FERTILISATION GUIDELINES FOR THE TABLE GRAPE INDUSTRY

TABLE 18: Norms for the K contents (%) of leaf blades and petioles of grapevines (Conradie, 1994).

Fruit set

Veraison

Calcium (Ca) Calcium is abundant in grapevine organs and the content thereof increases over the season. Calcium is found in the vine in soluble form, as well as fixed as oxalate crystals and as Ca pectate in the middle lamella of cells. It thus constitutes a part of cell membranes and plays a role in the transport of material in the conductive tissue, as well as the permeability of cell walls and the forming of certain proteins and enzymes. Calcium is considered as relatively immobile, even though it apparently goes into solution again in old tissue of perennial plants and moves predominantly apically and is inter alia involved in the development of growing tips. There is thus an upward flow of Ca from the soil through the vine, ultimately ending up on the soil as leaves and prunings, resulting in little Ca being removed by the crop (Table 9). Calcium is one of the most abundant elements in nature, but the presence thereof varies greatly, based on the composition of the parent material and the intensity of erosion and leaching. It is found in a large variety of secondary minerals, like calcite, dolomite and gypsum. It is also usually the most prominent exchangeable cation, even in acidic soils. Calcium deficiency symptoms have, as far as is known, not been observed in the field. In acidic soils, where Ca supply is low and deficiency symptoms can be expected, other aspects like aluminium and/or manganese toxicities prevail. In Southern France it has been found that acidic soil toxicities can be combatted more effectively with dolomitic rather than calcitic lime, probably due to an uptake antagonism between Mg from the dolomitic lime and the AI and Mn in the soil. The red leaf phenomenon exhibited by Barlinka table grapes in the Hex River Valley was initially considered to be a Ca deficiency, due to low Ca contents of affected leaves, while it could have serious negative effects on growth and bunch quality. It was found, however, that it was caused by a plant transferable pathogen, probably similar to the grapevine red blotch associated virus (GRBaV), which was identified in the USA two decades ago, and is only observed on red grape cultivars Berry stem necrosis is a physiological disorder found for certain cultivars under certain growing conditions (climate, N nutrition). It is characterised by black, necrotic girdling lesions around bunch stems and pedicels from veraison (in the Hex River Valley the symptoms appear considerably earlier) onwards, whereby the vascular bundles are abscised, resulting in berries wilting and not ripening (Figure 11). The necrotic tissue is characterised by low Ca contents, of which the causes and mechanisms are not understood. Petiole Minimum High Minimum High Minimum High Minimum High 0.65 1.30 1.00 3.00 0.60 1.05 0.90 2.30 Leaf blade Petiole Leaf blade

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