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

ABIOTIC ABNORMALITIES Wind, heat, sunburn and frost damage 6.1

Wind damage Depending on its speed/force, wind is considered to be one of the most important and best-known environmental conditions in terms of harmful impacts on grapevines. In extreme instances leaves, shoots, berries and bunches are torn off by gusts and gales, while situations have occurred where serious/destructive damage has been caused to grapevines and even trellis systems (Photos 391 - 393). Although grapevines are susceptible to wind damage throughout the growing season and individual cultivars display varying degrees of susceptibility, they are especially vulnerable during specific phenological stages and organs may be damaged to the extent that normal physiological activities are hampered – especially in areas where strong, but also more moderate prevailing winds occur. Typical examples include chafing of shoots, petioles and peduncles against foliage wires – in which case vascular tissue is damaged to the extent that it is unable to engage in optimal translocation processes (Photos 394 - 397). In addition wind during the flowering period may also contribute, inter alia, to poor(er) set of berries (Photo 398), while wilting and desiccation of young shoot tips on hot days is no exceptional phenomenon – especially in instances where soil water is limited. Furthermore indirect wind damage, as related to possible mealybug and leafroll spread, should never be disregarded, but considered a high priority throughout. Heat damage Although heat and sunburn damage may occur jointly, the former is usually associated with situations where grapevine tissue/organs are damaged by overheating without direct exposure to the sun. Typical of this is the wilting and drying out of actively growing shoot tips either during heatwave periods, or simply in instances where sudden increases in temperature occur after relatively cool spring conditions – a phenomenon that is obviously exacerbated by dry winds. Apart from inter alia Muscat d’Alexandrie, where older leaves are often subjected to the development of uneven, yellow to brown heat spots (Photo 399), the incidence of heat damage/spots is especially noticeable in bunches – the berries of which are afflicted to varying degrees at any stage shortly before ripening. In such instances (due to overheating of fruit tissue) characteristic indentations are caused – in due course the tissue turns brown from the inside – on berry surfaces, resulting in misshapen development and eventual drying out thereof (Photos 400 & 401). Shrivelling and eventual drying out of berries in reaction to the browning/drying out of sections of the peduncle/ framework may be singled out as an additional characteristic example of heat damage – a phenomenon to which individual, separate groups or even all berries on a bunch may be subjected (Photo 402).

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