FERTILISATION GUIDELINES FOR THE TABLE GRAPE INDUSTRY

In general, Zn deficiencies are difficult to correct with foliar sprays and a more effective treatment has proven to be the painting of spur pruned wounds with a strong Zn- sulphate solution (10%) within 3 – 4 hours after pruning, before strong sap flow commences. Spraying of pruned vines a few weeks before bud break with 4% Zn- sulphate or 1% Zn-oxide (together with 1.3% lime sulphur), is also recommended. Copper (Cu) The gap between sufficient Cu and a deficiency thereof in the plant, is very narrow and toxic concentrations are not much higher than those that are “normal”. Copper is normally assimilated by roots as the cupric ion, but also as certain chelates and uptake can also occur via the roots and bark. Copper has a profoundly essential role, as it is present in various proteins and is particularly found in oxidase systems, like inter alia cytochrome-, polyphenol- and ascorbic acid -oxidase. In nature, copper is found in a large variety of minerals and in soil in the organic fraction, adsorbed to the soil colloids. Various soil factors can restrict the plant availability of Cu, like the presence of lime (high pH), but also high AI concentrations in highly weathered, leached and/or sandy soils. Copper deficiencies have been observed for a considerable time in the Strandveld region of the South coast, in pastures and grain crops. Previously, however, attention has never been given to Cu nutrition of grapevines due to the total absence of reports about deficiencies, both locally and internationally. The apparent reason for this is the liberal use of Cu containing fungicides. Recently, however, Cu deficiencies have been identified on the highly weathered soils of the Cape south coast, as well as on sandstone material against higher mountain slopes. Young plantings on Cu poor soils are especially sensitive to Cu deficiencies and it appears as if cultivar differences also exist. Deficiency symptoms are not very specific; probably because poor root development is one of the effects of a Cu deficiency (typically only adventitious root development is observed on nodes and the bases of young vines). Consequently, other nutrients are then also poorly assimilated and deficiency symptoms like those of Fe, Zn and K can then be observed. A copper deficiency causes stunted growth and a general irregular leaf yellowing, very comparable to that of simazine damage, which can develop into necrotic zones on older leaves. Further characteristics are shortened internodes which result in a witches’ broom appearance, as well as dead growing tips (Figure 22).

72 | MAINTENANCE FERTILISATION