Handbook for Irrigation of Wine Grapes in South Africa

Chapter 7

If this water is accounted for, the PRD strategy will certainly save less than 50% water. In fact, the PRD strategy used only 35% less water when compared to the high frequency single line drip irrigation (Table 7.16).

7.2.9

IRRIGATION DURING THE POST-HARVEST AND DORMANCY PERIODS

During these periods, dry soil conditions or PAW depletion close to permanent wilting point will obviously not have any negative effects on grape yield, or end product quality. However, it reduced bud fertility, bunch mass and yield of Sultanina grapevines in a sandy soil in the Lower Orange River region drastically in the subsequent season (Table 7.17). This indicated that severe soil water deficits may affect bunch initiation, as well as differentiation in a negative way. Furthermore, the grapevines that were subjected to water deficits during the post-harvest and dormancy periods showed delayed budbreak symptoms (Fig. 7.21). There are indications that the low soil water content causes lower cane water content prior to budbreak (Fig. 7.22). In some cases, the unusually low yield is followed by vigorous vegetative growth after the grapes have been picked. The cane mass of Sultanina subjected to dry soil conditions was almost doubled due to the strong shoot growth after harvest, compared to grapevines that were irrigated during winter (Table 7.17). It is important to note that the strong growth in the post-harvest period did not reduce reserve accumulation, as quantified at the hand of cane starch content at pruning (Table 7.17). Excessively dry soil conditions during the post-harvest period and winter may not only reduce yield in sandy soils, but also in heavier soils (Myburgh, 2003a). TABLE 7.17. Effect of stage at which irrigation was terminated during the post-harvest period in 1997 on the 1998 fertility, bunch mass and yield, as well as cane mass and starch content at pruning of Sultanina in the Lower Orange River region (Myburgh, 2003b). Parameter Period following harvest when irrigation was terminated (weeks) 3 5 11 15 Not terminated Bunches/grapevine 14 cd* 8 d 20 bc 25 b 48 a Bunch mass (g) 150 b 146 b 181 b 199 b 299 a Yield (t/ha) 3.9 b 1.9 b 6.5 b 8.7 b 23.9 a Cane mass (t/ha) 4.1 b 4.1 b 3.3 b 4.4 b 2.0 a Cane starch content (mg/g dry mass) 16.9 a 16.5 a 14.1 a 14.2 a 14.6 a * Values followed by the same letter within a row do not differ significantly (p ≤ 0.05).

IRRIGATION OF WINE GRAPES 209