The San Joaquin Valley (SJV) of California leads the state in wine grape acreage and tons produced (California Department of Food and Agriculture, 2012). Previously, southern regions of the SJV were classified as only suitable for table grape production (Region V) as a result of their annual growing degree-day (GDD) accumulation (Amerine and Winkler, 1944). However, as the competition for arable land suitable for wine grape production increased in the state, areas that were previously not considered such as Region V are now planted to wine grapes. Much of the wine grapes planted in southern SJV are grown on a two- or three-wire single-curtain, non-shoot-positioned trellis (Gladstone and Dokoozlian, 2003) commonly referred to as the California sprawl. This trellis type, although not capital-intensive to install but is often used improperly, results in excessive fruit zone shading with procumbent cultivars, i.e., Syrah (Dokoozlian and Kliewer, 1995; Terry and Kurtural, 2011). Because profit margins are narrow, and maximum yield is paramount, balanced cropping in this region is not possible resulting in large canopies with declining yield and less than ideal fruit composition at the farm gate (Wessner and Kurtural, 2013). The declining yield in southern SJV wine grape vineyards is attributed to mechanical box pruning and irrigating to field capacity that exacerbates the excessive fruit zone shading for procumbent (drooping/trailing growth habit) cultivars (Wessner and Kurtural, 2013). This also results in mutual shading within the canopy in the current season and depresses bud fruitfulness in subsequent seasons (Sanchez and Dokoozlian, 2005).
Several studies have shown that node number per vine is a not an accurate or a precise regulator of the final crop level in vineyards (Bernizzoni et al., 2011; Geller and Kurtural, 2013; Poni et al., 2004; Terry and Kurtural, 2011). There is agreement in recent literature that attempts to balance grapevine yield with increased pruning severity fails because of the unpredictable vegetative and reproductive compensating responses from the grapevine (Bernizzoni et al., 2011; Geller and Kurtural, 2013; Kurtural et al., 2006). The compensating responses may include enhanced bursting of secondary shoots (Kurtural et al., 2006; Main and Morris, 2004) as well as more fruitful shoots stemming from latent buds on the cordon and basal buds that are not counted during balanced pruning procedures (Kurtural et al., 2006; Poni et al., 2004). Further enhancements to achieve specific yield targets are therefore achieved with shoot and cluster thinning through manual and mechanical means (Geller and Kurtural, 2013; Kurtural et al., 2006; Terry and Kurtural, 2011). However, these practices require rigorous crop estimation and maybe economically prohibitive (Kurtural et al., 2012) because their application by manual methods may increase labor operation time in the vineyard up to 50 to 60 h·ha−1 (Geller and Kurtural, 2013; Intrieri and Poni, 1995).
Shoot thinning in vineyards is applied when average shoot length is between 15 and 25 cm. Smart (1988) reported optimal shoot densities to achieve desired fruit composition and wine sensory properties for cool climate viticulture. He emphasized improved canopy microclimate and function can be achieved with shoot densities of 15 to 25 shoots/m of a row depending on cultivar and location influence. Recently, Geller and Kurtural (2013) recommended a shoot density of 35 shoots/m of a row with a hypocumbent (upright growth habit) cultivar to improve canopy microclimate and to achieve optimum Ravaz Index with a sustainable yield in a warm climate region (Region V of California). Geller and Kurtural (2013) further reported that decreasing shoot density did not provide any further benefit to canopy function. Increasing the severity of shoot thinning resulted in a lack of physiological response as a result of vegetative compensation by the grapevine generating larger leaf area by the remaining shoots on a sparsely populated canopy. Terry and Kurtural (2011) reported a shoot density of 23 shoots/m of optimized canopy architecture and maintained yield in a Region IV vineyard with improved berry phenolic composition at the farm gate when irrigation was reduced to 50% of ETo demand between fruit set and veraison for ‘Syrah’ grapevine.
Although there have been various reports on comparisons of mechanical box and spur pruning as well as determination of optimal shoot density in wine grape vineyards, most of these trials were conducted in cooler climates and did not investigate the interaction between them. There are not many specific studies published on mechanical shoot thinning effects on the grapevine grown in Region V with a procumbent grape cultivar such as Syrah. Therefore, our study was designed to investigate how the interaction of pruning systems and mechanical shoot removal affected canopy performance, yield components, fruit phenolic composition at harvest, and production efficiency of a procumbent cultivar in a warm climate region.
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