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

FUNGAL DISEASES Downy mildew 2.2

J ust like oidium, downy mildew occurs in all viticultural regions in South Africa and although it does not necessarily occur each season (depending on the region), unlike oidium, it causes such crop losses that it may be singled out as one of the most important fungal diseases from an economic point of view. In contrast to oidium, fungal spores require free water to germinate after hibernating on dead plant material in vineyards. Under favourable climatic conditions and from approximately 10 cm shoot lengths, downy mildew is able to attack and spread quickly and, depending on the rainfall pattern in particular, it will remain sporadic or take on epidemic dimensions in the course of the growing season. Additional aspects that accentuate the comprehensive impact of downy mildew, include inter alia that all grapevine cultivars of Vitis vinifera are susceptible and that young bunches (even before the flowering stage) often succumb without any signs being visually detectable on the leaves. Such situations may therefore result in total crop losses. The first signs of downy mildew are usually visible on the leaves and are associated with the appearance of irregular, pale yellow spots on the upper parts – sometimes referred to as oil spots (Photos 31 & 32) – which turn red fairly rapidly in red juice cultivars such as Alicante Bouschet (Photo 33). During humid conditions a characteristic white, downy fungal growth develops on the undersides (Photo 34), which represents millions of spore- producing organs of the fungus and spores. In due course the yellow spots become brown and/or reddish brown and the white downiness disappears (Photo 35). In the absence of control, fungal growth will resume when subsequent conditions are favourable and cause the vicious circle to be perpetuated, eventually causing total leaf surfaces to appear brown and desiccated. Late in the season – usually after the harvest – the entire or partial undersides of leaves may be covered in a white fungal growth, without yellow oil spots being visible on the upper parts of the leaves, which is quite unlike the visual appearance early in the season (Photos 36 & 37). Although fairly rare, young, green shoots may be infected under very favourable climatic conditions whereafter they turn black, dry out and die (Photos 38 & 39). In such instances young vines (one to three years old) may be wiped out completely. The economic impact of downy mildew can be ascribed especially to the drastic effect it has on the crop. Young bunches especially are very susceptible – even before flowering – which causes entire bunches to be wiped out at that stage (Photos 40 - 44). Although the susceptibility of berries decreases in the course of their development, infection can still occur at a fairly advanced stage. Typical reactions include shrivelling and browning of berries, which is often accompanied by the development of purplish colour hues (Photos 45 - 48). At that stage, parts of bunches or individual berries succumb, whereas

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