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

Root formation problems (3); withered shoot growth (4); vascular tissue problems (5).

FUNGAL DISEASES Anthracnose 2.4

A lthough the onslaught of anthracnose on susceptible grape cultivars in the Western Cape has practically ceased since 1968 (with the onset of regular spraying against downy mildew), it remains important to recognise its characteristic traits – especially because of the destructive impact it may have on grapevines and the accompanying crop losses. It goes without saying that anthracnose remains problematic in the summer rainfall areas especially. Hibernation takes place on infected wood – which once again emphasises that it is imperative to remove such material from vineyards during pruning. In spring when moist, warmer conditions prevail, spores are released which are able to infect all green parts of grapevines. According to the available literature, the distribution of anthracnose from one grapevine to another is slow because spore distribution occurs mainly through water. As with downy mildew, spores require free water to germinate, which is mostly enabled by rain, irrigation, dew and foggy weather. Although the importance of anthracnose (just like dead-arm disease) is often underestimated, serious damage may nevertheless be inflicted – especially when spring is exceptionally wet and in instances where it has already occurred in the past. Consequently it is important that expert advice be sought in terms of successful prevention and control practices. Typical of anthracnose infection, is the irregular appearance on leaves of pale grey spots (bigger than in dead-arm disease) with reddish brown to purple edges (Photo 63), which may assume significant sizes, often with a characteristic round appearance in the case of older leaves (Photo 64). In petioles, peduncles and veins, the spots are considerably smaller and more elongated (Photo 65). Tearing and shedding of dead tissue from the spots is very characteristic, with the result that the leaves assume a broken (tattered) appearance – especially where spots occur close or adjacent to each other (Photo 66). On shoots spots are initially small and dark brown to black in colour, sunken in the middle with slightly raised edges (Photo 67). In due course the spots become bigger, join each other and cause shrivelling and destruction of tissue so that shoots die back (Photos 68 & 69). Such shoots eventually appear black and dry with a hard texture. Although typical anthracnose spots are less accentuated on peduncles and pedicels, young bunches (even before flowering) may become entirely shrivelled and die back (Photo 70). After set and berry enlargement typical dark brown to black spots occur on the peduncles (Photo 71), which may later result in the dieback of the entire bunch or parts thereof (which were meant to get nutrients from the infected peduncle). The occurrence of round spots with pale grey centres surrounded by darker edges is very characteristic as far as the infection of older berries is concerned (Photo 72). This phenomenon has given the disease the appropriate name “birds-eye rot”.

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