Grapevine Red Blotch-Associated Virus, an Emerging Threat to the Grapevine Industry

Grapevine is a deciduous woody perennial vine for which the cultivation of domesticated species began approximately 6,000 to 8,000 years ago in the Near East, likely in a region between the Black sea and Iran (Myles et al. 2011; This et al. 2006). It is one of the earliest domesticated crops (Myles et al. 2011). Grape crops are primarily grown for fruit, juice, raisin, and wine production throughout the world. Thus far, 65 viruses have been described in grapevines (Martelli 2014). Several of them affect vine health and result in notable losses by reducing vigor and yield, altering fruit juice chemistry, shortening the productive lifespan of vineyards, or even causing vine mortality (Martelli 2014). Grapevine viruses belong to 27 genera of 15 plant virus families but some of them are yet to be taxonomically assigned to a genus or a family (Martelli 2014). Based on the diseases they cause, these viruses can be grouped into four major categories, including those that are (i) responsible for infectious degeneration or decline disease (15 viruses), (ii) associated with leafroll disease (five viruses), (iii) associated with the rugose wood complex (six viruses), and (iv) associated with the fleck complex (seven viruses). Additional viruses are associated with diseases that are geographically more restricted than the four abovementioned widespread ones, cause latent infections, or have an unknown pathogenicity (Martelli 2014).

Among the four major disease categories, leafroll disease occurs in all grape-producing regions of the world. Viruses associated with leafroll elicit interveinal reddening of leaves in combination with cupping, and primary veins remain green in red-fruited Vitis vinifera cultivars. In white-fruited V. vinifera cultivars, symptoms are less pronounced, consisting of a slight chlorosis and cupping (Martelli 2014; Naidu et al. 2014). In spite of causing distinct disease symptoms, viruses associated with leafroll affect yield and grape juice quality parameters, such as total soluble solids (TSS), anthocyanin content, and total phenolics (Naidu et al. 2014). Diagnostic assays are available for leafroll disease-associated viruses but, in recent years, many grapevines exhibiting symptoms similar to leafroll did not test positive for any of the leafroll-associated viruses (Greenspan 2013). This suggested the presence of previously unknown viruses or virus strains undetectable by existing diagnostic tests, or the occurrence of a hitherto undescribed pathogen.

In spite of the wealth of knowledge about the nature of virus diseases that have been recognized for extended time in grapevines, such as those associated with leafroll disease, new virus species and diseases of importance to grapevine health and productivity have continued to emerge. Among these is red blotch disease, a recently recognized threat to the grapevine industry. This review summarizes the current state of knowledge about the emergence of red blotch disease by describing the discovery of a new circular, single-stranded (ss)DNA virus that was first isolated from diseased grapevines in 2012, and highlighting gaps in research needed to address critical issues surrounding disease management.

DISCOVERY

Disease symptoms somewhat resembling those of leafroll disease were observed in an 8-year-old Cabernet Sauvignon vineyard at the experimental research station of the Department of Viticulture and Enology, University of California, Oakville in 2008 (Calvi 2011). The foliar red blotch symptoms started in the middle or at the edges of leaf blades and coalesced as the season progressed (Fig. 1A). Initially, symptoms were largely restricted to the basal leaves but leaves in the middle of the shoots became symptomatic as the season progressed. In symptomatic leaves, the primary, secondary, and tertiary veins often became red, and the veinlets within blotchy areas also turned red (Fig. 1B). Extensive testing for all known leafroll-associated viruses revealed no infection by any of the leafroll-associated viruses in symptomatic grapevines (Calvi 2011; Al Rwahnih et al. 2013). Diseased grapevines with similar red blotch symptoms that also tested negative for leafroll viruses were noticed in other commercial vineyards. At all locations, TSS were reduced in fruit juice of symptomatic grapevines. The affected vineyards were planted to red fruit ‘Cabernet franc’, ‘Cabernet Sauvignon’, and ‘Zinfandel’ that were grafted onto rootstocks 101-14 Mgt, O39-16, or AXR-1. The virus nature of the disease was determined in 2011 using a metagenomics approach by next-generation sequencing (NGS). Three cDNA libraries, each generated from purified double-stranded (ds)RNA preparations, not subjected to DNAse or RNAse treatment, and isolated from dormant canes of three scions were sequenced by Illumina sequencing. Sequence analysis revealed that all three libraries contained the sequence of a previously unknown ssDNA virus similar to viruses in the family Geminiviridae that have a monopartite genome organization. Two libraries yielded complete genome sequences, and the other contained nearly 70% of the genome based on de novo assembled contigs. This newly discovered virus was first reported in October 2012 and was given the provisional name of Grapevine red blotch-associated virus (GRBaV) (Al Rwahnih et al. 2012a).

