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

Anatomical and physiological reactions/changes Characterisation studies of grapevines that are visually infected with Shiraz disease (Goussard & Bakker, 2006) have revealed that the cambium and cambium derivatives of shoots and bunches especially are affected – giving rise to disturbed differentiation patterns in the secondary phloem especially, but also in the secondary xylem. Characteristic anatomical reactions/changes that are thus caused (Photos 236 - 240), reflect the following: (i) the cambium which is indented between the xylem rays in the direction of the pith imparts an intensely wavy appearance to the former; (ii) differentiation of abnormally large amounts of phloem at the expense of xylem; (iii) absence of secondary phloem fibres; (iv) no or poorly developed cork cambium and cork; (v) underdevelopment or absence of primary phloem and (vi) atypical xylem with small, compressed wood vessels. With regard to physiological reactions in terms of grape composition (at the time of harvesting), sugar and anthocyanin concentrations especially were significantly lower, in conjunction with a higher concentration of titratable acid. Significantly reduced starch content in canes was particularly obvious during winter pruning. Based on the above-mentioned results/ observations, it is possible to explain the effect(s) caused by these reactions/changes – individually, but also interactively – on grapevine sizes, as well as shoot, leaf and bunch characteristics. Globally considered anatomical reactions/changes of the cambium with subsequent impaired differentiation and functioning of the cambium derivatives can be singled out as important, if not the most important causes of insufficient and disturbed physiological activities, in turn resulting in abnormal morphological characteristics. The unpredictable targeting and peculiar spreading pattern with the accompanying destructive impact (Photo 241) of Shiraz disease on grapevines are comprehensive – even if such grapevines had been established as certified material. Intensive research is being conducted to formulate scientifically-based declarations that will shed light on: (i) reasons for the peculiar spreading pattern; (ii) why the disease remains mainly restricted to Shiraz and Merlot noir; (iii) the incidence of other vectors in addition to mealybug and (iv) the virus and/or virus-like entities specifically associated with Shiraz disease. Much progress has been made in respect of (iv) and it is hoped that the findings in this regard may cause the other aspects to be clarified in the near future.

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