Technical Yearbook 2023

PHOTO 7.1. The start of magnesium deficiency, red dots within yellowing patches, stretching inward from the outer edges of the leaf. Veins are still green in this example of Shiraz.

Phosphorous deficiency Leaves expressing phosphorous deficiency are generally characterised by various dark red dots forming on the tips of the leaf lobes. As deficiency symptoms intensify, the dark red dots merge to form dark patches clearly delineated by laminar veins (photo 6). Symptoms of phosphorous deficiency are not very common and seem to affect both red and white grape varieties similarly. Magnesium deficiency Magnesium deficiency in red grape varieties often displays striking symptoms and is common in the acidic soils of the Western Cape. Certain varieties are also prone to displaying symptoms, such as Grenache. Symptoms start as red spots accumulating in the interveinal area (photo 7.1) to later form distinctive red wedges between main veins. Importantly veins do not remain green as in the case of GLRaV-3 expression (photo 7.2). PHOTO 6. Phosphorous deficiencies cause blotchy dark-coloured pigmentation on the ends of leaf lobes, discolouration is not confined to white varieties, this for example is Tempranillo.

PHOTO 7.2. More developed magnesium deficiency, marked by clear red wedges between completely discoloured vines in Merlot.

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TECHNICAL YEARBOOK 2023