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Concurrently, a Cabernet franc vineyard in New York showed symptoms suggestive of leafroll but the vines tested negative for all leafroll-associated viruses (Krenz et al. 2012a). In March 2011, a collection of dormant canes from this vineyard was tested by rolling circle amplification (RCA), a method that efficiently amplifies small, circular DNA genomes such as those of phages and geminiviruses, among others (Dean et al. 2001; Haible et al. 2006; Inoue-Nagata et al. 2004). DNA sequencing of the endonuclease-digested RCA products revealed the presence of a geminivirus-like ssDNA genome of 3,206 nucleotides (nt). The genome sequence of this virus, tentatively named “Grapevine cabernet franc-associated virus”, was reported in spring 2012 (Krenz et al. 2012a). Because this was likely to be of significance to the grapevine industry, the Cornell group sent a copy of the manuscript submitted for publication to the grapevine virologists at Davis, CA. At this point, it became apparent that the same virus had also been discovered and was being worked on in California.

During fall 2012, the California and New York research groups presented their findings at the 17th congress of the International Council for the Study of Virus-like Diseases of the Grapevine (Al Rwahnih et al. 2012a; Krenz et al. 2012b), and to avoid confusion, both groups decided to henceforth refer to the virus as “Grapevine red blotch-associated virus”. Subsequently, reports confirmed the occurrence of GRBaV in Washington State and in Oregon vineyards, where it had been called “Grapevine red leaf-associated virus” (Poojari et al. 2013) and “Grapevine geminivirus: (Seguin et al. 2014), respectively. In these latter investigations, the virus was detected using deep sequencing of either purified total RNA (Poojari et al. 2013) or small RNAs (Seguin et al. 2014) to generate the cDNA libraries used for sequencing.

The discovery of GRBaV provided a plausible explanation for the abundance of grapevine samples with leafroll-like symptoms that had previously tested negative for leafroll-associated viruses by enzyme-linked immunosorbent assay (ELISA) or reverse-transcription polymerase chain reaction (RT-PCR). Since then, many symptomatic, leafroll-negative vines have been shown to be positive for GRBaV (Greenspan 2013; Stamp and Wei 2013; Sudarshana et al. 2013). In spite of the detection of GRBaV in vines exhibiting red blotch disease symptoms (Sudarshana et al. 2013), there has been resistance to accept the causal relationship between the virus and disease in the absence of the completion of Koch’s postulates. This is understandable, given the economic importance of the disease, the consequences of the discovery of a new virus for the nursery industry and clean plant programs, and the unexpected presence of a gemini-like virus in grapevines. Nonetheless, growers viewed red blotch as a new disease that required guidance with respect to diagnosis and management but recognized, along with extension educators and researchers, that disease symptoms in some instances resembled those caused by a variety of biotic and abiotic factors.

