Red Blotch – Associated Virus

Grapevine red blotch virus (GRBV), the causative agent of red blotch disease, is a member of the genus Grablovirus in the family Geminiviridae, and the first known geminivirus of Vitis spp.
(Cieniewicz et al. 2017a; Sudarshana et al., 2015; Varsani et al., 2017). Limited information is available on disease biology and epidemiology. Analysis of the spatiotemporal incidence of
GRBaV over a three-year period (2014–2016) in a selected 5-acre vineyard of Cabernet franc in California was consistent with the occurrence of virus spread. The incidence of diseased plants
increased by 1-2% annually. Spatial analysis of diseased plants in each year using ordinary runs analysis within rows and Spatial Analysis by Distance IndicEs (SADIE) demonstrated
aggregation, particular at the edge of the vineyard proximal to a riparian area, and isolated diseased vines. Analysis of epidemic spread fitting a stochastic spatiotemporal model using the
Monte Carlo Markov Chain method identified strong evidence for localized (within vineyard) spread. A spatial pattern consisting of a combination of strongly aggregated and randomly isolated symptomatic vines within 8-years post-planting suggested unique epidemic attributes compared to those of other grapevine viruses vectored by mealybugs and soft scales or by dagger nematodes for which typical within-row spread and small-scale autocorrelation are well documented (Cieniewicz et al., 2017b). These findings are consistent with the existence of a new type of vector for a grapevine virus. In contrast, similar spatiotemporal analyses of virus incidence in a diseased Merlot vineyard New York from 2014 to 2016 did not provide any evidence of GRBV spread. An insect vector of epidemiological relevance remains elusive for GRBV although the three cornered alfalfa treehopper (Spissistilus festinus [Say]) was recently shown to transmit GRBV in the greenhouse. To determine the diversity and distribution of potential vector candidates in the Cabernet franc vineyard in California, sticky cards were placed
from March to November in 2015 and 2016 in the area where disease incidence increased by nearly 20% between 2014 and 2016. Insects on sticky card traps were identified to species when possible by morphological characteristics and sequencing of the mitochondrial COI barcode region. Among the subsets of insect species/taxa that were removed from sticky cards and tested
by multiplex polymerase chain reaction, GRBV was consistently detected in Spissistilus festinus (Membracidae), Colladonus reductus (Cicadellidae), Osbornellus borealis (Cicadellidae) and a
Melanoliarus species (Cixiidae). Populations of these four candidate vectors peaked from June to September with viruliferous S. festinus culminating from late June to early July in both years. An assessment of co-occurrence and co-variation between GRBV-infected vines and viruliferous insects using the association function of SADIE identified a significant association between the
spatial distribution of infected vines and viruliferous S. festinus. These findings revealed the epidemiological significance of S. festinus as a vector of GRBV and the need for testing the
transmission capability of C. reductus, O. borealis, and the Melanoliarus species (Cieniewicz et al., 2017c). Altogether, our insights into the spread of GRBV and into population dynamics of S.
festinus and three other candidate vectors are important to inform epidemiological features of red blotch disease and devise disease management strategies.