Grapevine red blotch-associated virus is present in free-living Vitis sp. proximal to cultivated grapevines
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Authors: Keith L. Perry,1 Heather McLane,1 Muhammad Z. Hyder,1 Gerald S. Dangl,2 Jeremy R. Thompson,1 and Marc F. Fuchs3
1Department of Plant Pathology and Plant-Microbe Biology, 334 Plant Science, Cornell University, Ithaca, NY 14853
2Foundation Plant Services, University of California Davis, One Shields Ave., Davis, California 95616
3Department of Plant Pathology and Plant-Microbe Biology, Cornell University, New York State Agricultural Experiment Station, Geneva, NY 14456
Phytopathology April 2016
Summary by Alex Koeberle
Grapevine red blotch is an emerging disease of grapevine found throughout North America. It was first noticed in 2008 in Napa Valley, California and described with symptoms resembling grapevine leafroll. Initial tests indicated that vines showing symptoms of this disease tested negative for leafroll viruses. However, upon further testing, grapevine red blotch-associated virus (GRBaV) was shown to be present in these infected grapevines. Since then, GRBaV has been detected over a wide geographical range in a relatively short time span from 2008-2015. GRBaV is now distributed across grape growing regions in the United States and has significant adverse impacts on production and commercial value. Given its prevalence and economic implications, two important questions emerge: How does GRBaV spread and what is its origin?
In 2012, a California vineyard had a cluster of vines with severe red blotch symptoms located along one edge of the block close to an uncultivated riparian habitat where wild vines (free-living) were growing. The researchers in this study sampled and mapped the distribution of GRBaV-infected vines and conducted a genetic analysis of these vines. The objectives of this study were: 1) to assess the genetic diversity of GRBaV isolates within a single vineyard block, 2) to determine if GRBaV was present in wild vines close to a vineyard with GRBaV-infected vines, and 3) to determine how GRBaV isolates from wild vines and cultivated vines were related genetically.
Because vines infected with GRBaV exhibit “leaf-reddening” symptoms that are sometimes similar to leaf symptoms associated with grapevine leafroll, GRBaV presence must be confirmed by DNA testing. For this study, leaf samples from vines in adjacent blocks of Cabernet franc, Cabernet Sauvignon, and Merlot were collected at a single vineyard site in the Napa Valley and processed for nucleic acid extraction (the extraction yields both DNA and RNA – but since GRBaV is a DNA virus, researchers did not need to look at the RNA). DNA sequencing was then used to assess the genetic variation and phylogenetic origin of GRBaV isolates within the single vineyard.
- Cabernet franc block was heavily infected with GRBaV. 60 out of 65 symptomatic vines, visually assessed as having red blotch, and 5 out of 122 asymptomatic vines, tested positive for gene fragments of GRBaV, thus showing a strong correlation between red blotch symptom presence and GRBaV detection. In addition, symptomatic and infected vines of Cabernet franc clone 214 were clustered at one end of the block closest to a riparian habitat.
- GRBaV isolates from Cabernet franc clone 214 were genetically homogeneous. The Cabernet franc clone 2014 planting showed high sequence identity, but was genetically distinct from isolates collected from other clones and cultivars planted adjacent to this block.
- GRBaV detected in wild vines in riparian area. Canes collected from 10 wild vines were sampled and four tested positive for GRBaV. DNA sequences in isolates were genetically similar to those found in GRBaV isolates collected from cultivated vines.
- Wild Vitis sp. in riparian areas were hybrids. DNA markers from wild GRBaV infected vines were evaluated and found to be V. californica x V. vinifera hybrids.
- GRBaV isolates from wild vines are closely related to those collected in cultivated vines. Ten isolates from the study site were sequenced to determine the relationship between GRBaV isolates found in the wild and cultivated vines. With one exception, wild vine isolates were found to be more genetically similar to each other than to the isolates collected from cultivated vines, although one isolate was almost identical to an isolate from Merlot. Visual observations suggested movement from riparian vines into the vineyard, although sequencing did not reveal any source relationship.
This study revealed that GRBaV is in fact present in wild (free-living) Vitis sp. As such, one may infer that GRBaV can be spread by natural, non-human-mediated means such as insect vectors. Mechanisms that may spread this virus include pollen, seed, or vector transmission, or local spread via foot grafting. Researchers also found a “strong correspondence” between symptom ratings and virus detection.
At the vineyard study site, researchers hypothesized GRBaV had spread, but could not determine the direction of movement between wild and cultivated grapevines. Due to the close overall relationships of GRBaV genomes, it can be difficult to differentiate separate populations. An exception to this was seen in isolates from the Cabernet franc clone 214 block, which was studied most intensively; all showed complete genetic identity, suggesting that the virus originated with the initial planting stock.
The Bottom Line:
The most effective strategy for effective GRBaV management is to select and plant healthy nursery stock that has been tested and certified to be virus-free. GRBaV origin across North America is best explained by transmission via nursery stocks, however GRBaV can also spread in the field. Managing within-vineyard transmission and spread will require a better understanding of how natural spread can occur.
Alex Koeberle ’13 is a writer and managing editor for Appellation Cornell