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

caused by the well-known erinose bud mites on the top sides of leaves. Although the leaf variation is a common occurrence in most overseas countries, it is an extremely rare phenomenon locally. Margarodes Even though margarodes as a below soil-surface pest is probably not as well known as phylloxera and nematodes, it is widespread in practically all viticultural areas, with serious damage being caused to grafted grapevines. Unlike phylloxera, there are no known resistant rootstocks, with the result that this pest – which may contribute to total dieback of grapevines over the short and longer term – is subject to extremely difficult control/ curtailing measures. In view of the fact that the extent of the damage caused by margarodes and the distribution thereof in recent years has increased rather than decreased, cutting- edge research is being undertaken at ARC Infruitec-Nietvoorbij (Stellenbosch) to unravel various umbrella aspects relating to the complexities of margarode issues. Emanating from this research, much elucidating and industry applicable information has already been published, inter alia clarifying the life cycle and seasonal occurrence thereof – without which the implementation of effective control strategies would not be possible (De Klerk & Vermeulen, 2007; De Klerk & Carstens, 2010; De Klerk et al ., 2011). Above-soil-surface abnormalities closely resemble those of phylloxera and are initially associated with less vigorous growth (shorter and thinner shoots with smaller leaves), whereafter dieback of one or more arms/cordons occur, eventually to be followed by overall demise of affected grapevines, often in spot patterns (Photo 282). No characteristic visually detectable galls or other abnormalities – as in the case of phylloxera – are encountered on the roots. Feeding actions are undertaken by the larval stage of the margarodes’ lifecycle, during which long sucking mandibles are inserted into the roots, cell sap extracted and a harmful substance (or substances) presumably deposited in the tissue – with the result that the growing ability of grapevines is impaired to such an extent that they die back in due course. At the same time several layers of hard wax are secreted (which seal in the larvae) resulting in the formation of cysts (ground pearls). The occurrence of such cysts – which may be observed with the naked eye – in the proximity of the roots of affected grapevines, may be singled out as the most characteristic indication of the presence of this pest (Photo 283). Nematodes Nematodes are microscopic in size (Photo 284) and affect the root systems of practically all crops. Locally this widespread pest phenomenon is mainly associated with the incidence of: (i) root-lesion; (ii) root-knot; (iii) spiral and (iv) dagger nematodes – in which case grapevine roots are weakened to such an extent by the feeding actions that both

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