FERTILISATION GUIDELINES FOR THE TABLE GRAPE INDUSTRY

CHAPTER 7

FIGURE 30: White meat, characteristic of firm table grape berries – the so called “calcium ring” (Du Plessis, 2008).

The appearance of water berry is sometimes ascribed to low Ca contents in bunches. However, the mechanism is unknown, and because water berry is generally associated with vigorous growth, or situations of poor carbohydrate supply to bunches, the validity of this is questionable. Foliar and bunch sprays, aimed at increasing Ca content of berries to improve quality, is likewise inefficient. The management of water berry should comprise a strategy to control excess vigour, and to ensure, by means of good canopy management, that all leaves remain photosynthetically active for as long as possible. MAGNES I UM Magnesium has no direct effect on grape quality. It does however have an indirect effect on sugar accumulation in the berry – Mg plays an important role in photosynthesis and therefore on the production of carbohydrates (sugars). It has however been found that Ca and Mg can reduce the pH of the juice in the grapes, but the reason for this is unclear. A soil application of Mg-sulphate yielded the best reaction, but excessively high applications of Mg-salts may be detrimental for the soil structure. In certain ranks it is also believed that the application of Mg sprays (MgSO 4 ), sprayed before or during ripening, will prevent or reduce the development of water berry, even though water berry, as mentioned in Chapter 5, does not occur as a consequence of Mg deficiency. On account of Ca being immobile and sprays thereof being inefficient, it is claimed that Mg can replace Ca to some extent, thus having a strengthening effect on cell walls, due to its greater mobility within the plant. Compared to Europe, water berry symptoms have been found earlier in the season in South Africa, hence the practice to apply bunch directed MgSO 4 sprays of as strong as 15% preferably before bunch closure, rather than after veraison. However, the success of any form of Mg spray is questionable, because empirical data to confirm this practice is thus far not available.

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