Spatial and Temporal Distribution of North American Grapevine Yellows Disease and of Potential Vectors of the Causal Phytoplasmas in Virginia

North American grapevine yellows is a destructive, phloem-limited disease of winegrapes in the mid-Atlantic and northeastern regions of the United States caused by at least two phytoplasmas (aster yellows and X-disease Candidatus phytoplasma spp.). Because insects have been identified as vectors of grapevine yellows in several countries, one or more insect vectors are suspected of transmitting the phytoplasmas in North American vineyards. Adding to evidence in support of insect transmission of North American grapevine yellows were observations that diseased vines often occurred in clusters frequently near vineyard edges that bordered woodland vegetation, suggesting movement of insect vectors into vineyards. The spatial distribution of North American grapevine yellows-affected vines was annually assessed in two northern Virginia vineyards over an 8-yr period. Spatial analysis indicated that diseased vines were significantly clustered, or their distribution was nonrandom, in both vineyards. Clustering was observed more frequently in the larger of the two vineyards. In addition, we surveyed the abundance of leafhoppers and planthoppers in and around three North American grapevine yellows-affected Chardonnay vineyards. The relative abundance of captured candidate vector species in and around North American grapevine yellows-infected vineyards differed among sampling methods. Agallia constricta was the most commonly captured leafhopper in sweep sampling in all years. Several species known to transmit phytoplasma were also captured, including Scaphoideus titanus, a grapevine yellows vector in Europe, Graminella nigrifrons, and Deltocephalus flavicosta. Three leafhopper species, S. titanus, Osbornellus auronitens, and Jikradia olitorius, exhibited seasonal movement into the vineyard that may account for the observed clustering of diseased vines.