A Guide to Grapevine Abnormalities in South Africa - P.G. GOUSSARD

Nitrogen Generally speaking nitrogen deficiencies – in practice, associated abnormality phenomena are very seldom visually detectable – are characterised by diminished shoot growth and a general yellowing of leaves. On the other hand high nitrogen levels induce excessive shoot growth combined with intense, dark green leaf discolourations (Photo 464). In serious cases toxicities are associated with shiny blemishes on leaf surfaces and salt deposits on leaf edges, whereafter tissue is subjected to browning and withering (Photos 465 & 466). Furthermore berry shattering is caused, among others, as well as a reduction in potassium levels in leaves, where deficiency abnormalities may even occur. These are considered characteristic phenomena resulting from excessive applications of nitrogen. Potassium In contrast to the familiar problems caused when grapevines are subjected to excesses of potassium, characteristic, visually identifiable abnormality phenomena are mostly absent under normal cultivation conditions. On the other hand, although potassium deficiencies are associated with the onset of typical characteristics, manifestation thereof can at times be widely divergent, thereby causing greater confusion with a variety of other causes/problems. Potassium deficiencies normally manifest themselves during the early part of the growing season through curling leaves (both upwards and downwards are options), a situation that usually goes hand-in-hand with yellowing of leaf edges, followed by gradual browning and withering. Later in the season deficiencies are associated with browning of older leaves especially, thereby contributing to early leaf losses and impaired growth (Photos 467 - 470). Magnesium Abnormalities associated with magnesium deficiencies are characteristic and can usually be observed from véraison onwards on older leaves, where interveinal yellowing in white grape cultivars and a similar reddening in red grape cultivars can be observed (Photos 471 & 472). In extreme cases leaf edge necrosis may be induced (Photo 473). Although production levels may be affected by magnesium deficiencies, generally the onset of associated abnormality phenomena are detectable in good time – which enables an appropriate course of action. Iron Iron deficiency abnormalities are visually detectable as an interveinal yellowing of young, rapidly growing leaves while the primary, as well as secondary and finer veins remain green (Photo 474). In due course the yellowing becomes more intense, eventually imparting a pale yellow and even white appearance to leaves. Towards the end of the season the entire grapevine may be affected, in which case it is transformed into a stunted, bush-like format as a result of abnormal development of lateral shoots (Photo 475). Obviously yields are

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