VIT Report
This report will provide a brief discussion on the implications of pruning techniques and management practices on a number of different aspects of grapevines. Pruning is one of the main factors in vine management that allows for the manipulation of vine balance and grape composition. It enables the careful selection of spurs and canes, thereby influencing the location and development of the canopy (Reynolds & Vanden Heuvel, 2009). By changing the number of buds and the type of bearing wood, the pruning methods can modify the yield and the leaf area to fruit ratio of grapevines, which also alters grape berries maturation (Miele & Rizzon, 2013; Greven et al., 2015; Marcon Filho et al., 2016b). A primary aim when choosing the pruning method, is to achieve balance between vegetative and fruit growth whilst simultaneously providing appropriate fruit composition for winemaking (Jackson & Lombard 1993, Tassie & Freeman 1988). Increasing pruning severity can increase shoot number per node, increase bunch weight (Jackson et al. 1984) and increase fruitfulness (Byrne & Howell 1978). The yield of vines is dependent on the number of buds left on the vine at pruning. After bud burst, bunch number on the developing shoots determines potential crop, and leaf area determines how much crop can be ripened (PETRIE et al. 2000). Lighter pruning, in general, increases yield but it may also be associated with negative effects on fruit quality such as lower total soluble solids and modified pH and titratable acidity (JACKSON et al. 1984). Higher bud numbers left at pruning do not always give a linear yield response (WOLPERT et al. 1983), and the vine tends to compensate by reducing the number of buds producing shoots, the number of bunches per shoot and average bunch weight. From the considerable literature on pruning levels and yield it is clear that different cultivars respond differently to different levels of pruning, within the same vine management system.
The pruning method and approach outlined below will aim to maintain a balance between vegetative growth and fruit production. Where a vine is under pruned, (too many buds left) the vine will produce many small clusters of small grapes that may fail to ripen properly. If the vine is over pruned, (too few buds left) the yield will be low and the vegetative growth excessive. To ensure balance is achieved in a vine, the number of buds left will be adjusted according to the amount of one-year-old wood removed in pruning. The pruning of vines seeks to eliminate vine shoots, vine leaves, premature shoots, latent buds, leaves and roots in order to modify the natural growth of the vine (Aliquo et al., 2010). Taking into account the fruit production habit wherein the plants produce clusters in the last growth branches (vine leaves) that originate in the development of the previous season (vine shoots), pruning is used to limit the number and length of vine leaves, creating a balance between vigor and production, regulating the productive and vegetative potential and avoiding the aging of the vine (Aliquo et al., 2010).
Bud fruitfulness generally increases with increasing shoot light exposure with at least one-third to one-half of the available sun. Bud fruitfulness is only one component of yield. The other factors that vary are flower number, berry number and berry weight. The main branches of flower clusters are formed by the time vines enter dormancy, but additional branching occurs during bud swell in the spring. By budburst, flower number is set. Fruit set following bloom varies from 20 to 70%, and is influenced by many factors such as variety and weather around bloom.
Shading and its impact on bud fruitfulness is the reason why it is important to select well-exposed, moderate diameter canes for retention at pruning. Cane diameter is a good indicator of shoot vigour. Both low-vigour (small diameter) and high-vigour (large diameter) canes tend to have lower bud fruitfulness. Training systems that work with the vine’s growth habit are designed to provide adequate sunlight exposure to the renewal zone (area retained for next year’s crop).
Many studies across a range of varieties have shown the sensitivity of yield and yield components in response to variation of node number (Jackson & Lombard 1993, Jackson et al. 1984, Tassie & Freeman 1988, Zamboni et al. 1996). Early studies of vine balance have shown an increase in yield per vine with an increase in node number retained at pruning (Kimball & Shaulis 1958, Shaulis & Oberle 1948, Tompkins & Shaulis 1955), however more recent studies have refined this response showing that increasing node number generally increases yield until a plateau is reached, after which yield increases at a much slower rate (Bindon et al. 2008, Jackson & Lombard 1993, Jackson & Steans 1983-4, Jackson et al. 1984, Tassie & Freeman 1988).
Yield component compensation can result in pruning level alone being an inadequate management tool for yield manipulation as berry number can change at fruitset and berry size can change during the season (Dry et al. 2004). Dunn et al. (2004) found that more severe pruning increased berry size in Chardonnay and Cabernet Sauvignon and warned that for cool- climate cane pruned vines, responses were less consistent than in warmer climates. Bindon et al. (2008) found only a weak relationship between berry size and secondary metabolite concentration in Shiraz utilising a wide range of berry sizes as a result of winter pruning treatments.
Early studies found increasing yield delayed maturity by lowering sugar (total soluble solids [TSS]) and titratable acidity (TA) in berries at harvest (Winkler 1970). High yields have been shown to delay maturity and to decrease wine quality for some red varieties (Chapman et al. 2004, Jackson & Schuster 1987, McCarthy et al. 1987, Winkler 1954, Winkler 1970). Bravdo et al. (1984) and Bravdo et al. (1985) found that the wine colour density of Carignane and Cabernet Sauvignon decreased when yields were promoted beyond the upper yield limit for each variety.
The treatment of 40 nodes per vine has been observed to be an excessively high pruning level, however results from the current study suggest that vines of moderate vigour may well be able to sustain these yields without negatively impacting on fruit and wine composition. However, in warm, moist seasons, the denser canopies associate with these higher node numbers may prove to be difficult for disease management. The lack of decline in quality in this study supports the statement of Keller (2010) that ‘vines of medium vigour often produce higher yields and better quality fruit than vines at either end of the vigour spectrum’. There were many indications that pruning levels of between 30 and 40 nodes per vine produced balanced vines, or vines of matched crop size and vegetative growth. This was in part due to the use of the Scott Henry trellis system in the experimental vineyard, which increases exposed canopy surface area and improves the yield potential, in part by reducing shading of leaves (Smart & Robinson 1991).