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

ABIOTIC ABNORMALITIES Drought, drowning and salinity 6.2

Drought On the whole, drought induced water deficits are fairly easy to recognise, being associated with a yellowing of older leaves in particular. This often occurs in conjunction with prior desiccation/scorching of leaf edges. In instances where grapevines are subjected to sustained drought conditions, such situations are usually followed by total desiccation and premature leaf fall, whereafter grapes (usually in a wilted condition) are subjected to the consequences of sunburn damage. In this category, manifestations of abnormalities associated with drought are visually portrayed and elucidated by succinct captions, also explaining specific causes of/reasons for individual cases (Photos 411 - 417). Drowning Although grapevines display resistance to drowning during the dormant period, there is significant vulnerability in the course of the growing season however – with specific reference to active root growth phases, in which case peaks are usually reached during flowering, but also in the course of several post-harvest weeks. As with drought and salinity, drowning induced water deficits are associated with yellowing and desiccation of leaves, often in conjunction with drying/scorching of leaf edges (Photos 418 & 419). In view of the fact that serious situations of drowning, where water is persistently accumulated either above or just below the surface of the soil (Photos 420 & 421) – thus contributing to the eventual dying of grapevines – visual detectability is compromised where above-ground abnormalities only become obvious once such conditions have come to an end. Root and soil investigations are therefore highly recommended where drastic drainage problems/ drowning conditions go hand-in-hand with a sour smell (Photo 422). Salinity According to available literature delayed shoot growth, followed by the prevalence of darker green, dull leaves as a result of a thicker layer of wax, may be considered commonplace with the onset of abnormalities associated with saline damage. Obviously it is often not easy to observe the above-mentioned manifestation(s) visually and furthermore confusion with other causes is also likely. The transition to desiccation/scorching of leaf edges (Photo 423), eventually to penetrate between main veins (in conjunction with leaves that curl upwards), is very characteristic of salinity and therefore also easier to interpret visually (Photo 424). At the same time the onset of leaf fall is moved forward, while shoot tips tend either to remain green, die back or experience renewed bud burst, resulting in the occurrence of green leaves (Photos 425 - 427). Under such conditions fewer or even no bunches are produced – and where these do materialise, optimal ripeness is not achieved (Photo 428).

262 • A Guide to Grapevine Abnormalities in South Africa