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.

The transmission mode of grapevine red blotch virus (GRBV, genus Grablovirus, family Geminiviridae) by Spissistilus festinus, the three-cornered alfalfa hopper, is unknown. By analogy with other members in the family Geminiviridae, we hypothesized circulative, nonpropagative transmission. Time-course experiments revealed GRBV in dissected guts, hemolymph, and heads with salivary glands after a 5-, 8-, and 10-day exposure to infected grapevines, respectively. After a 15-day acquisition on infected grapevines and subsequent transfer on alfalfa, a nonhost of GRBV, the virus titer decreased over time in adult insects, as shown by quantitative PCR. Snap bean proved to be a feeding host of S. festinus and a pseudosystemic host of GRBV after Agrobacterium tumefaciens-mediated delivery of an infectious clone. The virus was efficiently transmitted by S. festinus from infected snap bean plants to excised snap bean trifoliates (90%) or grapevine leaves (100%) but less efficiently from infected grapevine plants to excised grapevine leaves (10%) or snap bean trifoliates (67%). Transmission of GRBV also occurred trans-stadially but not via seeds. The virus titer was significantly higher in (i) guts and hemolymph relative to heads with salivary glands, and (ii) adults emanating from third compared with first instars that emerged on infected grapevine plants and developed on snap bean trifoliates. This study demonstrated circulative, nonpropagative transmission of GRBV by S. festinus with an extended acquisition access period compared with other viruses in the family Geminiviridae and marked differences in transmission efficiency between grapevine, the natural host, and snap bean, an alternative herbaceous host.

Spissistilus festinus (Hemiptera: Membracidae) transmit grapevine red blotch virus (GRBV, Grablovirus, Geminiviridae) in greenhouse settings; however, their role as a vector of GRBV in vineyards is unknown. Following controlled exposures of aviruliferous S. festinus for two weeks on infected, asymptomatic vines in a California vineyard in June and a 48 h gut clearing on alfalfa, a nonhost of GRBV, approximately half of the released insects tested positive for GRBV (45%, 46 of 102), including in the salivary glands of dissected individuals (11%, 3 of 27), indicating acquisition. Following controlled exposures of viruliferous S. festinus for two to six weeks on GRBV-negative vines in vineyards in California and New York in June, transmission of GRBV was detected when two S. festinus were restricted to a single leaf (3%, 2 of 62 in California; 10%, 5 of 50 in New York) but not with cohorts of 10–20 specimens on entire or half shoots. This work was consistent with greenhouse assays in which transmission was most successful with S. festinus exposed to a single leaf (42%, 5 of 12), but rarely occurred on half shoots (8%, 1 of 13), and never on entire shoots (0%, 0 of 18), documenting that the transmission of GRBV is facilitated through the feeding of fewer S. festinus on a restricted area of grapevine tissue. This work demonstrates S. festinus is a GRBV vector of epidemiological importance in vineyards.

Grapevine red blotch disease emerged within the past decade, disrupting North American vine stock production and vineyard profitability. Our understanding of how grapevine red blotch virus (GRBV), the causal agent of the disease, interacts with its Vitis hosts and insect vector, Spissistilus festinus, is limited. Here, we studied the capabilities of S. festinus to transmit GRBV from and to free-living vines, identified as first-generation hybrids of V. californica and V. vinifera ‘Sauvignon blanc’ (Vcal hybrids), and to and from V. vinifera ‘Cabernet franc’ (Vvin Cf) vines. The transmission rate of GRBV was high from infected Vcal hybrid vines to healthy Vcal hybrid vines (77%, 10 of 13) and from infected Vvin Cf vines to healthy Vcal hybrid vines (100%, 3 of 3). In contrast, the transmission rate of GRBV was low from infected Vcal hybrid vines to healthy Vvin Cf vines (15%, 2 of 13), and from infected Vvin Cf vines to healthy Vvin Cf vines (19%, 5 of 27). No association was found between transmission rates and GRBV titer in donor vines used in transmission assays, but the virus titer was higher in the recipient leaves of Vcal hybrid vines compared with recipient leaves of Vvin Cf vines. The transmission of GRBV from infected Vcal hybrid vines was also determined to be trans-stadial. Altogether, our findings revealed that free-living vines can be a source for the GRBV inoculum that is transmissible by S. festinus to other free-living vines and a wine grape cultivar, illustrating the interconnected roles of the two virus hosts in riparian areas and commercial vineyards, respectively, for virus spread. These new insights into red blotch disease epidemiology will inform the implementation of disease management strategies.

