released in 2015 as the first natural dry-on-vine, or NDOV, raisin grape. That means berries can dry on the vine without the cane-cutting step used in standard dry-on-vine systems. The trait matters because California raisin growers have been under pressure for years from labor shortages, rising labor costs, and competition from other producing countries. A variety that can be harvested and pruned more easily by machine could lower costs and change how raisin vineyards are managed.

The study found that the T-trellis raised yield by about 10% compared with the high single-wire system. That gain came with a slight decline in raisin quality. Rootstocks also lifted yield by about 10%, compared with own-rooted vines. The paper said none of the treatments changed preharvest fruit drop, an important point for Sunpreme because the variety has been associated with shatter concerns before harvest.

Nitrogen produced a more complicated result, but the direction was clear. The higher annual nitrogen rate, 90kg per hectare, brought only modest production benefits at most and reduced raisin quality. In one part of the paper, the authors said the higher rate did not increase raisin yield; elsewhere they described the yield effect as modest. In both cases, they said quality moved lower with more nitrogen. Their overall recommendation was that annual applications of 44.8kg to 56kg N per hectare appear sufficient to sustain yield while maintaining raisin quality in NDOV vineyards.

The experiment compared vines on their own roots with vines grafted to Freedom and 1103P rootstocks. Those choices matter in the San Joaquin Valley because rootstocks are commonly used to protect vines from soil pests, and because different root systems change how vines take up water and nutrients. Freedom has long been widely used in raisin vineyards, though growers have also been looking more closely at other options, including 1103P, as sudden vine decline has become a concern in some plantings.

The nutrition results showed that rootstock had a strong effect on the mineral profile of the vines. Vines on Freedom had the highest boron, potassium, and total nitrogen levels, and the lowest zinc. Vines on 1103P had the lowest potassium and the highest magnesium. Own-rooted vines had the lowest nitrate nitrogen, calcium, and magnesium, but the highest zinc. The researchers said those differences could affect management decisions depending on soil type.

Most measured nutrients were generally in ranges considered adequate. Potassium and zinc stood out as the main nutrients that may still need close attention. That is especially relevant in high-yield systems, where potassium shortages can slow ripening, and in vineyards using rootstocks with known zinc issues.

The team also calculated how much nutrition leaves the field in harvested raisins, a practical measure for fertilizer planning. For each metric ton of raisins, the fruit contained about 7.3kg of potassium, 5.8kg of nitrogen, 0.345kg of magnesium, 0.0089kg of boron, and 0.0025kg of zinc. Depending on nitrogen treatment, harvested raisins removed about 44.8kg to 58.3kg N per hectare. The authors said those values offer working estimates for nutrient replacement in Sunpreme vineyards.

The field work was done in a vineyard planted in 2019. Researchers used a split-plot design across four blocks. Rows were spaced 3.5 meters apart, with vines 2 meters apart within rows. Nitrogen was applied through fertigation at two rates, 45kg or 90kg N per hectare, split into two applications, one about a month after budbreak and another at fruit set. Irrigation followed a deficit program that reflected common raisin vineyard practice in the region.

To measure fruit drop, the team placed weed mats under selected vines during the summer and collected berries that fell before harvest. To measure fruit quality, they harvested raisins when moisture was at or below 16%, or sooner if poor drying weather was expected, and sent samples to the USDA Processed Products Division in Fresno for official grading. They also collected petiole and leaf blade samples at bloom and veraison to track nutrient status through the season.

The authors said published research on cultural practices for NDOV raisin grapes has been limited, especially for trellis choice, rootstock selection, and fertilizer needs. That makes the new findings useful beyond Sunpreme alone. Even though the trial focused on raisins, the results may also matter for beverage businesses that rely on California grape acreage and watch fertilizer use, fruit quality, and vineyard economics closely. Decisions on trellis layout, rootstock, and nitrogen can shape returns per acre and input efficiency in regions where land, water, and labor often shift among raisins, juice grapes, wine grapes, and other grape-based products.

The study also helps define a more practical path for growers considering mechanized raisin systems. The researchers noted that currently available vineyard equipment tends to work better with single-curtain trellis systems, while early Sunpreme testing had been done on broader quadrilateral systems. Their results suggest there is room to improve yield with trellis design, but also that management tradeoffs remain, especially when the goal is to protect quality while pushing production.

For growers choosing among rootstocks, the paper points to a second tradeoff. Freedom delivered strong nutrient uptake for boron, potassium, and nitrogen, which can support vigorous production, but it also showed the lowest zinc levels. By contrast, 1103P improved magnesium status but carried the lowest potassium levels, a factor that could matter in soils where potassium supply is already tight or in vineyards with heavy crop loads.