Genetic variability of natural populations of Grapevine leafroll-associated virus 2 in Pacific Northwest vineyards

A Chronological Study on Grapevine Leafroll-Associated Virus 2 in Australia

Grapevine leafroll disease affects the health status of grapevines worldwide. Most studies in Australia have focused on grapevine leafroll-associated viruses 1 and 3, while little attention has been given to other leafroll virus types, in particular, grapevine leafroll-associated virus 2 (GLRaV-2). A chronological record of the temporal occurrence of GLRaV-2 in Australia since 2001 is reported. From a total of 11,257 samples, 313 tested positive, with an overall incidence of 2.7%. This virus has been detected in 18 grapevine varieties and Vitis rootstocks in different regions of Australia. Most varieties were symptomless on their own roots, while Chardonnay showed a decline in virus-sensitive rootstocks. An isolate of GLRaV-2, on own-rooted Vitis vinifera cv. Grenache, clone SA137, was associated with severe leafroll symptoms after veraison with abnormal leaf necrosis. The metagenomic sequencing results of the virus in two plants of this variety confirmed the presence of GLRaV-2, as well as two inert viruses, grapevine rupestris stem pitting-associated virus (GRSPaV) and grapevine rupestris vein feathering virus (GRVFV). No other leafroll-associated viruses were detected. Among the viroids, hop stunt viroid and grapevine yellow speckle viroid 1 were detected. Of the six phylogenetic groups identified in GLRaV-2, we report the presence of four groups in Australia. Three of these groups were detected in two plants of cv. Grenache, without finding any recombination event. The hypersensitive reaction of certain American hybrid rootstocks to GLRaV-2 is discussed. Due to the association of GLRaV-2 with graft incompatibility and vine decline, the risk from this virus in regions where hybrid Vitis rootstocks are used cannot be overlooked.

Distribution and Genetic Diversity of Grapevine Viruses in Russia

Viral diseases can seriously damage the vineyard productivity and the quality of grape and wine products. Therefore, the study of the species composition and range of grapevine viruses is important for the development and implementation of strategies and tactics to limit their spread and increase the economic benefits of viticulture. In 2014–2019, we carried out a large-scale phytosanitary monitoring of Russian commercial vineyards in the Krasnodar region, Stavropol region and Republic of Crimea. A total of 1857 samples were collected and tested for the presence of Grapevine rupestris stem pitting-associated virus (GRSPaV), Grapevine virus A (GVA), Grapevine leafroll-associated virus-1 (GLRaV-1), Grapevine leafroll-associated virus-2 (GLRaV-2), Grapevine leafroll-associated virus-3 (GLRaV-3), Grapevine fanleaf virus (GFLV), and Grapevine fleck virus (GFkV) using RT-PCR. Out of all samples tested, 54.5% were positive for at least one of the viruses (GRSPaV, GVA, GLRaV-1, GLRaV-2, GLRaV-3, GFLV, GFkV) in the Stavropol region, 49.8% in the Krasnodar region and 49.5% in the Republic of Crimea. Some plants were found to be infected with several viruses simultaneously. In the Republic of Crimea, for instance, a number of plants were infected with five viruses. In the Krasnodar region and the Republic of Crimea, 4.7% and 3.3% of the samples were predominantly infected with both GFkV and GRSPaV, whereas in the Stavropol region, 6% of the selected samples had both GLRaV-1 and GVA infections. We carried out a phylogenetic analysis of the coat protein genes of the detected viruses and identified the presence of GVA of groups I and IV, GRSPaV of groups BS and SG1, GLRaV-1 of group III, GLRaV-2 of groups PN and H4, GLRaV-3 of groups I and III. The results obtained make it possible to assess the viral load and the distribution of the main grapevine viruses on plantations in the viticultural zones of Russia, emphasizing the urgent need to develop and implement long-term strategies for the control of viral diseases of grapes.