Since the initial reports of the virus associated with red blotch disease (Al Rwahnih et al. 2013; Krenz et al. 2012a; Poojari et al. 2013), a cloned GRBaV genome has been shown to be infectious and to incite red blotch symptoms following agroinoculation of presumably GRBaV-free grapevine plants (M. F. Fuchs and K. L. Perry, unpublished data). Additionally, geminivirus-like particles have been observed in purified extracts from infected grapevine tissues when viewed by transmission electron microscopy (S. Bag, A. Rowhani, T. Tian, and M. R. Sudarshana, unpublished data). Taken together, these findings support the causal role of GRBaV in red blotch disease, and underscore the need for research to develop management programs and limit virus spread.

The discovery of GRBaV in grapevines was surprising and unexpected. Although several new viruses have been reported in grapevine in recent years (Al Rwahnih et al. 2009, 2012a,b, 2013; Giampetruzzi et al. 2012; Zhang et al. 2011), newly identified viruses associated with recently recognized diseases are uncommon, and GRBaV was only found because of its association with what came to be recognized as a new disease. Prior to the recent reports of GRBaV and Citrus chlorotic dwarf-associated virus (CCDaV) in citrus trees affected by chlorotic dwarf disease in Turkey (Loconsole et al. 2012), only a few other geminiviruses have been reported from fruit crops (Cheng et al. 2014; Ferreira et al. 2010; Saxena et al. 1998; Wang et al. 2004). Interestingly, the genomes of GRBaV at 3,206 nt (Krenz et al. 2012a), and CCDaV at 3,614 nt (Loconsole et al. 2012) are somewhat larger than those of previously reported monopartite geminivirus genomes, which range from 2.7 to 3.0 kb in size (Brown et al. 2012; Rojas et al. 2005). As large-scale sequencing technologies are more commonly employed, the discovery of increasing numbers of circular ssDNA and other virus genomes associated with woody plant hosts might be expected (Loconsole et al. 2012; Rosario et al. 2012).

GENOME STRUCTURE, GENETIC VARIABILITY, AND TAXONOMY

The genome of GRBaV is composed of a single molecule of circular ssDNA (Al Rwahnih et al. 2013; Krenz et al. 2012a, 2014; Poojari et al. 2013; Seguin et al. 2014). RCA did not reveal the presence of a DNA-B molecule (Al Rwahnih et al. 2013; Krenz et al. 2012a), the second DNA found in bipartite begomoviruses (Brown et al. 2012). This was consistent with all of the NGS sequencing datasets (Al Rwahnih et al. 2013; Poojari et al. 2013; Seguin et al. 2014). Similarly, DNA satellites that are often associated with monopartite geminivirus genomes (Zhou 2013) were not detected in grapevines affected by red blotch disease (Al Rwahnih et al. 2013; Poojari et al. 2013; Seguin et al. 2014).

Analyses of the GRBaV genome led to the prediction of six open reading frames (ORFs) (Fig. 2). The genome sequence contained the nonanucleotide sequence TAATATT|AC that serves as the nick site for the origin of replication, and is characteristic of all members of the family Geminiviridae (Brown et al. 2012; Rojas et al. 2005; Stanley et al. 2005). Consistent with other geminiviruses, the orientation of the predicted ORFs is bidirectional. There are three ORFs in the viral sense orientation (V1, V2, and V3) and three in the complementary orientation (C1, C2, and C3), although none of these genes or their predicted products has been experimentally confirmed (Fig. 2). By analogy with mastreviruses (Wright et al. 1997), the complementary-sense ORFs encode replication-associated proteins (RepA); C1 encodes the RepA protein and C1 and C2 give rise to a spliced transcript encoding the replication protein (Rep) (Krenz et al. 2014). C3 is internal to and in the same frame as the C1 ORF. BLASTP analyses (Altschul et al. 1997) reveal that, among all ORFs, the C2 ORF shares the greatest amino acid sequence identity with other geminiviruses, with at least 50% identity. Among the top hits were Euphorbia caput-medusae latent virus, presumably from the genus Begomovirus, and several members of the genus Mastrevirus, including Miscanthus streak virus, Sugarcane streak virus, and Oat dwarf virus. The C1-encoded protein shares 32 to 36% amino acid identity with presumed members of the genus Begomovirus (French bean severe leaf curl virus and Euphorbia caput-medusae latent virus), and with members of the genus Mastrevirus (Chickpea chlorosis virus and Chickpea chlorotic dwarf virus) (Krenz et al. 2014).

