Impact of Shoot and Cluster Thinning on Yield, Fruit Composition, and Wine Quality of Corot noir

Yield components and vine growth.

In 2008, CL decreased yield through a decrease in cluster number per vine compared to the control (Table 1). However, individual cluster weight was increased by both ST and CL. Cropload ratio was decreased by both ST and CL. The ST by CL interaction was significant and had an additive effect for cluster number per vine and berry weight.

In 2009, CL resulted in fewer clusters per vine compared to the control, but the increased average cluster weight resulted in CL having no impact on yield per vine, similar to results reported for Seyval blanc (Reynolds et al. 1986). ST significantly reduced yield per vine in 2009 through a decrease in cluster number (Table 1). Crop-load ratio was reduced by ST, while CL surprisingly had no impact on crop load at harvest. Individual cluster weight was increased 20 g by ST and CL separately (Table 1). The ST*CL interaction was significant with an additive effect for yield per vine, clusters per vine, and berry weight in 2009.

Pruning weights and yields in both years, but particularly in 2009, confirmed the grower reports of high vigor in Corot noir. Pruning weights reported here for 2008 (ranging from 2.6 to 4.5 kg/vine, or 1.1 to 1.8 kg/m canopy) were considerably higher than those reported for other hybrids on nondivided systems in other cool-climate growing regions (Dami et al. 2006, Reynolds et al. 1995, 2004) as well as in New York (Sun et al. 2011). Concord in New York is generally reported to have pruning weights below ~2 kg/vine (0.8 kg/m canopy) (Bates 2008, Poni et al. 1994), suggesting that Corot noir, in some instances, may be higher in vigor than Concord. However, hybrids in Arkansas have been reported to have similar vigor to that reported here for Corot noir (Morris et al. 1984, 2004). Vines in all treatments of this study had crop loads that were below the generally accepted range for hybrids (Dami et al. 2006, Reynolds 1989, Reynolds et al. 2004), likely resulting in increased vegetative growth of vines.

ST and CL separately reduced canopy density as reflected by the reduction in OLN and a similar decrease in CEL in 2008. While there were fewer shade-producing contacts in the canopy as well as fewer shading layers between clusters and the nearest canopy boundary, the percentage of photon flux that reached the clusters (CEFA) and leaves (LEFA) did not differ among treatments, suggesting that if changes in canopy density impacted fruit growth and composition, it was not through changes in cluster or leaf light environment. ST and CL had no impact on OLN, CEL, CEFA, or LEFA in 2009 (Table 2). CEL and OLN were generally higher and CEFA and LEFA were lower in 2009 compared to 2008, suggesting a more dense canopy through the fruiting zone despite the fact that pruning weights in 2009 were less than half of those reported for 2008 (Table 1).

Differences between years in yield per vine and pruning weight were striking, with vines in 2009 producing less than half the yield and pruning weight of 2008. A spring frost in May 2009 resulted in loss of ~30% of primary shoots. Our observations indicated that secondary shoots were extremely fruitful; however, the average cluster weight in 2009 was generally at least 100 g less than in 2008, and berry number per cluster was also considerably lower. Additionally, yield was likely reduced in 2009 as a result of the excessive vigor of the vines in 2008, resulting in poor floral bud initiation in that year.

Fruit and wine composition.

In 2008, soluble solids was increased by 2.5 Brix with CL compared to the control (Table 3), but was not affected significantly by ST. However, pH increased slightly and TA decreased as a result of ST, while CL had no impact on either parameter. In 2009, berry pH and soluble solids were increased by ST and CL, although the increases were low (<0.8 Brix compared to the control). TA decreased slightly as a result of ST (Table 3), but was unaffected by CL, a result that has been similarly reported in other hybrid studies investigating CL (Dami et al. 2006, Morris et al. 1987). Decreases in TA during ripening are due to both K⁺/H⁺ exchange and respiration of malic acid (Boulton et al. 1996). Malic acid respiration increases at higher berry temperatures, which can result from greater cluster exposure. However, since the treatments did not alter cluster light environment, greater potassium uptake may be a more plausible explanation for the decrease in TA observed with ST. Greater potassium uptake by vines subjected to ST and/or CL is also the most likely explanation for the increase in pH observed with both practices. However, the biological relevance of a pH of 3.53 compared to 3.58 is questionable.

Berry anthocyanin concentration, expressed as malvidin-3-glucoside equivalents, ranged from 990 to 1260 mg/kg berry weight across treatments and years (Table 4). These concentrations are comparable to several of the more intensely pigmented V. vinifera cultivars such as Cabernet Sauvignon (Boulton et al. 1996). The concentration of anthocyanins in wines ranged from 670 to 900 mg/L, or a mean of 67% of the concentration observed in grapes, with a maximum of 86% observed for the ST + CL treatment. Both of these values are about a factor of 2 greater than typical anthocyanin concentrations (300 to 500 mg/L) and typical extraction efficiency (20 to 50%) for red wines (Boulton et al. 1996). Anthocyanin extraction efficiency is known to vary among cultivars and sites (Romero-Cascales et al. 2005), although the factors that limit anthocyanin extractability are not well understood. The pH differential method used here is designed to quantify monomeric anthocyanins, and a possible explanation for our observed results is that the low tannin concentration of the Corot noir wines (described below) resulted in reduced polymeric pigment formation and consequently less immediate loss of the anthocyanins. Interestingly, we also observed a significantly higher extraction ratio in 2009 than in 2008 (75% vs. 59%, p < 0.05), although the reasons for this difference are not obvious, as the same fermentation procedures were followed in both years. No significant difference in berry skin weight (data not shown) or berry weight (Table 1) was observed, so greater adsorption to solids seems unlikely.

