Abstract
A production trial in the San Joaquin Valley of California was conducted where canopy microclimate of Syrah 05/SO4 grapevines was altered through three pruning systems and two leaf removal treatments arranged factorially to rejuvenate vineyards with declining productivity. Vines were either pruned by hand to 44 nodes each, mechanically box-pruned to a 10 cm hedge, or cane-pruned by hand to six 8-node canes arranged in opposing directions of the row with horizontal canopy separation. Outer surface layer of leaves were either removed mechanically 20 days postbloom on the east side of the canopy in a 45 cm zone above the cordon in the fruit zone or not removed. Yields from spur- and mechanically box-pruned vines were considered too low for the study area, and leaf removal had no effect on yield components. Spur-pruned vines reached 24 Brix earlier than mechanically box-pruned and cane-pruned vines in each year. Leaf removal had no effect on fruit composition of Syrah at harvest. Berry skin phenolics were not consistently affected by treatments applied. Cane pruning resulted in 3.8 leaf layers, with 32 shoots per 30 cm of row, 7.77 kg/kg Ravaz index, and consistently ripened 22 tons/ha to 24 Brix and should therefore be used in the San Joaquin Valley to improve yields in vineyards with declining productivity. The study identified a pruning system for vineyards in warm climates that can sustain yields and provides management information for growers on how to rejuvenate vines that have declined in productivity.
For many San Joaquin Valley (SJV) winegrape growers in California, management of their vineyards with insufficient guidelines for vigorous cultivars, such as Syrah, to produce high-quality fruit is a challenge. Winegrapes planted in the SJV are often grown on a two- or three-wire single-curtain, non-shoot-positioned trellis (Gladstone and Dokoozlian 2003). This trellis type, while not capital intensive to install, is often used improperly, resulting in excessive fruit-zone shading under vigorous conditions (Dokoozlian and Kliewer 1995, Terry and Kurtural 2011). Since profit margins are narrow and maximum yield is paramount, balanced cropping is not possible, resulting in large canopies with declining yield and less than ideal fruit composition at the farm gate (Terry and Kurtural 2011). The declining yields in the SJV are also attributed to mechanical box pruning, which also exacerbates the excessive fruit-zone shading that retains more than 30 buds per 30 cm of cordon, whereby fruit-zone shading in the current season depresses fruitfulness of buds in subsequent seasons (Sanchez and Dokoozlian 2005). Literature indicates consistent production in vineyards is achieved by balanced cropping (Howell 2001, Morris 2007, Terry and Kurtural 2011). Balanced cropping aims to achieve equilibrium between vegetative and reproductive growth of the grapevine, and thus ensures sustained vineyard production. Post-fruit-set leaf removal has been shown to improve canopy microclimate such as percent gaps and amount of light transmitted through the fruiting zone without adversely affecting yield in warm climate (Main and Morris 2004).
There does not appear to be a method developed for warm climate winegrape growers to rejuvenate yields after declining productivity. The objective of this study was to test the interactive effects of pruning systems and leaf removal on canopy architecture and microclimate, yield, fruit composition, and Ravaz index of Syrah grown in a warm climate and identify a method where yield can be sustained without adversely affecting fruit and berry skin phenolic composition.
Materials and Methods
Plant materials and site. This study was conducted from 2010 to 2011 at a commercial vineyard planted with Syrah 05/SO4 grapevines at 2.3 m × 3.4 m (vine × row) spacing in north-south oriented rows. The research site was located in Kern County, CA (35°00.322’N; 118°53.808’W, elev. 137 m) and was planted in 1999 on Premier sandy-loam soil, a coarse-loamy, mixed, superactive, calcareous thermic Xeric Torriorthent and described as a deep, well-drained soil formed in alluvium from granite rocks (www.nrcs.usda.gov/). The rowing degree day (GDD) accumulation at the research site was 2566 and 2561 (10°C base), and 404 mm and 178 mm of precipitation were received in 2010 and 2011, respectively. The vines were trained to a bilateral cordon at 1.35 m with two foliage support wires at 1.70 m and a 20 cm U-top. The yield at the vineyard increased steadily to 22 tons/ha by the fifth leaf. After the sixth leaf the vineyard was mechanically box-pruned where yield increased initially to 26.2 tons/ha, after which it declined to 11.2 tons/ha by the time the study commenced.
The vineyard was drip-irrigated with pressure compensating emitters delivering 2.1 L/h/vine. Precipitation was insufficient to fill the soil profile in both years. The root zone was irrigated beginning at budburst based on a crop coefficient (Kc) of 0.2 and 80% of the daily evapotranspiration (ETo) obtained from the CIMIS station in Arvin, CA. Irrigation was interrupted before bloom until midday leaf water potential (Ψ) was below -1.2 MPa. After that point, 50% of daily ETo was applied until veraison. Irrigation was triggered when midday Ψ was below -1.4 MPa. Starting at veraison, irrigation was raised to 80% of daily ETo with a Kc of 0.8 and irrigation was triggered when midday leaf Ψ was below -1.2MPa.
