Handbook for Irrigation of Wine Grapes in South Africa

The vegetative growth and yield of Merlot in the sandy soil near Lutzville did not respond to medium frequency irrigation before, or after véraison (Table 7.11). Pre- or post-véraison water constraints also had no effect on juice pH and TTA, as well as sensorial wine colour. In contrast, pre-véraison medium frequency irrigation reduced the berry character, whereas post-véraison water constraints had the opposite effect, compared to high frequency irrigation throughout the season (Table 7.11). Post-véraison water constraints also enhanced the berry and spicy character of the wine. Consequently, medium frequency irrigation in the post- véraison period produced the best overall wine quality. Similar to the Pinotage near Robertson, water constraints before véraison reduced wine quality of the Merlot, compared to higher water constraints during berry ripening (Table 7.11). In the sandy soil, pre-véraison water deficits reduced the vegetative growth of Shiraz, compared to high frequency irrigation throughout the season (Table 7.11). Unlike the Sauvignon blanc and Merlot, medium frequency irrigation during ripening reduced the berry mass of the Shiraz, compared to the other strategies (Table 7.11). Visual observation revealed typical berry shrivelling upon soil water deficits where more water deficits occurred in the post-véraison period. Medium frequency irrigation in the pre- or post-véraison periods did not affect the pH of the Shiraz juice (Table 7.11). However, post-véraison water constraints increased juice TTA, compared to high frequency irrigation throughout the season and post- véraison. Similar to the Merlot, pre-véraison water constraints resulted in the lowest TTA. This suggested that berries were more exposed to the sun due the lower vegetative growth, which caused higher malic acid respiration (Fig. 7.10). Post- véraison water constraints also improved sensorial wine colour and spicy character, compared to pre-véraison water constraints, but did not affect the berry character (Table 7.11). Medium frequency irrigation in the post-véraison period enhanced the overall wine quality of the Shiraz. In this case, better wine quality was related to smaller berries. Furthermore, berry shrivelling did not seem to have a negative effect on wine quality. In another scenario, Shiraz in a fine sandy loam soil near Robertson in the Breede River valley was irrigated at a low frequency, i.e. 75 or 90% PAW depletion, followed by high frequency irrigation during berry ripening (Fig. 7.17). Irrigating at the high level of PAW depletion before véraison reduced vegetative growth, compared to high frequency irrigation throughout the season (Table 7.12). It must be noted that the vegetative growth of the Shiraz was substantially stronger in the heavier soil near Robertson, compared to the sandy soil near Lutzville (Table 7.11). Low frequency irrigation before véraison reduced berry mass, compared to high frequency irrigation throughout the season (Table 7.12). Pre- véraison water constraints only caused a yield reduction where the PAW was depleted to 90%. Due to less vigorous growth, more exposed leaves probably caused the lower juice pH where pre-véraison water constraints occurred. The juice TTA seemed to be unaffected by the water constraints. Irrigation at 75 or 90%

202 CHAPTER 7 – IRRIGATION STRATEGIES