Ecology of grapevine red blotch disease

Grapevine red blotch virus (GRBV) is the type member of a new genus- Grablovirus in the family Geminiviridae. GRBV causes grapevine red blotch disease, which was identified within the last decade as a threat to grape production in North America. GRBV is primarily spread via grafting and dissemination of infected planting material, but it can also be vectored under greenhouse conditions by the three-cornered alfalfa hopper (Spissistilus festinus). The epidemiology and ecology of this disease is not well understood. In order to gain a comprehensive understanding of GRBV spread in vineyards and the factors mediating secondary spread, we conducted several multi-year studies in vineyards with varying levels of GRBV incidence. We mapped red blotch symptoms for up to five years, and conducted insect surveys for two years in two vineyards in California and one vineyard in New York. These studies collectively demonstrated that GRBV is spreading in California vineyards, but not in New York. They also demonstrated that spread of GRBV is associated with the spatial distribution of S. festinus. Based on the frequent detection of GRBV in specimens from these surveys, three additional insect vector candidates were identified- Colladonus reductus, Osbornellus borealis (Cicadellidae) and a Melanoliarus spp. (Cixiidae), though these and S. festinus were not observed in the New York vineyard. GRBV and a related grablovirus, wild Vitis virus 1 (WVV1), were detected in wild grapevines throughout northern California, in higher density closer to grape production areas. The diversity of GRBV in wild grapevines and grape cultivars is similar but the distribution and diversity patterns of GRBV in wild grapevines suggest that the direction of GRBV spread is predominantly from vineyards to wild grapevines. Studies on diversity of S. festinus throughout California and the southeastern U.S. revealed two distinct genotypes, in which the southeastern populations are distinct from the California populations, regardless of the host plant or time of collection. These studies on the epidemiology and ecology of red blotch disease have informed disease management recommendations with efforts that should be focused on removal of inoculum sources (i.e. infected vines and wild grapevines) rather than controlling S. festinus populations