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

Phylloxera.

the vigour and productive life expectancy of grapevines are seriously impaired. As with phylloxera and margarodes, above-ground abnormalities are associated with a gradual decrease in the growth and bearing abilities of affected grapevines, often in spot patterns. In the case of root-knot nematodes, below soil-surface abnormalities are mainly reflected by the incidence of round galls on young roots (Photo 285), as well as a lack of lateral roots, while adult females can sometimes be observed as small, white pearls in the galls. Although possibly less prominent, hook-like galls, similar to those caused by phylloxera, are also induced by dagger nematodes on young root tips (Van Zyl et al ., 2012). Overall the damage caused by nematodes may be ascribed to the following aspects in particular, namely: (i) direct damage (mechanical penetration by means of stylets) of the root by feeding on it, resulting in accessible wounding ports for easy infection by harmful rotting organisms and (ii) depositing of substances in the saliva during feeding whereby cells either die back (even over a distance) or are stimulated to divide abnormally – whereby root growth/development is arrested and the uptake of water and nutrients is seriously impaired. Additionally the transfer of fanleaf-associated viruses by dagger nematodes – which operate as acknowledged vectors thereof – should be seen in a serious light, inasmuch that grapevines may even die. Seeing that Vitis vinifera does not offer any resistance in this case either, the only method to fight this pest in the long-term remains the use of resistant rootstocks. In view of the fact that varying degrees of resistance should be maintained, however, and that nematodes (apart from via plant material) can be distributed extremely effectively via cultivation implements, drainage and irrigation water especially, it is recommended that for curtailing methods to be truly effective, one should follow applicable guidelines emanating from recent research findings (Hugo & Malan, 2011a, b, c) – with the emphasis on irrigation water.

PHOTO 275. This photo illustrates the occurrence of phylloxera (as yellowish-green plant lice) on a young root tip. Note that gall-forming has already begun, whereby further root growth is totally restricted. The white crystals on the photo are symbolic of sand-quartz particles and should not be confused with any stage of the life cycle of phylloxera.

180 • A Guide to Grapevine Abnormalities in South Africa