Monitoring nematode populations to adapt fallow periods againstXiphinema vectors of Grapevine fanleaf virus (GFLV)

Abstract: The nematode Xiphinema index is, economically, the major virus vector in viticulture,transmitting specifically the Grapevine fanleaf virus (GFLV), the most severe grapevine virusdisease worldwide. A second virus (ArMV) and its vector (X. diversicaudatum) do exist in theBordeaux region but seem of lower importance. Plots that have become completely infected, nolonger providing sufficient yield or quality, have to be uprooted to be replaced by new healthyplants.Increased knowledge of the spatial distribution of this nematode, both horizontally andvertically, and of correlative GFLV plant infections, is essential for efficient control of thedisease. Vertical distribution data showed that the highest numbers of individuals occurred at 40to 110cm depth, corresponding to the layers where the highest densities of fine roots wereobserved. Horizontal distribution revealed a significant aggregative pattern with patches of 6 to8m diameter, together with a significant neighbourhood structure of nematode densities, thusidentifying the relevant sampling scale to describe the nematode distribution. Nematode patchesoften, but not always, correlate significantly with those of GFLV-infected grapevine plants. Thenematodes are able to survive in the soil even without food for several years. Since nematodeslive deep in the soil, they cannot be attained by nematicides (that are, anyway, progressivelyforbidden). Because of this lack of efficient control measures of the vector, a fallow period of 7years is advised to farmers but rarely observed for economic reasons. In some cases, high reinfestationlevels have occurred with short fallow periods but this does not always occur.Sampling of numerous plots in the Bordeaux region, uprooted because of virus infestations,showed that nematode numbers are unpredictable. No clear relation was found between thepercentage of each virus and the population of its vector. X. diversicaudatum was rarely presentin significant amounts, even though infestation percentage of plants with ArMV could be high.X. index was sometimes present in high numbers (up to 300 per litre of soil) in some samples butmostly numbers were quite low or nematodes even not detected. These results suggest that thefallow period can be adapted following nematode sampling. In the case of very low nematodenumbers it could be reduced to 1.5 to 3 years whereas longer periods need to be observed whenhigh numbers are found. Nematode sampling grid should be rather dense, and sampling shouldideally be done twice, just after uprooting and before replanting. The use of ‘nematicidal’antagonistic plants against X. index is currently being investigated in our lab.