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Protein homologs of the encoded GRBaV virion-sense ORFs were difficult to detect. Sequence similarity was observed between the putative GRBaV coat protein and that of several begomoviruses, including the old world Mesta yellow vein mosaic virus at 28%, a value that supports the extreme divergence of GRBaV with previously studied geminiviruses. Based on this limited homology and its position in the genome, the V1 ORF is predicted to encode the GRBaV coat protein. No putative conserved domains were observed for the V2 and V3 predicted ORFs. By analogy with mastreviruses and other geminiviruses, the V2 and V3 ORFs of GRBaV have been suggested to encode movement-related proteins (Al Rwahnih et. al. 2013; Krenz et. al. 2012a, 2014; Poojari et. al. 2013). The low nucleotide identity and protein similarity with other geminiviruses suggest that GRBaV could be recognized as a member of a new genus in the family Geminiviridae (Varsani et al. 2014).

Since the discovery of GRBaV, the genome sequence of 15 isolates has been determined (Table 1). Neighbor-joining analyses indicated two divergent groups of GRBaV isolates (Fig. 3) (Krenz et al. 2014). Among the available GRBaV genomes, the nucleotide identity between the two clades ranges from 91 to 93%. This is consistent with intraspecies variation within the family Geminiviridae (Rojas et al. 2005; Varsani et al. 2014). Although no biological differences between isolates are recognized at present, additional studies of biological characteristics are required to determine the existence of two distinct strains of GRBaV. The majority of available GRBaV isolates belongs to clade 2 and share approximately 98% nucleotide identity; isolates of clade 1 are somewhat more divergent at approximately 95%. Also, within clade 1, putative recombinants of GRBaV have been described (Fig. 3) (Krenz et al. 2014).

TABLE 1. Isolates of Grapevine red blotch-associated virus with sequenced genomes

Isolate	Cultivar	Origin	GenBank accession number	Reference
NY135	Merlot	Pennsylvania	KF147917	Krenz et al. 2014
NY137	Cabernet franc	Pennsylvania	KF147918	Krenz et al. 2014
NY147	Pinot noir	New York	KF751708	Krenz et al. 2014
NY175	Merlot	Maryland	KF147916	Krenz et al. 2014
NY271	Cabernet franc	Pennsylvania	KF147915	Krenz et al. 2014
NY358	Cabernet franc	New York	JQ901105	Krenz et al. 2012a
Canada	Unknown	Canada	JX559642	M. Rott and M. Belton, unpublished
CF214	Cabernet franc	California	KC896623	Al Rwahnih et al. 2013
CS337	Cabernet Sauvignon	California	KC896624	Al Rwahnih et al. 2013
Z1A	Zinfandel	California	KC896625	Al Rwahnih et al. 2013
WA1	Unknown	Washington	KC427993	Poojari et al. 2013
WA2	Unknown	Washington	KC427994	Poojari et al. 2013
WA-MR	Merlot	Washington	KC427995	Poojari et al. 2013
WA-CF	Cabernet franc	Washington	KC427996	Poojari et al. 2013
Or1a	Pinot noir	Oregon	KF137562	Seguin et al. 2014