Berry skin tannin and seed tannin concentrations ranged from 0.2 to 0.3 mg/g berry and 0.8 to 1.2 mg/g berry, respectively, which are comparable to some V. vinifera cultivars (Harbertson et al. 2002). Berry skin tannin was not affected by ST or CL in 2008, although berry seed tannin was reduced slightly by ST (Table 4). Berry skin tannin was decreased by CL in 2009, but was unaffected by ST; seed tannin was not affected by either practice. Wine tannin was decreased by both ST and CL in 2009, but the concentrations of wine tannin (42 to 64 mg/L) were an order of magnitude below typical concentrations observed in red wines made from V. vinifera cultivars (Harbertson et al. 2008). The tannin extractability was very low (6%) (Table 4) compared to the wine made from V. vinifera species (4.9 to 61%) (Harbertson et al. 2002, Bindon et al. 2010, Adams and Scholz 2008). This finding is consistent with a previous study that reported a tannin extractability of 2 to 4% for the hybrid cultivar Marechal Foch (Sun et al. 2011). The low tannin extractability may be partly due to low skin tannin concentrations in Corot noir (0.2 to 0.3 mg/berry) as compared to V. vinifera, as skin tannin appears to be more rapidly and effectively extracted during fermentation (Sampaio et al. 2007). Additionally, insoluble grape cell wall material can bind to tannins and limit its extractability (Bindon et al. 2010). Potentially, the cell wall material of hybrid winegrapes may bind tannins more strongly than in V. vinifera, although that would need to be investigated.

ST and/or CL has been reported to improve fruit composition (particularly soluble solids) in the more vigorous hybrid cultivars, at least in some years (Reynolds et al. 1986, Wolpert et al. 1983, Prajitna et al. 2007, Dami et al. 2006). The results of this study were variable with respect to yield components and vine size, but CL increased soluble solids in both years while ST reduced TA (Table 3). CL also reduced wine tannin in both years as indicated by p values compared to the control (Table 4), which could be considered a detrimental impact.

Cost of implementing practices.

A simple accounting analysis of the costs of implementing CL and ST was performed in order to depict a realistic financial scenario for commercial Corot noir growers of the potential costs and benefits of adopting these practices. Variable production costs of growing winegrapes in the New York Finger Lakes region were estimated based on a published report of V. vinifera production (White 2008). The two primary costs associated with implementing ST and/or CL are (1) additional vineyard labor hours required to complete the thinning in consort with other ongoing field practices and (2) potential opportunity cost if total yield is decreased as a result of either practice and thus there are fewer grapes to sell and/or make into wine. It is likely that the cost of labor estimates used in our analysis that were determined for V. vinifera (White 2008) are lower than would be required for hybrids due to the generally larger canopy size of hybrids; therefore, required prices to compensate for labor costs may be higher than reported here. If a grower has to spend money to implement cluster and/or ST, and then has fewer grapes to sell at the standard industry price, then the grower can expect to receive lower total revenue for their crop. Logically, if a grower expects to maintain the same level of welfare (total revenue) following implementation of ST and/or CL, the price they charge for Corot noir grapes would need to be above market price.

In 2008 and 2009 the average prices for Corot noir grapes in the Finger Lakes were $570 and $503 per tonne, respectively (Walter-Peterson 2008, 2009). The price per tonne of Corot noir grapes in 2008 that a grower would need to charge in order to maintain constant welfare would increase from a base market price of $570 per tonne for the control up to $865 per tonne for the ST+CL vineyard (Table 6). For the same parameters in 2009, the price per tonne to maintain constant grower welfare would need to increase from $503 to $738 per tonne. The variability in these prices reflects the specific practices implemented (Table 6). The increase in price per 750 mL bottle required to compensate for the higher fruit costs plus additional vineyard labor ranged from $0.09 to $0.41 in 2008 and from $0.02 to 0.32 in 2009. It is unclear whether hybrid fruit growers who implement ST and/or CL could recapture their costs by charging substantially higher market prices or whether grape buyers would be willing to pay such a premium without compelling enhancements to fruit quality or flavor. However, the sensory analysis results in this study indicate a consistent increase in fruitiness with CL (alone in 2008 or combined with ST in both years), a characteristic positively associated with consumer preference (Bogart and Bisson 2006). Wine consumers may be willing to pay more for such wines or may be more likely to purchase the wine again. Ultimately, a hybrid grapegrower’s decision of whether to implement thinning practices is best determined based on a rational analysis of the potential costs and benefits similar to the one presented here.