Expression of the p24 silencing suppressor of Grapevine leafroll-associated virus 2 from Potato virus X or Barley stripe mosaic virus vector elicits hypersensitive responses in Nicotiana benthamiana

The 24-kDa protein (p24) encoded by Grapevine leafroll-associated virus 2 (GLRaV-2) is an RNA-silencing suppressor (RSS), but its effect on active viral infection is unclear. Using a Potato virus X (PVX)-based expression system, we demonstrated that p24 elicits lethal systemic necrosis in Nicotiana benthamiana, sharing typical characteristics of the hypersensitive response (HR), and that NbRAR1 (a cytoplasmic Zn2+-binding protein) is involved in the PVX-p24-mediated systemic necrosis. Moreover, expression of p24 from Barley stripe mosaic virus (BSMV) vector triggered local necrosis in infiltrated patches of N. benthamiana, likely inhibiting viral systemic spread. By deletion analysis, we demonstrated that amino acids (aa) 1 to 180, which are located in the region (aa 1-188) previously shown to be necessary for p24's RSS activity, is sufficient for p24 to elicit systemic necrosis in the context of PVX infection. Using substitution mutants, we revealed that silencing-suppression-defective mutants R2A and W54A induce only a mild necrotic response; two mutants without self-interaction ability previously shown to lose or retain weak suppression function also displayed decreased pathogenicity: W149A without RSS activity elicited a mild necrotic response, whereas V162H/L169H/L170H which retains weak RSS activity was able to induce systemic necrosis, but with a 1- to 2-day delay. Taken together, p24 plays an important role in GLRaV-2 pathogenesis, triggering HR-like necrosis in N. benthamiana plants when expressed from PVX or BSMV vector; both the silencing suppression and self-interaction are crucial for p24's pathogenicity activity, and the region of p24 for determining systemic necrosis is mapped to aa 1-180.

A Novel Genetic Variant of Grapevine leafroll-associated virus-3 (GLRaV-3) from Idaho Grapevines

Grapevine leafroll-associated virus-3 (GLRaV-3) is a major constraint on profitable grapevine cultivation. The virus is transmitted efficiently by mealybugs and soft scale insects, or through vegetative propagation by cuttings, and is present worldwide, wherever grapevines are grown. GLRaV-3 exists as a complex of genetic variants currently classified in several phylogenetic groups that can differ from each other by as much as 30% in nucleotide sequence of the whole genome. In the course of the GLRaV-3 testing of wine grapes in southern Idaho, plants of two grapevine cultivars were found to harbor a novel genetic variant of GLRaV-3, named ID45, which exhibited ≤80% nucleotide sequence identity level to the known GLRaV-3 isolates in its most conserved HSP70h gene. The ID45 variant caused no foliar symptoms in 'Cabernet Sauvignon' in the fall, and was demonstrated to have poor reactivity to commercial virus-specific antibodies. The entire 18,478-nt genome sequence of the GLRaV-3-ID45 was determined using a combination of high-throughput and conventional Sanger sequencing, and demonstrated to have typical organization for the genus Ampelovirus (family Closteroviridae), with only 70 to 77% identity level to the GLRaV-3 genomes from other established phylogroups. We concluded that ID45 represented a new phylogenetic group IX of GLRaV-3. Database search using ID45 nucleotide sequence as a query suggested that this novel ID45 variant is present in at least one other grape-growing state in the U.S., California, and in Brazil. An RT-PCR based test was developed to distinguish ID45 from the predominant GLRaV-3 phylogroup I found in Idaho in single and mixed infections.

The Relative Occurrence of Grapevine leafroll-associated virus 3 and Grapevine red blotch virus in Washington State Vineyards

Vineyard surveys were conducted for three consecutive seasons in eastern Washington State, the major grapevine-growing region in the state, to document the occurrence of Grapevine leafroll-associated virus 3 (GLRaV-3) and Grapevine red blotch virus (GRBV). The majority of samples were collected from red-berried wine grape (Vitis vinifera) cultivars exhibiting symptoms of or suspected for grapevine leafroll (GLD) and red blotch (GRBD) diseases. A limited number of samples from white-berried cultivars were collected randomly due to the lack of visual symptoms. Samples were collected from a total of 2,063 grapevines from 18 red-berried cultivars and seven white-berried cultivars planted in eight American Viticultural Areas and tested for GLRaV-3 and GRBV using RT-PCR and PCR, respectively. The results showed 67.77% and 6.01% of total samples positive for GLRaV-3 and GRBV, respectively, and 9.06% of samples positive for both viruses. About 17% of samples tested negative for the two viruses, but some of these samples were positive for GLRaV-2 and GLRaV-4. Overall results indicated that GLRaV-3 was more common than GRBV, independent of cultivars and the geographic origin of samples. Due to variability in symptoms in red-berried cultivars, virus-specific diagnostic assays were deemed necessary for reliable identification of GLRaV-3 and GRBV and to differentiate GLD and GRBD symptoms from those induced by biotic and abiotic stresses in vineyards. A multiplex PCR protocol was developed for simultaneous detection of GLRaV-3 and GRBV in grapevine samples. A global phylogenetic analysis of GRBV genome sequences revealed segregation of virus isolates from Washington State vineyards into two distinct clades, with the majority of isolates belonging to clade II.

