Effect of Variety, Cordon Height, and Irrigation on Mechanical Pruned Vineyard in Southern SJV

George Zhuang, UCCE Viticulture Farm Advisor in Fresno County

Introduction

Mechanical pruning (box pruning) is the standard practice for newly planted wine vineyards in the San Joaquin Valley (SJV). According to the 2019 SJV South winegrape cost study (Zhuang et al. 2019), mechanical pruning can reduce production costs from $3,000 to $2,500 per acre resulting in 17% cost savings. With the California minimum wage again increasing to $16 per hour in 2024, the labor savings from mechanical pruning could be even greater now. An understanding of how different varieties perform under mechanical pruning systems is desirable to advise growers on cultural practices for these varieties. Currently, the most used trellis system on wine vineyards are single-cordon or quadrilateral cordons without any catch wire. Cordon height varies from 52-72” above the vineyard floor across various vineyard sites. Also, irrigation management on mechanically pruned vines might be different from hand-pruned vines due to the different canopy shapes and shoot density.

Experimental Design

Four wine varieties including Cabernet Sauvignon, Petite Sirah, Petite Verdot, and Pinot Gris were planted in August 2017, hand pruned in 2018 and 2019, with mechanical pruning starting in 2020. The rootstock used was 1103P for Cabernet Sauvignon, Petite Verdot, and Pinot Gris, while 5BB was used for Petite Shira. The vines were planted with a 6’ by 10’ vine by row spacing. A split block design (2 ×2) was applied for Cabernet Sauvignon and Petite Shira with two cordon heights and two irrigation treatments replicated three times. The High Cordon height was set at 68”, while the Low Cordon height was set at 52” above the vineyard floor (Figure 1). Sustained Deficit Irrigation (SDI) and Regulated Deficit Irrigation (RDI) were applied to both varieties. SDI was maintained at 80% crop evapotranspiration (ETc) through the entire growing season, while RDI used 60% crop ETc from berry set to veraison then switched to 80% ETc from veraison to harvest. The field plot is illustrated in Table 1.

Figure 1. Vines on low and high cordons side by side

Results

Data collected on Cabernet Sauvignon show no yield differences between the cordon heights in any of the 4 years (Table 2). The only consistent difference across all years that the data was collected was in leaf area. In all three years that that data was collected (2021 – 2023) the high cordon had more leaf area per vine. Clusters per vine did show differences in 2020 and 2021. However, in 2020 it was the high cordon with more clusters per vine, while in 2021 it was the low cordon that had more clusters per vine. There were no differences in cluster numbers for 2022 and 2023. Berry weight did not show differences across all four years of data. Berry ripeness (Brix, pH, and TA) only showed differences in 2021 where the high cordon was more mature (higher Brix and pH, and lower TA). In the remaining years, no differences in berry ripeness were found.

Data collected on Pinot Gris showed even fewer differences between high and low cordon heights. Clusters per vine, Berry weight, Brix, pH, and TA all showed no differences across all three years of data collection. The only data to show differences was for yield in 2022 where the high cordon height outproduced the low cordon height.   

Summary

·        Cordon height mainly affected the leaf area for Cabernet Sauvignon but not for Petite Sirah (Petite Sirah data not shown). High vigor is the key driver to take maximum advantage of a higher cordon to establish the full canopy (Petite Sirah is less vigorous than Cabernet Sauvignon).

·        No yield difference was found between high and low cordon for red cultivars in our study; however, high cordon did increase the yield for Pinot Gris. Higher harvest Brix was found from higher cordon due to its larger canopy (more leaf area per vine). Higher cordon tended to produce better color (berry anthocyanins) due to the elevated cluster zone far away from the vineyard floor resulting in cooler cluster temperature.

·        No yield difference was found between RDI and SDI, although smaller berry size was noticed in RDI which tended to produce higher color (smaller berry with higher skin/pulp ratio).

·        With the recent advancements, mechanical pruning can now be also applied to CA raisin production. This is due to the revolutionary natural dry-on-the-vine (DOV) raisin varieties such as Ras-1 (BLOOM Fresh) and Sunpreme (USDA ARS) which do not require cane cutting, and whose basal buds are fruitful. The results from this wine grape trial might also be implied to natural DOV raisin production.

Further readings

Zhuang, S., Fidelibus, M., Kurtural, K., Lund, K., Torres, G., Stewart, D. and Sumner, D. 2019. Sample costs to establish a vineyard and produce winegrapes in southern San Joaquin Valley (Fresno, Madera, Merced, and Stanislaus Counties) – Cabernet Sauvignon, Rubired, Colombard, and Chardonnay varieties. Grapes–Wine | Cost & Return Studies (ucdavis.edu)