The experimental design was a 3 × 2 factorial (three pruning systems and two leaf removal types) with four replications of 386 vine plots of which 24 were sampled based on a grid pattern of every 76th vine or 21 m apart. An integrated pest management program was used. All other cultural practices were standard for the area and conducted according to the University of California Cooperative Extension guidelines.
Pruning treatments. Three pruning treatments were applied: spur, cane, and mechanical box (“box pruning”). Spur-pruned vines were manually pruned to retain 44 nodes on 22 spurs (control). Cane-pruned treatments were manually pruned to six 8-node canes with one renewal spur with two nodes retained per spur. Four of these canes were tied onto foliage catch wires separated 25 cm horizontally at 1.7 m above vineyard floor, on either the north or south side of the vine, and the other two canes were tied to the cordon wire that was 25 cm below the catch wires, creating horizontal and vertical separation of canopy. The box-pruning treatment consisted of hedging to a 100 mm spur height with a 600 mm sprawl-pruner (model 63700; Oxbo International, Kingsburg, CA) to a node density of 15 nodes per 30 cm of row.
Leaf removal treatments. Two leaf removal treatments were applied in the fruiting zone. Leaves were either removed 20 days postbloom in a 45-cm zone in the fruiting zone above the cordon or were not removed. The surface layer of leaves were removed with a vacuum-type mechanical leaf puller (model 62084; Oxbo International) that consisted of a rotating drum that drew in air and leaves that were sheared from the vine with a sickle bar. Leaves pointing to the interior of the canopy were not removed.
Canopy architecture and microclimate. Shoot density was measured two weeks after budburst as described elsewhere (Terry and Kurtural 2011). Total shoots per vine were counted by the addition of count shoots (borne from count positions >5 mm distal to the base of the cane) and noncount shoots (borne from positions <5 mm distal to the base of the cane and secondary shoots). Leaf layer number and percent photosynthetically active radiation (PAR) in the fruiting zone were measured using the methods described by Gladstone and Dokoozlian (2003).
Yield, fruit composition, and Ravaz index. Fruit yield and cluster numbers for each treatment were measured on different dates as they reached 24 Brix by hand-harvesting 24 vines from each experimental unit. Average cluster weight was calculated by dividing fruit yield per vine by the number of clusters harvested. Ravaz index was calculated by dividing the yield per vine by the dormant pruning weight per vine and is expressed as kg/kg. Fruit composition at harvest was measured using standard methods as reported elsewhere (Terry and Kurtural 2011). Samples of 20 random berries at 24 Brix for each treatment at harvest were used for determination of anthocyanins, total iron reactive phenolics, and tannins as reported elsewhere using the University of California assay (http://wineserver.ucdavis.edu/adams/tannin/totalassay.pdf).
Statistical analysis. The data were initially subjected to three-way analysis of variance (year × pruning × leaf removal). Year × pruning failed the Levene’s test. Therefore, the results were subjected to two-way (pruning method × leaf removal) type III test of fixed effects using the MIXED procedure of SAS statistical software (ver. 9.2.; SAS Institute, Cary, NC), appropriate for randomized complete block with a factorial arrangement of treatments. Treatment means were considered significantly different by Tukey’s honestly significant difference test at p < 0.05.
Results
Canopy architecture and microclimate. The pruning method used in 2010 and 2011 affected the number of total shoots borne per 30 cm of canopy in Syrah grapevines (Table 1). In 2010, the percent PAR in the fruit zone of Syrah was higher in box-pruned vines compared to spur- or cane-pruned vines. In 2010, the leaf removal treatment also increased percent PAR in the fruit zone of canopy compared to no leaf removal. In 2011, pruning method and leaf removal interacted to affect percent PAR in the fruit zone, where it was 66% higher for box-pruned vines with leaf removal than for cane-pruned vines without leaf removal. In 2010, cane-pruned Syrah vines had higher leaf layer numbers than spur- and mechanically box-pruned vines. In both years leaf removal decreased leaf layer numbers in the canopy.
Effects of pruning system and leaf removal on canopy architecture, microclimate, and yield components of Syrah 05/SO4 in 2010 and 2011.
