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

Sunburn damage Sunburn by itself is usually associated with situations where serious damage may be inflicted on bunches of susceptible cultivars especially as a result of direct exposure. Abnormality phenomena associated with this are largely very characteristic and mostly related to wilting, shrivelling and eventual desiccation of berries on one or more parts of the bunch (Photo 403). In addition light brown to orange-yellow discolouration in individual berries – which may be accompanied by varying degrees of shrivelling – may be considered characteristic of exposure to direct sunrays on very hot days (Photos 404 & 405). A common occurrence in bush vines is to fall apart in the centre, causing ripening grapes to be severely exposed to sunburn – a similar situation may occur when shoots of trellised grapevines hang free (Photo 406). Although leaves may be affected/damaged by sunburn to a lesser degree, they are not immune and when affected, display characteristic traits. In typical examples one notices the emergence of pale brown spots between veins, encircled by a dark brown line – thus creating a distinct separation between the brown and green parts (Photos 407 & 408). Frost damage Although grapevines are usually not subject to frost during the dormant period (June to August), trunks, cordons and one-year-old canes may in extreme cases be affected to the extent that rifts/cracks develop – a situation that, in addition to creating convenient entry points for crown gall bacteria inter alia, may even contribute to dieback phenomena in grapevines. In contrast to the above, the incidence of spring frost is extremely harmful in September and October, when sensitive green shoots, flower clusters and even buds that are just starting to burst, may be destroyed (Photos 409 & 410).

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