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

4.5

VIRUS AND VIRUS-LIKE DISEASES Shiraz decline

Morphological and anatomical reactions/changes A characteristic of grapevines associated with Shiraz decline, is the incidence of abnormally thickened (swollen) graft unions with cracks that appear in most instances from 5 - 6 years after planting (Photos 242 & 243). This trend – which is exposed by bark removal especially – is continued on trunk sections at the top ends of graft unions (visually the rootstock components appear normal) and represented by deep cracks/grooves (even on cordon arms) in more sensitive clones (Photos 244 & 245). At the same time hardening of affected wood sections occurs, accompanied by the development of necrotic areas – in varying degrees of intensity. In conjunction with trunk abnormalities a red discolouration of all leaves on infected grapevines is observed from mid- to late summer, which, unlike leafroll for example, includes total leaf surfaces (without curling of leaf edges) (Photos 246 & 247). Anatomical studies (Grenan et al ., 2007; Goszczynski, 2011 and others) of grapevines associated with Shiraz decline revealed abnormal functioning of the cambium as reflected in the fact that cells (individual or in adjacent groups) within the cambial zone are gradually subjected to necrosis – resulting in an intermittent cambium ring/girdle. Eventually such a situation causes a serious decrease to be induced, on an annual basis, in the differentiation of new, normally functioning vascular tissue (secondary xylem and phloem) in the vicinity of the affected areas – obviously this can also offer an explanation for the visual appearance of cracks/grooves/ridges (Photo 248). Although the above-mentioned findings may be considered an important cause of decline, the actual reason(s) for cambium necrosis remain(s) unknown and research continues. Globally considered – in grapevines associated with Shiraz decline – anatomical changes or necrosis of the cambium with subsequent disturbed differentiation and impaired functioning of the cambium derivatives may be singled out as important, if not the most important causes of insufficient and impaired physiological activities which in turn result in abnormal morphological characteristics. In instances where uncertainties with regard to identification issues should occur – such as when this decline phenomenon (which appears to be closely related to stress) may be confused with, inter alia, stem grooving, or girdling/pinching/ strangulation of trunks and shoots, or even with crown gall, it is recommended that expert advice be sought in confirmation. One can only hope that the findings which may emanate from intensive ongoing research will prove helpful to pinpoint the true cause or causes of this destructive disease that decimates Shiraz.

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