Excessive vine vigor often leads to issues such as poor canopy ventilation, inconsistent fruit ripening, and a high incidence of cluster rot. Hedging, or shoot tipping, is one common approach to managing vigorous vegetative growth in vertical shoot-positioned grapevines (Smart and Robinson, 1991; Wolf, 2008). A short-term solution, top hedging of positioned shoots, reduces canopy density only temporarily in the fruit zone if the shoots are hanging downward over the top wire. In addition, hedging ultimately results in more vigorous vegetative growth through newly emerged lateral shoots following the loss of apical dominance from shoot tip removal (Mason et al., 2014; Reynolds and Wardle, 1989). Clusters can also be affected by hedging. When shoot tips were removed early in the season, translocation of assimilates to the cluster improved the fruit set, leading to greater cluster compactness (Molitor et al., 2015; Vasconcelos and Castagnoli, 2000).
Emerging lateral shoots and compact clusters tend to exacerbate fungal disease incidence such as botrytis bunch rot caused by Botrytis cinerea and resulting from fruit zone shading, limited air circulation, and reduced fungicide penetration into clusters (Hed et al., 2009; Molitor et al., 2015). Symptoms of botrytis bunch rot include gray–brown mold sporulating growth covering grape berries that then shrink, shrivel, and change color to brown (white grapes) or red (purple grapes) when mature. Dull-green or reddish brown spots cover leaves, and brown spots cover pedicels and rachises, killing them and causing portions of the cluster to shrivel and drop.
Preliminary findings of one study showed that spray penetration into the clusters decreased linearly when cluster compactness increased (Hed et al., 2009). In addition, fruit zone leaf removal at trace bloom reduced cluster compactness by up to 26% and bunch rot by up to 83% (Hed et al., 2009).
There are few published studies on the impact of hedging as a canopy management tool. The timing of shoot tip manipulation can impact cluster and canopy growth. Early-season hedging at bloom or postbloom led to greater total leaf area and canopy density in the fruiting zone (Koblet, 1987; Molitor et al., 2015; Reynolds and Wardle, 1989). Delaying hedging to full canopy reduced total leaf area per shoot and per vine quantified about 1 month before veraison, possibly limiting the growth of the photosynthesizing leaf area of ‘DeChaunac’ vines trained on a high wire cordon (Reynolds and Wardle, 1989). Delaying hedging can also help reduce cluster compactness (berry number per centimeter rachis length). Vines topped 4 weeks after the end of flowering had the lowest cluster compactness in vertical shoot-positioned trained ‘Pinot gris’ in 1 year of a 2-year study in Luxembourg (Molitor et al., 2015). However, there is new interest in the wine grape industry regarding the use of shoot wrapping or shoot tucking as possible alternatives to hedging (France et al., 2018). This technique of wrapping shoots along the top catch wire or tucking shoots into the catch wires is known in the wine grape industry as palissage or tressage, respectively.
Molitor et al. (2015) found that shoot wrap (when used as a control to quantify total shoot growth in a season) reduced cluster compactness in ‘Pinot gris’ compared with hedging 1 week before and at bloom. France et al. (2018) evaluated the shoot wrap and shoot tuck techniques preliminarily in New York, noting six fewer lateral shoots per vine with the shoot wrap technique, and longer rachises by 1.4 cm in vines with the shoot tuck technique, compared with vines in the hedged treatment. In their study, cluster compactness was not reduced consistently by either technique compared with hedging in ‘Riesling’ wine grapes. A long-term study comparing the shoot tuck, shoot wrap, and hedge techniques is lacking in the literature.
The objective of our 4-year study was to investigate effects of shoot tip management and timing of application on vine growth, fruit composition, cluster morphology, and disease severity/incidence in ‘Cabernet franc’ wine grapes. We hypothesized that implementing either shoot wrap or shoot tuck early during the season would reduce lateral emergence and cluster compactness in ‘Cabernet franc’ vines compared with hedging.
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