Grapevine red blotch virus (GRBV) causes red blotch disease and is transmitted by the three-cornered alfalfa hopper, Spissistilus festinus. GRBV isolates belong to a minor phylogenetic clade 1 and a predominant clade 2. Spatiotemporal disease dynamics were monitored in a 1-hectare ‘Merlot’ vineyard planted in California in 2015. Annual surveys first revealed disease onset in 2018 and a 1.6% disease incidence in 2022. Ordinary runs and phylogenetic analyses documented significant aggregation of vines infected with GRBV clade 1 isolates in one corner of the vineyard (Z = −4.99), despite being surrounded by clade 2 isolates. This aggregation of vines harboring isolates from a non-prevalent clade is likely due to infected rootstock material at planting. GRBV clade 1 isolates were predominant in 2018–2019 but displaced by clade 2 isolates in 2021–2022, suggesting an influx of the latter isolates from outside sources. This study is the first report of red blotch disease progress immediately after vineyard establishment. A nearby 1.5-hectare ‘Cabernet Sauvignon’ vineyard planted in 2008 with clone 4 (CS4) and 169 (CS169) vines was also surveyed. Most CS4 vines that exhibited disease symptoms one-year post-planting, likely due to infected scion material, were aggregated (Z = −1.73). GRBV isolates of both clades were found in the CS4 vines. Disease incidence was only 1.4% in non-infected CS169 vines in 2022 with sporadic infections of isolates from both clades occurring via secondary spread. Through disentangling GRBV infections due to the planting material and S. festinus-mediated transmission, this study illustrated how the primary virus source influences epidemiological dynamics of red blotch disease.

Grapevine red blotch virus (GRBV) is the causative agent of red blotch disease. Limited information is available on the seasonal variation of GRBV titer in relation to disease symptom expression in vineyards across the United States. In this study, no statistically significant difference in GRBV titer was found among asymptomatic infected vines in June ( P = 0.451) and among symptomatic infected vines in October ( P = 0.068) in a diseased ‘Cabernet franc’ vineyard in California, regardless of the years symptomatic, one to seven, as shown by qPCR. Similarly, there were no statistically significant differences in GRBV titer as it related to isolates of the two phylogenetic clades in asymptomatic infected ‘Cabernet franc’ and ‘Cabernet Sauvignon’ vines in June ( P = 0.138 and P = 0.778, respectively) and in symptomatic infected vines in October ( P = 0.806 and P = 0.490, respectively). GRBV titer differed among cultivars in diseased California vineyards ( P < 0.001) and increased over the course of the growing season in infected ‘Merlot’ and ‘Cabernet franc’ vines, but not in ‘Cabernet Sauvignon’ vines. Patterns observed in California were consistent in New York and Georgia vineyards. In a Geneva double curtain-trellised ‘Cabernet Sauvignon’ vineyard in Georgia, GRBV distribution was uneven between cordons, and virus titer was variable within the vine canopy in June ( P = 0.017) but not in October ( P = 0.107). This work revealed consistent patterns of GRBV titer during a growing season in different vineyards across the United States. It also highlighted relatively high virus titer in symptomless grapevines in June, when Spissistilus festinus-mediated GRBV transmission is documented in northern California.