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DIAGNOSIS

Red blotch disease can be diagnosed by monitoring foliar symptoms that first appear on older leaves at the base of the canopy in late spring to early summer (June to July in the Northern Hemisphere) and are progressively observed toward the top of the canopy in later months (August to October in the Northern Hemisphere). Red spots or blotches occurring early in the season typically coalesce, with most of the leaf blade becoming red later in the season. Shades of red varied from crimson to purple (Fig. 1A–G and J). Late in the season, heavily symptomatic leaves often drop off prematurely. On white-berried V. vinifera cultivars, foliar symptoms are less conspicuous and generally involve irregular chlorotic areas (Fig. 1H) that can become necrotic later in the autumn (Fig. 1I). At times, diseased vines show cupping of the leaf blade, and veins underneath the leaf blade of red-berried V. vinifera cultivars become red. In addition to foliar symptoms, delayed fruit ripening and altered fruit juice chemistry indices are characteristic of red blotch. The severity of blotches and the onset of symptoms can vary among grapevine cultivars (Fig. 1) and growing seasons, indicating that a reliable diagnosis based on characteristic symptoms can be challenging due to confounding factors, including the striking similarities between the foliar symptoms caused by red blotch- (Fig. 4A) and leafroll-associated (Fig. 4B and C) viruses. This is likely one reason for the recent discovery of GRBaV, even though vineyard managers and vintners were cognizant of a serious pathological problem since the early 2000s. Also, there are similarities between foliar red blotch symptoms and symptoms caused by abiotic factors such as poor root health, physical injuries that result in trunk or shoot girdling (Fig. 4E and F), mineral deficiencies (Fig. 4H–J), biotic factors such as mite damage (Fig. 4G), and crown gall caused by Agrobacterium tumefaciens (Fig. 4D). Because symptom variation makes the visual diagnosis of GRBaV-infected vines difficult, only DNA-based assays have proven reliable for accurate diagnosis.

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Determining the GRBaV genome sequence has facilitated the design of diagnostic primers that target different viral ORFs (Fig. 2), including the predicted coat protein (V1), the RepA (C1) (Krenz et al. 2014), and the putative movement protein (V2) (Al Rwahnih et al. 2013). Primers are used in simplex or duplex conventional PCR (Table 2) with nucleic acid extracts from various tissues, including cortical scrapings of dormant canes, leaf petioles, and blade tissue (Al Rwahnih et al. 2013; Krenz et al. 2014). The virus is also detected in roots, fruit clusters, fruit skin, and fruit juice (M. R. Sudarshana and A. Gonzalez, unpublished data). A real-time PCR using primers to probe the V2 ORF has been developed to detect the virus (M. R. Sudarshana, A. Dave, and A. Gonzalez, unpublished data). Another approach for the detection of GRBaV utilizes a macroarray of 60- to 70-mer virus-specific oligonucleotide probes spotted onto a nylon membrane (Thompson et al. 2014). Unlike most grapevine viruses that are optimally detected in young tissue during the spring season (nepoviruses) or in old tissues in the late summer to fall (vitiviruses and closterovirids), preliminary observations indicate that GRBaV can be detected at any time of the year, although dormant cane material provides the most consistent results (unpublished data).

TABLE 2. Diagnostic primers for the detection of Grapevine red blotch-associated virus by polymerase chain reaction

Name	Sequence (5′→3′)	Position (nucleotides)	Amplicon (bp)	Reference
GVGF1	5′-CTCGTCGCATTTGTAAGA-3′	255–272
GVGR1	5′-ACTGACAAGGCCTACTACG-3′	793–812	557	Al Rwahnih et al. 2013
CPfor	5′-AGCGGAAGCATGATTGAGACATTGACG-3′	1,073–1,099
CPrev	5′-AACGTATGTCCACTTGCAGAAGCCGC-3′	1,329–1,304	231	Krenz et al. 2014
Repfor	5′-CAAGTCGTTGTAGATTGAGGACGTATTGG-3′	2,567–2,595
Reprev	5′-AGCCACACCTACACGCCTTGCTCATC-3′	2,884–2,850	318	Krenz et al. 2014

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GEOGRAPHIC DISTRIBUTION AND IMPACT

