Furthermore, the leaf area required to ripen one gram of fruit also depends on the trellis and training system; it appears to be considerably higher in a typical vertically shoot-positioned canopy than in vines that are sprawltrained or trained to divided canopies.

Overcropping ordinarily delays fruit maturation and therefore decreases grape sugar and color if harvest cannot be delayed.1,36,38,40 However, the effect of crop load on berry composition depends on how a difference in crop load is achieved. An outbreak of pests or diseases, or a hail storm, can reduce photosynthetically-active leaf area after the yield potential has been established, which results in reduced sugar accumulation.

If an increase in yield is accompanied by deterioration in the canopy microclimate, then the changes in fruit and wine composition are negative.29 In other words, so-called overcropping effects are often actually shade effects caused by poor pruning practices or other vineyard management errors.

For example, if pruning is too light (too many buds left), then there may be too many shoots, which leads to a dense canopy. Alternatively, if pruning is too severe (too few buds left), then the few remaining shoots may grow too vigorously and produce too many laterals, which leads to shade in the fruiting zone. In contrast, where an increase in yield is accompanied by an improvement in the canopy microclimate, berry composition may also be improved.

For example, the concentration of undesirable methoxypyrazines in grapes grown on high-yielding minimally pruned vines can be dramatically reduced compared with lower-yielding spur-pruned vines. Indeed, when yields are increased by decreasing the pruning level (leaving more buds), the resulting wine is often fruitier and less vegetative, perhaps due to the reduced shoot vigor (also see Chapter 6.1).4,5 If, on the other hand, yields are stimulated by abundant water supply, the result is usually the opposite.

Therefore, balancing shoot growth and fruit production is an important viticultural goal. For an individual vine, the crop-load/fruit-quality relationship generally follows an optimum curve with increasing quality as crop load is increased from a very low level, followed by a plateau, and finally a reduction in quality when crop load is further increased. Under changing external conditions (including cultural practices), this curve can be shifted upward or downward.

Rather than setting a specific, inflexible target yield, economically-minded vineyard managers aim to achieve the highest possible yield without compromising quality. Moreover, where weather conditions permit, delayed ripening can sometimes be compensated by delayed harvest.

On overcropped vines, cluster thinning is typically employed to reduce the crop load and enhance ripening. This can be especially beneficial in cultivars that are prone to overcropping due to their large clusters, such as Syrah, Mourvèdre, Grenache, or Zinfandel.

One form of cluster thinning is cutting away the distal one-third or half of the flower clusters during bloom, which not only decreases yield but also often leads to less compact clusters due to the compensatory stretching of the rachis. It is possible that the advanced maturity observed following such early thinning may be attributed less to the smaller crop than to the tendency for berries in the proximal portion of a cluster to ripen more rapidly than those in the distal portion.37 In table grapes, where large berries are desirable, cluster thinning is sometimes supplemented by berry thinning.

The size of individual berries is often more important than the crop level or even crop load in determining fruit composition. For instance, large increases in berry size after veraison may be coupled with concomitant increases in berry sugar and K due to phloem import.30 The concentration of K in the berries decreases as the crop load increases.15

However, because tartrate synthesis inside the berry ceases at veraison, an increase in berry size can lead to a substantial decrease in tartrate due to a “dilution effect.” Of course, more K and less tartrate in the berries will result in a corresponding increase in juice and wine pH. In addition, larger berries have a relatively smaller skin:pulp ratio, which has implications for red wine composition and quality due to the importance of the extraction of skin-derived compounds (mainly anthocyanins, tannins, and flavonols) during fermentation. In contrast to grapes used for red winemaking, large size and crispness are important quality traits of table grapes.