Yield components. In 2010 and 2011, the average number of clusters harvested was higher on cane- and box-pruned vines than on spur-pruned vines (Table 1). In both years, the average berry weight at harvest was 15% higher on spur-pruned vines than cane- and box-pruned vines. In 2010, pruning method and leaf removal interacted to affect average cluster weight where average cluster weight of spur-pruned vines without leaf removal was higher compared to cane-pruned vines without leaf removal. In 2011, the average cluster weight was higher on spur- and caned-pruned vines than on box-pruned vines. In both years of the study, cane pruning increased yield by 26% compared to the other two pruning treatments and the Ravaz index of cane-pruned vines was higher.
Fruit composition. Regardless of treatment combination, the Syrah grapevines reached the harvest target of 24 Brix in 2010 and 2011. However, in 2010 and 2011, cane-pruned vines reached 24 Brix five to seven days later than spur- and box-pruned vines, respectively. In both years, leaf removal did not affect Brix accumulation (Table 2). In 2010, juice pH of spur-pruned vines was consistently higher than cane- and box-pruned vines throughout the ripening period. In 2011, titratable acidity (TA) of box-pruned vines was higher than spur- and cane-pruned vines. In 2011, TA of juice from vines with leaf removal was higher than that from vines without leaf removal early in the ripening period, but leaf removal had no effect on TA at harvest.
Effects of pruning system and leaf removal at harvest on fruit composition and berry skin phenolics of Syrah 05/SO4 grapevines, 27 Aug 2010 and 27 Aug 2011.
In 2010, pruning and leaf removal interacted to affect total phenolics and tannins in the berry at harvest. Anthocyanins in the berry skin were not affected in 2010 (Table 2). Total phenolics of cane-pruned vines with no leaf removal were 34% higher than spur-pruned vines with no leaf removal. Tannins of spur-pruned vines with leaf removal were 54% higher than that of box-pruned vines with or without leaf removal. In 2011, there was no effect of pruning method or leaf removal on berry skin phenolics.
Discussion
Canopy architecture and microclimate. Results from this study indicate that dormant pruning alone was not enough to manipulate canopy architecture and microclimate, therefore leaf removal was implemented to create a more favorable canopy microclimate by improving light exposure and air circulation within the canopy (Bledsoe et al. 1988, Poni et al. 2009, Tardaguila et al. 2010). Mechanical box pruning decreased leaf layer number to 3.2, which is consistent with previous reports (Reynolds 1988). All pruning methods were within the optimal range for leaf layer number between 3.0 and 4.0 (Reynolds and Vanden Heuvel 2009, Terry and Kurtural 2011). Leaf removal decreased the leaf layer number in both years to 2.8, as reported elsewhere (Bledsoe et al. 1988).
Yield components. Spur pruning reduced yield and clusters harvested, while berry and cluster weights were increased compared to other reported treatments that retained more nodes (Bates 2008). The final berry weight indirectly affected the phenolic concentration of the wine; therefore, the concentration depended on the skin surface-to-berry volume ratio as reported by Ojeda et al. (2002). Therefore, smaller berries, as seen with cane pruning and box pruning, may be more desirable because of a higher skin-to-pulp ratio (Morris 2007). According to Shaulis and May (1971) and Smart (1988), reducing shoot crowding improved radiation microclimate and hence an increase in yield was seen. Mechanical box pruning resulted in higher yields than spur pruning, and a decrease in yield in the second year of the study was seen as reported elsewhere (Andersen et al. 1996), indicating that selective thinning of shoots from the bearing surface was eventually needed to sustain yield (Keller et al. 2004, Terry and Kurtural 2011) with mechanical box pruning.
There was a strong positive relationship between the number of clusters harvested and yield in both years of the trial as reported elsewhere (Bates 2008, Terry and Kurtural 2011). Bates (2008) also reported results similar to the strong positive relationship between yield and shoots per vine in 2011. Therefore, it was important to find the appropriate number of exposed shoots per unit of canopy in order to achieve the maximum yield without increasing shade within the canopy, which would have reduced quality and/or bud fruitfulness in the following year (Smart 1988). A reduction in berries per cluster and yield per vine would have been expected with early leaf removal. Our results confirm that postbloom leaf removal did not reduce yield, as such was our aim (Bledsoe et al. 1988), or its components (Main and Morris 2004).
The pruning system and leaf removal treatments applied in this study were able to bring Ravaz index close to the optimum value of 10 kg/kg. Spur and box pruning resulted in 0.5 to 1.0 kg/m pruning weight per meter (data not shown), a range identified as sufficient to achieve vine balance (Kliewer and Dokoozlian 2005). In 2010 all pruning methods and canopy management practices resulted in a Ravaz index within the optimal range. However, in 2011 all treatments except for cane pruning with leaf removal were less than 5.0 kg/kg. Cane pruning in 2011 led to a decrease in exposed shoots per vine, resulting in minimal changes in pruning weight per meter of row and yield. Although the Ravaz index did decrease in 2011, it was maintained within the optimal range of 5 to 10 kg/kg.