Epidemiology and Genetic Diversity of Grapevine Leafroll-Associated Viruses in British Columbia

Grapevine leafroll disease (GLD) is a complex associated with one or more virus species belonging to the family Closteroviridae. The majority of viruses in this complex are vectored by one or more species of mealybugs (Pseudococcidae) and/or scale insects (Coccidae). Grape-growing regions of British Columbia (BC), including Okanagan, Similkameen, and Fraser valleys and Kamloops (BC central interior), Vancouver, and Gulf islands, were surveyed during the 2014 and 2015 growing seasons for the presence of four major grapevine leafroll-associated viruses, including Grapevine leafroll-associated virus 1 (GLRaV-1), GLRaV-2, GLRaV-3, and GLRaV-4. In total, 3,056 composite five-vine samples were collected from 153 Vitis vinifera and three interspecific hybrid vineyard blocks. The results showed GLRaV-3 to be the most widespread, occurring in 16.7% of the composite samples, followed by GLRaV-4 (3.9%), GLRaV-1 (3.8%), and GLRaV-2 (3.0%). Mixed infections of two or more GLRaVs were found in 4.1% of the total samples. The relative incidence of GLRaVs differed among regions and vineyard blocks of a different age. Characterization of partial CO1 region from a total of 241 insect specimens revealed the presence of Pseudococcus maritimus, Parthenolecanium corni, and other Pulvinaria sp. in BC vineyards. Spatial patterns of GLRaV-3 infected grapevines in three vineyard blocks from three different regions in the Okanagan Valley showed variable degrees of increase in disease spread ranging from 0 to 19.4% over three growing seasons. Regional differences in the relative incidence and spread of GLD underline the need for region-based management programs for BC vineyards.

Johnsongrass mosaic virus Contributes to Maize Lethal Necrosis in East Africa

Maize lethal necrosis (MLN), a severe virus disease of maize, has emerged in East Africa in recent years with devastating effects on production and food security where maize is a staple subsistence crop. In extensive surveys of MLN-symptomatic plants in East Africa, sequences of Johnsongrass mosaic virus (JGMV) were identified in Uganda, Kenya, Rwanda, and Tanzania. The East African JGMV is distinct from previously reported isolates and infects maize, sorghum, and Johnsongrass but not wheat or oat. This isolate causes MLN in coinfection with Maize chlorotic mottle virus (MCMV), as reported for other potyviruses, and was present in MLN-symptomatic plants in which the major East African potyvirus, Sugarcane mosaic virus (SCMV), was not detected. Virus titers were compared in single and coinfections by quantitative reverse transcription-polymerase chain reaction. MCMV titer increased in coinfected plants whereas SCMV, Maize dwarf mosaic virus, and JGMV titers were unchanged compared with single infections at 11 days postinoculation. Together, these results demonstrate the presence of an East African JGMV that contributes to MLN in the region.

Comparative detection of a large population of grapevine viruses by TaqMan® RT-qPCR and ELISA

Grapevine (Vitis spp.) can be infected by numerous viruses that are often widespread and of great economic importance. Reliable detection methods are necessary for sanitary selection which is the only way to partly control grapevine virus diseases. Biological indexing and ELISA are currently the standard methods for screening propagation material, and PCR-methods are becoming increasingly popular. Due to the diversity of virus isolates, it is essential to verify that the tests allow the detection of the largest possible virus populations. We developed three quadruplex TaqMan^® RT-qPCR assays for detecting nine different viruses that cause considerable damage in many vineyards world-wide. Each assay is designed to detect three viruses and the grapevine Actin as an internal control. A large population of grapevines from diverse cultivars and geographic location was tested for the presence of nine viruses: Arabis mosaic virus (ArMV), Grapevine fleck virus (GFkV), Grapevine fanleaf virus (GFLV), Grapevine leafroll-associated viruses (GLRaV-1, -2, -3), Grapevine rupestris stem pitting-associated virus (GRSPaV), Grapevine virus A (GVA), and Grapevine virus B (GVB). In general, identical results were obtained with multiplex TaqMan^® RT-qPCR and ELISA although, in some cases, viruses could be detected by only one of the two techniques.

