Handbook for Irrigation of Wine Grapes in South Africa

TABLE 7.12. Effect of low frequency irrigation until véraison, followed by high frequency irrigation during berry ripening, compared to high frequency irrigation throughout the season on midday Ψ S , cane mass, berry mass, yield and sensorial wine quality of Shiraz in the Breede River valley (Lategan, 2011). Data are means for three seasons. Parameter RAW depletion pre-véraison→post-véraison 40%→40% 75%→40% 90%→40% Pre-véraison Ψ S (MPa) -0.89 a* -1.27 b -1.29 b At harvest Ψ S (MPa) -0.82 a -0.79 a -0.79 a Cane mass (t/ha) 3.6 a 2.6 b 2.7 b Berry mass (g) 1.4 a 1.2 b 1.1 c Yield (t/ha) 25.6 a 25.4 ab 22.5 b Juice pH 3.84 a 3.74 ab 3.70 b Juice TTA (g/  ) 5.2 a 5.0 a 5.0 a Wine colour (%) 38.6 ab 34.4 b 48.5 a Berry character (%) 42.0 b 41.0 b 48.4 a Spicy character (%) 36.7 ab 33.0 b 39.0 a Wine quality (%) 40.2 ab 34.3 b 42.3 a * Values followed by the same letter within a row do not differ significantly (p ≤ 0.05). Considering the three scenarios above, wine quality of the red cultivars was almost consistently reduced where low water constraints before véraison were followed by more frequent irrigation during berry ripening. The only exception was the Shiraz near Robertson where the PAW was depleted to 90% before véraison, followed by high frequency irrigation during berry ripening. Wine quality of Sauvignon blanc seemed to be less sensitive to different pre- and post-harvest irrigation frequencies, except where sudden water deficits after véraison in the sandy soil near Lutzville resulted in poorer wine quality. The Shiraz near Robertson was also subjected to low frequency irrigation before véraison, followed by a top up irrigation at véraison and deficit irrigation until harvest (Fig. 7.18). This combination of strategies reduced cane mass, berry mass and yield where irrigation was applied at 90% PAW depletion before véraison, compared to irrigation at a high frequency throughout the season (Table 7.13). Irrigation at 75% PAW depletion before véraison caused similar trends. Low frequency irrigation followed by deficit irrigation reduced the juice pH, but had no effect on the TTA (Table 7.13). In fact, irrigation at 90% PAW depletion caused a further pH reduction, compared to 75% depletion. Irrigation at 90% PAW depletion before véraison followed by DI during berry ripening almost invariably improved the wine colour, berry character, spicy character and overall wine quality, compared to high frequency irrigation throughout the season, as well as 75% depletion before véraison (Table 7.13). In this case, better wine quality was also related to smaller berries.

204 CHAPTER 7 – IRRIGATION STRATEGIES