Surveys of vineyards and germplasm collections have indicated the presence of GRBaV in wine and table grape cultivars in numerous states in the United States, including California, Maryland, New York, New Jersey, Oregon, Pennsylvania, Virginia, Washington (Krenz et al. 2014), Arkansas (K. L. Perry and M. F. Fuchs, unpublished data), Georgia (Brannen et al. 2013), Idaho (A. Karasev, personal communication), and Texas (National Clean Plant Network 2013). The virus has also been detected in interspecific hybrids (K. L. Perry and M. F. Fuchs, unpublished data), in rootstocks in the United States (Stamp and Wei 2013), and in wine grape cultivars in British Columbia (GenBank JX559642) and Ontario, Canada (M. F. Fuchs, unpublished data). There are no reports yet on the presence of GRBaV outside of North America; however, given the extensive exchange of propagation material globally, the virus is expected to be found in other major grape-growing countries. At the National United States Department of Agriculture–Agricultural Research Service (USDA-ARS) Clonal Germplasm Repository—Tree Fruit & Nut Crops & Grapes in Davis, CA, several table grape accessions collected from different countries have been found to be infected by GRBaV (Al Rwahnih et al. 2015). It is not known whether these accessions were infected at the time of their introduction or after establishment in the vineyard.

A few studies have addressed the effect of red blotch on vine health. For example, prior to the discovery of GRBaV, fruit juice of diseased Cabernet Sauvignon vines showed a 2.3°Brix reduction of TSS compared with fruit juice of asymptomatic vines (Calvi 2011). Photosynthesis and stomatal conductance, berry weight and anthocyanin level at harvest, and pruning weight were reduced in diseased vines. Midday leaf water potential, juice acidity, and pH were apparently unaffected in symptomatic vines. Also, lower ratios of leaf/petiole potassium and higher berry potassium levels in diseased vines were observed, suggesting vascular blockage at the leaf-petiole boundary, with both the translocation of solutes and berry metabolism being affected (Calvi 2011). In a study conducted after the discovery of GRBaV, a 2.65 and 1.0°Brix reduction was measured in fruit juice of Cabernet Sauvignon and ‘Chardonnay’ grapevines, respectively. GRBaV seems to affect fruit yield in Chardonnay and Zinfandel but not in Cabernet Sauvignon (R. J. Smith, M. L. Cooper, M. M. Anderson, and M. R. Sudarshana, unpublished data). Some effects observed for GRBaV-infected grapevines are similar to those for grapevines affected by leafroll-associated viruses (Naidu et al. 2014).

SPREAD AND MANAGEMENT

A clustering of GRBaV-infected vines within healthy vineyards adjacent to or proximal to infected vineyards and their spatiotemporal increase are consistent with short-distance spread of the virus (unpublished data). Also, the aggregated patterns of infected vines in some vineyards suggest the possible existence of a vector. Some members of the family Geminiviridae are transmitted by leafhoppers (Brown et al. 2012), and the Virginia creeper leafhopper (Erythroneura ziczac Walsh) has been reported to be capable of GRBaV transmission under experimental conditions (Poojari et al. 2013). However, the epidemiological significance of insect vectors transmission of GRBaV in vineyards is presently unknown. The identification of an insect vector in vineyards would be desirable not only to augment our understanding of the epidemiology but also to aid in the development of comprehensive management programs.

GRBaV is graft transmissible (Al Rwahnih et al. 2013; Poojari et al. 2013) and widely distributed in North American vineyards (Al Rwahnih et al. 2013; Krenz et al. 2014). It has been reported in vineyards of grafted (Al Rwahnih et al. 2013; Krenz et al. 2014) and self-rooted (Poojari et al. 2013) vines. These observations are consistent with the dissemination of the virus through planting material, making each step of vine production such as vegetative propagation of scion or rootstocks and grafting potentially involved in long-distance spread of the virus. These findings highlight the need to revisit the health status of foundation stocks, and reinforce the production of clean vines, possibly through therapeutic treatments and extensive indexing efforts carried out by laboratories of the National Clean Plant Network that are supported, in part, by USDA Animal and Plant Health Inspection Service.

