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

FUNGAL DISEASES Oidium 2.1 O idium occurs in all viticultural regions of South Africa and depending on climatic conditions, intensive control programmes, drawn up in conjunction with consultants, are normally required to keep the disease in check. Fungal spores (do not require free water for subsequent germination) usually hibernate in winter buds and are subsequently released during bud burst, when vines are already susceptible to infection. Although all green parts of grapevines are affected, the first signs are usually observed on the upper surfaces of leaves as small, pale yellow spots (not oil spots) which are especially noticeable when holding the latter at a slant against the light (Photos 10 & 11). A white, powdery fungal growth develops in due course and keeps developing until the entire leaf surface displays a powdery appearance (Photos 12 & 13). It goes without saying that in such instances normal physiological functions of leaves are seriously hampered, thus impeding the normal ripening process. Intense leaf infection (Photos 14, 15 & 16) may result in early leaf fall, whereby bunch exposure to sunburn or heat damage may assume serious dimensions. In addition the essential accumulation of reserves for initial growth in the subsequent growing season is also compromised. As on the leaves, superficial powdery growth in the form of irregular spots occurs on green shoots (become darker in colour when rubbed) (Photos 17, 18, 19 & 20). These spots are retained with lignification and can therefore also be observed on canes during winter. In serious infections shoots may be covered entirely with this powdery growth, causing limited growth or even dieback in young shoots especially. Furthermore buds on such canes may experience problems with bud burst in the subsequent growing season. In bunches oidium can be observed over the entire period from flowering until just before ripening. Inflorescences may wilt with subsequent poor, or even no berry set. Small, green berries are rapidly covered in white powder, whereafter they turn brown and are usually either dropped or develop abnormally and unevenly (Photo 21). Bigger berries usually display, in addition to the white powderiness (Photo 22), a kind of brown (sometimes black) corking on the surface (Photo 23). If the powdery “dust” is rubbed off, darker colour scars appear on the berries, which will subsequently burst and expose the seeds (Photos 24, 25, 26 & 27). Pedicels may also be affected, which hampers the translocation of nutrients to such an extent that ripening cannot occur normally (Photo 28). As ripening progresses (increase of sugar in berries), a decrease in susceptibility to oidium is experienced (Photo 29), which causes the appearance thereof in such cases, when the bloom is rubbed off, to be associated with cobweb-like, superficial brown networks (Photo 30).

16 • A Guide to Grapevine Abnormalities in South Africa