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

BACTERIAL DISEASES Crown gall 3.2

A lthough crown gall can occur in all the cultivation areas, it is especially problematic in the Orange River Region – due to climatic conditions that may be conducive – where it is able to bring about considerable losses in susceptible cultivars. According to literature on this subject, the bacteria may survive in the soil for long periods (in the absence of any vegetation) to be distributed successfully – even over great distances – by drift actions, implements, vehicles and animals. Additionally, a wide range of host plants may give rise to a considerable accumulation of population numbers. As with bacterial blight, penetration of material occurs through wounds (which are incurred, inter alia, by weeding actions, frost, wind and insects) whereafter the bacteria spread systemically through the grapevine. Since the disease may also be latent in grapevines, and in view of its systemic nature, excellent opportunities are harboured for distribution via nursery material. The establishment of young infected grapevines usually results in poor initial vigour, which may be followed by early dieback (within two to three years) – this kind of situation is once again indicative of the priority value that should be given to the ongoing use of certified propagation material. Infected older vines, on the other hand, may survive longer, however production will decline and insufficient lignification of shoots is likely. Obviously such shoots can be damaged and wounded by early autumn frost. Despite the fact that crown gall is associated with characteristic, visually perceptible traits, the expression thereof can at times be confusing and varying in intensity. Consequently it is important in this instance also to obtain expert advice for accurate identification in order to implement the appropriate preventative measures. After penetration of the material through wounds, moist local tissue is stimulated by the bacteria and so multiplies abnormally rapidly, whereafter the tissue protrudes outwards in the form of galls/tumours. Such situations therefore give rise to the appearance of characteristic galls on trunks of grapevines near the surface of the soil (Photos 154 & 155). Initially galls are creamy white to pale green, with a round, soft and smooth appearance that changes, with ageing, into dark-coloured, hard and wart-like structures, with parts that fall off and become rotten. Galls can also develop higher up on trunks, canes and even susceptible rootstocks below the graft unions (Photos 156 & 157). The incidence of galls on parts of the trunk causes grapevines to die back in due course, depending on the intensity of the affliction. In cases where crown gall occurs in grafted nursery vines, the formation of galls is mostly restricted to wounds on the rootstock components – with the emphasis on the graft unions and positions where incisions were made to initially remove the buds. According to literature on this subject, galls do not occur on roots, but they may develop on the basal poles of cuttings and rootstock components, where uniform root formation did not occur. Obviously such situations will play an important role in the distribution of this disease. However, thanks to the high standards that have been set locally

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