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

BACTERIAL DISEASES Bacterial blight 3.1

B acterial blight occurs in practically all winegrowing regions of the Western Cape and – depending on climatic conditions – it can destroy grapevines with accompanying crop losses. Although the onslaught on susceptible cultivars has been drastically reduced since about 1936, it is nevertheless still important to be aware of characteristic traits – especially in years when the onset of the growing season goes hand-in-hand with exceptionally wet climatic conditions. According to the literature, the bacteria survive during winter in the sap of infected grapevines and are translocated during bud burst to newly developed shoots. Subsequent rain or irrigation plays an important role in spreading the disease to adjacent shoots, leaves and bunches – in which case material is penetrated through wounds (no matter how small). Obviously the ability of bacteria to survive/move in the sap flow will result in the infection of pruning shears during winter pruning, whereafter neighbouring grapevines get infected in turn. If such infected material is therefore used for grafting, ideal conditions are created for distribution through nursery material. Although bacterial blight is associated with distinctive, visually perceptible characteristics, the expression thereof can sometimes be confusing and of varying intensity. Consequently it is important that expert advice in terms of correct identification be obtained without further ado, so that applicable preventative measures can be implemented as soon as possible. Delayed bud burst is typical of bacterial blight, followed by the development of shoots with short internodes (Photo 141). In instances where the disease already displays an advanced stage of distribution in the wood, bud burst may not occur, or where it has already occurred, stunted shoots may die back rapidly (Photos 142 & 143). Obviously such situations will/can contribute to the gradual, and eventually total dieback of grapevines. In the case of young shoots the occurrence of lenticular, fine elongated cracks, which gradually develop into necrotic cracks and cancers across entire shoot halves, is highly characteristic of bacterial blight (Photos 144, 145, 146 & 147). Similar necrotic cracks also occur on peduncles and lateral branches, in which instance desiccation/dieback of bunches – even at very early stages – will start (Photos 148, 149 & 150). Leaves can be infected either directly through the stomata (which will result in the occurrence of angular brown spots) or from infected shoots. Very characteristic of the latter scenario is the desiccation of entire leaves, or parts of leaves, from the edges inwards and where the necrotic tissue is bright brown in appearance, and separated from normal green parts by a yellow edge (white grape cultivars) and a red edge (red grape cultivars) (Photos 151, 152 & 153). In Afrikaans the disease is aptly named “vlamsiekte” for the flame-coloured discolourations of the leaves.

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