Fruit composition. Recent studies have confirmed the effects, especially of canopy architecture, on fruit composition (Keller et al. 2004, Bates 2008, Terry and Kurtural 2011). Consistent with this, Brix and pH of spur-pruned vines were slightly higher and TA was lower than of box-pruned vines (Keller et al. 2004). Contrary to one report (Keller et al. 2004), TA of box-pruned vines was higher than other treatments at veraison and was higher at harvest. In general, Brix, pH, and TA seemed to be related to berry size. Keller et al. (2004) found a negative correlation between Brix and yield. In 2011, this was true for spur and cane pruning, where spur pruning resulted in the lowest yield with the highest Brix at harvest and cane pruning resulted in the highest yield with the lowest Brix. Yield of spur- and box-pruned vines was comparable, but the Brix of box-pruned vines was similar to cane-pruned vines because berry weights of the two treatments were statistically the same. Contrary to Keller et al. (2004), who reported no relationship between berry size and fruit composition, larger berry size was associated with higher Brix and pH and lower TA as a result of spur pruning, in 2011.
Consistent with Bledsoe et al. (1988), leaf removal decreased canopy density and increased percent PAR transmission; however, it had no effect on juice pH and TA, as it did in the previous studies. Bledsoe et al. (1988) reported that leaf removal increased Brix at harvest. However, in this study there was no consistent effect on Brix accumulation by leaf removal treatments. Contrary to previous studies, postbloom leaf removal did not increase Brix (Bledsoe et al. 1988). Improved fruit composition was previously achieved with prebloom leaf removal by decreasing berry size and therefore increasing the skin-to-pulp ratio (Ojeda et al. 2002). However, in the current study, using postbloom leaf removal, there was no difference in berry size among leaf removal treatments that explained why leaf removal did not affect Brix. Postbloom leaf removal treatments had no added benefits on Brix, pH, and TA in either year. Therefore, leaf removal is not a reliable canopy management method for improving fruit composition in warm climates.
Spur pruning did not lead to increased berry skin anthocyanins compared to box pruning, contrary to Keller et al. (2004). Previous studies have found that increased sunlight exposure to the berries generally results in increased berry skin phenolics (Dokoozlian and Kliewer 1996, Bergqvist et al. 2001). However, in the current study there was no increase in berry skin phenolics with the increase of percent PAR transmission associated with box-pruned and leaf removal treatments. Mechanical box pruning decreased leaf layers to 3.2 increasing percent PAR transmission and reducing berry weight, but berry skin phenolics were not affected. This was attributed to the macroclimate of the region where excess day and night time temperatures might be affecting anthocyanin accumulation in the berry skin (Bergqvist et al. 2001).
Conclusion
The methods presented here demonstrate the potential effects of mechanical dormant pruning and cane pruning combined with leaf removal methods as compared to spur-pruning only, on Syrah grapevines. The cane-pruning method described here decreased berry and cluster weight while increasing yield. Although spur and mechanical box pruning are more cost efficient than cane pruning, the yields of cane pruning were superior. Leaf removal treatments improved the canopy microclimate, but the vines were physiologically unresponsive to this treatment, evidenced by inconsistent results on berry phenolic composition. Data from this study indicate that cane pruning and canopy separation should be used in the warm climates of the San Joaquin Valley to increase yields if they decline due to lack of management after mechanical box pruning in consecutive years. The key to obtaining higher yields was to expose more nodes per shoot, increasing the height of the canopy, and separating it with the cane-pruning method. However, the Ravaz index for cane-pruned systems was still lower than the 10 kg/kg optimum because there was not enough separation within the canopy to intercept the optimal amount of light to optimize yield. Therefore, we recommend that a quadrilateral training system be used with a cordon height of 1.8 m for an ameliorated ratio of row spacing to canopy height. However, in the absence of a financial incentive to install a horizontally separated canopy, the method of cane pruning and laying the canes on existing catch wires was successful in this study. The study identifies a pruning system for vineyards in warm climates that can be applied instead of spur or mechanical box pruning and that can sustain yields and provide canopy management information for growers on how to rejuvenate vines that have declined in productivity.
Acknowledgments
The authors acknowledge the American Vineyard Foundation, the Bronco Wine Company Research Chair Trust funds, and Oxbo International Corp. for partial funding during the execution of this project. The authors also acknowledge the technical assistance of Geoffrey Dervishian, Joseph P. Geller, Semih Tangolar, and Serpil Tangolar.
- Received April 2012.
- Revision received July 2012.
- Accepted August 2012.
- Published online December 1969
- ©2013 by the American Society for Enology and Viticulture
Literature Cited
Vol 64 Issue 1
Jump to section
Related Articles
Cited By...
More from this TOC section
Similar Articles
