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

4.7

VIRUS AND VIRUS-LIKE DISEASES Aster Yellows

Characteristic abnormalities/morphological reactions and changes Following the transmission to healthy plant material (whether by grafting or leafhoppers as sucking, phloem-feeding insects) Aster Yellows associated phytoplasmas move undisturbed from cell to cell in all directions, in order to – in systemic format – spread systematically in the phloem of all organs of a grapevine (Photo 261). Characteristic of this disease is seriously delayed, or even no bud burst (which may eventually contribute to the overall dieback of bearers) and dwarfed shoot growth on parts of grapevines – with the additional tendency to form numerous water shoots (often in bush-like format) everywhere on the trunks (and even cordons) (Photo 262). This is accompanied by the onset of characteristic, visually detectable abnormalities (early in the growing season especially) that appear initially as hardened and crackling leaves (Photos 263 & 264). Combined with this, one finds the yellowing and copper-coloured reddening of leaves in white and red grapevine cultivars respectively (Photos 265 - 267), whereafter the affected leaves look noticeably brittle and thicker than usual, with leaves curling downwards (Photo 268). Typical of Aster Yellows is the phenomenon of affected shoots not being subjected to normal lignification, apart from having short shoots and premature dieback of shoot tips (Photos 269 & 270) – consequently lignification does not take place, or only partially. The appearance of a lead-grey colour (which can be rubbed off with one’s finger) on the green shoots, which is accompanied in the case of Chardonnay especially by shallow longitudinal cracks along the surface, is deemed very characteristic of this disease (Photos 271 & 272). Globally considered the abortion of young bunches and partial dieback of bigger bunches (Photos 273 & 274) can be singled out as some of the most important contributing factors to reduced productivity in infected grapevines, while possible anatomical reactions/changes of the phloem may in turn give rise to insufficient and disturbed physiological activities – thereby contributing to the shortened lifespan of grapevines.

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