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

VIRUS AND VIRUS-LIKE DISEASES Stem grooving and corky bark ( rugose wood complex) 4.3

Stem grooving Stem grooving (also known as legno riccio or rugose wood ) is associated with a drastic decline in infected grapevines. Characteristic behavioural patterns associated with the disease are dwarfed growth (smaller grapevines), dieback, exceptionally delayed bud burst and extensive reductions in crop size – to be followed, in most instances, by the total dieback of grapevines (Photo 208). Typical of stem grooving is swelling (thicker bark with a coarse surface) which often occurs at the top end of graft unions – with scion trunks looking markedly thicker compared to those of the rootstocks (Photo 209). Characteristic of the visually detectable abnormalities, best revealed by bark removal, are lengthwise grooving and pitting of underlying wood, which may occur in rootstock or scion sections of the trunk or in both (Photos 210 - 213). Anatomical changes/reactions induced by stem grooving may be ascribed to an abnormally functioning cambium which results in abnormal grooving on the xylem cylinder. Exceptional amounts of parenchymatic phloem tissue is formed in the affected areas at the expense of xylem. In normal adjacent zones, normal phloem and xylem are differentiated, with the result that the cambium takes on a wavy appearance. The cork cambium follows the same topography as the vascular cambium with the result that the grooves in the phloem are formed directly opposite the scars in the xylem (Photo 214). When the dead, coarse bark (Photo 215) is removed, hollows and grooves are also visible on the exterior surface of the underlying tissue and with cross sections (sawing through trunk sections) it is possible to observe just how deep the grooves have often become (Photo 216). Obviously it can be expected that in view of such far-reaching anatomical changes/reactions – which in due course increase in intensity – any grapevine would find it impossible to function normally at a physiological level (especially if it is furthermore exposed to stress), eventually to be doomed, in most instances, to die. Corky bark Globally considered corky bark – in conjunction with delayed bud burst, dieback of shoots and late leaf drop – is associated with seriously impaired vigour and considerable/total crop losses. Characteristic traits are associated with uneven/insufficient ripening (lignification) of some shoots in infected grapevines (Photo 217). Such shoots, the bark of which appears thickened and spongy (rubbery), have a tendency to bend downwards (hang) with tips that grow straight upwards, which imparts a crescent-shaped appearance (Photo 218). Lengthwise cracks that occur mainly in internodal swellings – induced by excessive proliferation of secondary phloem – at the base of the above-mentioned shoots, are considered very characteristic of corky bark (Photos 219 & 220). Additionally and characteristic of corky bark is the phenomenon that normal shedding of outer layers of bark (cork) does not occur in wood that is older than a year, but remains intact in

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