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

to the actual impact of these diseases – especially when grapevines are under stress (Photos 132, 133, 134, 135, 136 & 137). Phytium and Sclerotium root rotting / dieback Root rotting associated with Phytium and Sclerotium is mostly problematic in nurseries or in young grapevines up to the age of five years. Seeing that rotting of finer roots in particular are implicated in both instances, one would expect such situations to make a significant contribution to the dieback of grapevines – thereby being associated with the trunk disease complex. Although characteristic abnormalities associated with these root rotting diseases have been elucidated in vividly illustrated studies (Marais, 1981; Ferreira & Venter, 1996), it remains practically impossible to ascribe general dieback phenomena to Phytium or Sclerotium specifically. Consequently it is strongly recommended here too that implicated plant material be subjected to diagnostic tests by disease clinics so as to accurately determine specific causes. Dead-arm disease / dieback / rotting When climatic conditions are conducive to dead-arm disease until late in the season, shoots may be seriously affected and subjected to dwarfing/dieback; even buds and shoots may die back – hence the name “dead-arm”. In susceptible cultivars the above-mentioned situations may cause specific bearers – on an apparently healthy cordon – to be systematically subjected to bearer build-up with the accompanying dieback due to rotting of wood (Photos 138 & 139). Slow dieback / dwarfing Slow dieback or dwarfing per se in normally performing grapevines is usually easy noticeable (Photo 140). Where characteristic traits associated with specific abnormalities are absent, accurate identification of actual causes is often problematic and may cause confusion and possibly erroneous identification. By means of illustration, one can mention that although dieback in the above-mentioned situation (based on visually detectable behavioural patterns of grapevines) could be associated with possible Phytophthora problems, vascular tissue blockage – related to Petri disease – has been revealed as the true cause following diagnostic examination.

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