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

Virus associated phenomenon (1); thickened graft union (2). FUNGAL DISEASES Dead-arm disease 2.3

U nlike oidium and downy mildew, the occurrence of dead-arm disease in susceptible cultivars is restricted mainly to the Western Cape, where outbreaks usually occur at the start and during spring – often in a sporadic manner and under wet conditions. In the summer rainfall regions dead-arm disease is usually not considered a serious problem because of spring normally being dry. During hibernation in winter fruiting bodies are formed in the bark sections of affected one-year-old or older wood, or on dead plant material lying on the ground. When these fruiting bodies become wet due to rain, irrigation, dew or foggy weather, spores are released that attach themselves to susceptible grapevine organs and successful germination takes place under conditions of high relative humidity. The significance of dead-arm disease is often underrated. It may, however, rapidly turn into an acute problem due to damage caused under conditions that are beneficial to infection (inter alia dwarfed shoot growth and die-back of buds and shoots). It is therefore essential to obtain expert advice with regard to successful long-term control programmes. The first signs of dead-arm disease are usually observed on the basal internodes of young shoots and are associated with dark brown, lens-shaped spots which expand to become longitudinal cracks, black discolouration and withering of the tissue (Photos 52, 53 & 54). Normally such cracks are fairly shallow and only as deep as the bark. Sometimes, especially during prolonged damp conditions, lesions are not quite visible as individual entities, with practically the entire circumference of the shoot, or large parts thereof, being covered by dark brown to black tissue discolouration (Photo 55). When climatic conditions favour infection until late in the season, shoots may be severely affected and dwarfed, with the accompanying die-back of buds and even shoots (Photo 56), hence the name “dead-arm”. Similar lesions also occur on peduncles, spreading to the rachis and other parts of the bunch framework (Photo 57). Such situations usually cause young berries in certain bunch zones to turn black and shrivel. Infected petioles and main veins are also subjected to having lesions forming on them, often affecting vascular bundles and causing certain parts of the leaf to wither (Photo 58). The occurrence of dead-arm disease on leaves can be identified by small, black spots (surrounded by pale yellow/white circles) which, if numerous, cause a wrinkled and abnormal appearance to the above (Photos 59, 60 & 61). Leaves on the basal internodes are the most severely affected with a decrease in intensity towards the growing shoot tip (Photo 62). The phenomenon that apical leaves appear to be healthy leads to the misconception that infected vines get cured in the course of the season. Should conditions that favour and promote infection re-emerge at a later stage, these characteristic signs of the disease resume on primary and lateral shoots, which may contribute to die-back of pertinent organs.

40 • A Guide to Grapevine Abnormalities in South Africa