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

FUNGAL DISEASES Botrytis (grey rot) 2.5

A lthough grey or botrytis rot occurs in practically all areas where grapevines are cultivated, the severity of the disease is far less in warm, dry areas – mainly due to the restrictive effect of prevailing low relative humidity on infection. Apart from favourable climatic conditions, the onset and course of the infection is determined by several factors, the most important being cultivar resistance and cultivation practices such as vine and bearer spacing, trellis systems and canopy management (suckering and leaf thinning). Following infection, the disease can progress in several ways – regulated mainly by climatic conditions – to cause the development of noble rot (cool conditions, high relative humidity, but not wet) or grey rot (hot, moist conditions). In the former instance it is possible to make special late harvest or noble wines, while in the latter case tissue degradation (rotting) of berries is the order of the day, causing considerable crop losses. Hibernation occurs through the formation of specific structures (sclerotia) in infected shoots and berries as well as on dead organic material in vineyards. Under favourable conditions sporulation takes place, whereafter spores land on berries and undergo successful germination in the presence of free run-off, subsequently penetrating the skin. In view of the fact that infection can even take place during the flowering stage whereafter the pathogens will remain latent until the onset of ripening, chemical control (in areas with conducive climatic conditions) should be carried out based on expert advice. In warmer and drier areas, meticulous and correct application of canopy management practices in particular should suffice to control botrytis rot. Although germinating spores are not dependent on wounds to penetrate the berry skin, the latter offers additional entry ports, which are liberally used, as a result of berry burst and bird pecks. The first signs of grey rot occur during ripening and are associated with light brown (white berries) and dull (coloured berries) skin discolourations, where the slightest pressure causes the skin to slip away from the underlying tissue – hence the names “glyskil” and “kraakdop” (Photos 73 & 74). This phenomenon is very typical of grey rot in contrast with sour rot, inter alia, where it does not occur. The rot slowly penetrates the berry tissue causing total browning, which results in tissue degradation under favourable conditions (Photos 75 & 76). Grey spores are formed on infected berries where they may be observed either in bunch format or in streaked development format (Photos 77, 78 & 79). In the event of dry, hot periods after infection, this is usually limited to a few berries per bunch, whereafter they either shrivel, turn brown and toughen (Photo 80), or the infection is followed by the development of other harmful secondary organisms (Photo 81). Noble rot is totally dependent on natural, conducive climatic conditions (as mentioned above) at which time infection causes fine cracks in berry skins in bunches that are left on

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