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

GRAPEVINE PESTS Phylloxera, margarodes and nematodes 5.1

Phylloxera It is common knowledge that phylloxera are small plant lice (Photos 275 & 276) which occur mostly below the soil surface, in which case grapevine roots are affected to varying degrees of intensity. Seeing that scion material of Vitis vinifera grapevines have no resistance to this pest, the only method by which it can be curbed remains the use of phylloxera- resistant rootstocks – so much so that one can rightly declare the practice of grapevine grafting to be practically based on this in totality. Although rootstocks and hybrids are also affected, varying degrees of genetic resistance are maintained, which in turn are influenced to a large extent by vigour, soil type and climate. In this regard research-based findings have shown that the more vigorous the rootstock, the greater the resistance to phylloxera, while the presence/occurrence thereof in itself is to a large degree associated with heavy versus light textured soils (high percentage of fine to medium sand). On the whole, phylloxera (which occurs in all the viticultural areas) should still be seen as a serious local problem – and its potential impact on annual yields is grave. Abnormalities above the soil surface are associated with weaker growth/decline of grapevines in spot patterns within existing vineyards – a situation that gets worse as time goes by and eventually contributes to complete dieback. Characteristically the affected grapevines experience arrested growth earlier in the season, with an accompanying dull green discolouration and eventual yellowing/desiccation of leaves – in conjunction with grapes that are often subjected to watery ripening (Photo 277). It goes without saying that such dieback/desiccation of grapevines may be due to various causes, thus requiring thorough root analyses before one is able to blame the above-mentioned visual observations specifically on phylloxera. Below soil-surface abnormalities are characterised by the incidence of: (i) hook-like galls/thickening (nodositites) on young, fine feeding roots and (ii) slightly raised, less obvious galls (tuberositites) on older roots (Photos 278 - 280). The formation of nodositites – the colour of which initially appears yellow, changing to yellowish brown in a short period and later to dark brown – on feeding roots is very characteristic and is considered the most obvious abnormality with regard to the presence of phylloxera. The extremely detrimental effect of this pest on the roots of non-resistant grapevines may be ascribed to the fact that saliva is deposited in the wounds during feeding (long, sucking mandibles are inserted into roots to extract cellular juices), which initiates abnormal cell division resulting in the formation of galls. This goes hand-in-hand with the arrest of further root growth/lengthening and the uptake of water and nutrients is seriously impaired/prevented. In addition wounds caused by the feeding actions of phylloxera present extremely comfortable ports of ingress to various rotting organisms – which may eventually contribute to the complete devastation of root systems. Phylloxera can also feed on grapevine leaves and may form galls on the undersides (Photo 281) – a situation that should not be confused with the protrusions/galls

178 • A Guide to Grapevine Abnormalities in South Africa