Limited information is available on the spread dynamics of grapevine red blotch virus (GRBV, genus Grablovirus, family Geminiviridae) in vineyards. In this study, we investigated red blotch disease progress in three vineyards with a disparate initial inoculum prevalence. Secondary spread was documented in Cabernet Sauvignon and Cabernet franc vineyards in California, but not in a Merlot vineyard in New York. Increase in annual disease incidence (4.8, 0.13, and 0%) was unrelated to the estimated initial source of inoculum at planting (1, 40, and 40%) in the Cabernet franc, Cabernet Sauvignon, and Merlot vineyards, respectively. Limited genetic diversity of GRBV populations in newly infected vines supported localized spread in California vineyards, and suggested the planting material as the primary source of inoculum. Among the community of hemipteran insects visiting two of the three study vineyards, populations of Spissistilus festinus, the vector of GRBV, were absent in the Merlot vineyard and low in the Cabernet Sauvignon vineyard. Furthermore, all cover crop samples collected from GRBV-infected California vineyards each spring of 2016 to 2018, particularly legume species which are preferred hosts of S. festinus, tested negative for GRBV, suggesting a minimal role, if any, in GRBV spread as inoculum reservoirs. Together our findings illustrate differential disease progress in distinct vineyard ecosystems, and support the elimination of virus inoculum sources as an actionable disease management strategy across vineyards.
Spissistilus festinus (Say, 1830) (Hemiptera: Membracidae) is a frequent pest of leguminous crops in the Southern United States, and a vector of grapevine red blotch virus. There is currently no information on the genetic diversity of S. festinus. In this study, populations of S. festinus were collected in 2015–2017 from various crops and geographic locations in the United States, and fragments of the mitochondrial cytochrome C oxidase 1 (mt-COI) gene and the nuclear internal transcribed spacer 2 (ITS2) region were characterized by polymerase chain reaction and sequencing. Maximum-likelihood and Bayesian analyses of the mt-COI and ITS2 sequences yielded similar phylogenetic tree topologies, revealing two distinct genetic S. festinus lineages with all of the specimens from California comprising one phylogenetic clade, alongside a single GenBank entry from Arizona, and all specimens from the Southeastern United States comprising a statistically-supported distinct clade, regardless of host and year of collection. The mt-COI gene fragment showed up to 10.8% genetic distance between the two phylogenetic clades. These results suggest the existence of two genotypes within S. festinus in the United States. The only distinct morphological trait between the two genotypes was a less elevated pronotum in the representative specimens from California, compared to the representative specimens from the Southeastern United States. Since this phenotypic feature is inconspicuous, a diagnostic polymerase chain reaction targeting a variable region of the mt-COI fragment was developed to reliably distinguish between the specimens of the two genotypes of S. festinus and to facilitate their specific identification.
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