Due to unacceptable fruit quality and resultant losses in revenue, 4- to 10-year-old California vineyards with a high (>50%) disease incidence have been removed in California. Efforts to manage red blotch have included attempts to improve the nutritional status of GRBaV-infected vines by supplementing with phosphates or potassium, adjusting fruit load, and delaying harvesting by 2 to 3 weeks so that TSS in fruit and the proportion of volatile compounds in the wine would be augmented (Calvi 2011). Nonetheless, there are no conclusive demonstrations of management options that restore fruit quality in infected grapevines to acceptable levels.

SEEING RED: CONCLUSIONS AND FUTURE PROSPECTS

At the whole-plant level, plants have a limited repertoire of responses to viruses (namely, stunting, growth deformation, chlorosis, and altered pigmentation) that can, in some cases, form specific patterns such as mosaic or blotches, among others. In grapevines, late-season changes in foliar pigmentation are characteristic of leafroll disease and are typified by a reddening of leaves in cultivars with red fruit. Leafroll disease is associated with a number of phloem-limited viruses (Martelli 2014; Naidu et al. 2014) and, in many plant species, infection by phloem-limited virus results in changes in leaf color and pigmentation. Thus, it should not be surprising that infection by GRBaV, a putative phloem-limited virus, could result in symptoms resembling leafroll. However, not all viruses infecting grapevine, including other phloem-restricted viruses (i.e., vitiviruses), have obvious effects on host growth or give rise to distinct symptoms. Thus, there remains much to be learned about how viruses affect vine growth and productivity. Because other biotic and abiotic factors are known to influence leaf reddening, the development of red blotch-like symptoms is not necessarily indicative of GRBaV infection in red-berried grapevine cultivars.

Diagnosis of GRBaV currently relies on DNA-based assays, primarily PCR (Al Rwahnih et al. 2013; Krenz et al. 2014), and this limits the number of samples that can be processed at once. A high-throughput indexing methodology such as ELISA using specific antibodies is desirable for large-scale testing to facilitate the rapid screening of germplasm, breeding material, and environmental samples. Although ELISA using monoclonal antibodies or polyclonal immunoglobulins is available for several monopartite and bipartite geminiviruses (Pico et al. 1999; Thomas et al. 1986), the purification of phloem-limited geminiviruses from woody plant hosts can be a challenging endeavor.

Currently, the only available management option for GRBaV (and other grapevine viruses) is ensuring that newly planted vines are derived from stocks that have tested free of the virus. Other virus-specific management strategies will require a better understanding of the biology of the virus and the epidemiology. Key aspects of the virus that need to be understood include (i) identifying the vectors of GRBaV, (ii) identifying epidemiologically important reservoirs of GRBaV, and (iii) determining the biological differences among virus isolates and putative strains. Additionally, the vexing question of the origin of GRBaV remains unanswered. Whether it originated in North America and, therefore, is likely absent from other grape-growing regions in the world, or whether the apparent absence from other grape-growing continents is a result of limited testing or a lack of open communication is not known.

Finally, the recent discovery of GRBaV and its detrimental impact on vine health calls for a revision of existing grapevine certification programs. Standards should now include GRBaV in addition to viruses responsible for fanleaf degeneration disease and those associated with leafroll disease and rugose wood complex.

ACKNOWLEDGMENTS

We thank our respective lab members J. van den Heuvel (Department of Horticulture, Cornell University), A. Walker and J. Wolpert (Department of Viticulture and Enology, University of California, Davis), and R. Smith (University of California ANR, Sonoma County, Santa Rosa); anonymous reviewers for their useful comments; and the numerous individuals who have contributed to this work, with our apologies to those left unmentioned. This research was supported, in part, by the American Vineyard Foundation; California Fruit Tree, Nut Tree, and Grapevine Improvement Advisory Board; USDA-NIFA-Specialty Crop Block Grant; New York Wine and Grape Foundation; and USDA-ARS CRIS 5306-22000-014-00D.