Maize Lethal Necrosis (MLN), an Emerging Threat to Maize-Based Food Security in Sub-Saharan Africa

In sub-Saharan Africa, maize is a staple food and key determinant of food security for smallholder farming communities. Pest and disease outbreaks are key constraints to maize productivity. In September 2011, a serious disease outbreak, later diagnosed as maize lethal necrosis (MLN), was reported on maize in Kenya. The disease has since been confirmed in Rwanda and the Democratic Republic of Congo, and similar symptoms have been reported in Tanzania, Uganda, South Sudan, and Ethiopia. In 2012, yield losses of up to 90% resulted in an estimated grain loss of 126,000 metric tons valued at $52 million in Kenya alone. In eastern Africa, MLN was found to result from coinfection of maize with Maize chlorotic mottle virus (MCMV) and Sugarcane mosaic virus (SCMV), although MCMV alone appears to cause significant crop losses. We summarize here the results of collaborative research undertaken to understand the biology and epidemiology of MLN in East Africa and to develop disease management strategies, including identification of MLN-tolerant maize germplasm. We discuss recent progress, identify major issues requiring further research, and discuss the possible next steps for effective management of MLN.

Grapevine leafroll disease and associated viruses: a unique pathosystem

Grapevine leafroll is the most complex and intriguing viral disease of grapevine (Vitis spp.). Several monopartite closteroviruses (family Closteroviridae) from grapevines have been molecularly characterized, yet their role in disease etiology is not completely resolved. Hence, these viruses are currently designated under the umbrella term of Grapevine leafroll-associated viruses (GLRaVs). This review examines our current understanding of the genetically divergent GLRaVs and highlights the emerging picture of several unique aspects of the leafroll disease pathosystem. A systems biology approach using contemporary technologies in molecular biology, -omics, and cell biology aids in exploring the comparative molecular biology of GLRaVs and deciphering the complex network of host-virus-vector interactions to bridge the gap between genomics and phenomics of leafroll disease. In addition, grapevine-infecting closteroviruses have a great potential as designer viruses to pursue functional genomics and for the rational design of novel disease intervention strategies in this agriculturally important perennial fruit crop.

Grapevine Leafroll: A Complex Viral Disease Affecting a High-Value Fruit Crop

Grapevine (Vitis spp.) is one of the most widely grown fruit crops in the world. It 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. Grapevines are broadly classified into red- and white-berried cultivars based on their fruit skin color, although yellow, pink, crimson, dark blue, and black-berried cultivars also exist. Grapevines can be subject to attacks by many different pests and pathogens, including graft-transmissible agents such as viruses, viroids, and phytoplasmas. Among the virus and virus-like diseases, grapevine leafroll disease (GLD) is by far the most widespread and economically damaging viral disease of grapevines in many regions around the world. The global expansion of the grape and wine industry has seen a parallel increase in the incidence and economic impact of GLD. Despite the fact that GLD was recognized as a potential threat to grape production for several decades, our knowledge of the nature of the disease is still quite limited due to a variety of challenges related to the complexity of this virus disease, the association of several distinct GLD-associated viruses, and contrasting symptoms in red- and white-berried cultivars. In view of the growing significance of GLD to wine grape production worldwide, this feature article provides an overview of the state of knowledge on the biology and epidemiology of the disease and describes management strategies currently deployed in vineyards.

Genetic diversity of Grapevine virus A in Washington and California vineyards

Grapevine virus A (GVA; genus Vitivirus, family Betaflexiviridae) has been implicated with the Kober stem grooving disorder of the rugose wood disease complex. In this study, 26 isolates of GVA recovered from wine grape (Vitis vinifera) cultivars from California and Washington were analyzed for their genetic diversity. An analysis of a portion of the RNA-dependent RNA polymerase (RdRp) and complete coat protein (CP) sequences revealed intra- and inter-isolate sequence diversity. Our results indicated that both RdRp and CP are under strong negative selection based on the normalized values for the ratio of nonsynonymous substitutions per nonsynonymous site to synonymous substitutions per synonymous site. A global phylogenetic analysis of CP sequences revealed segregation of virus isolates into four major clades with no geographic clustering. In contrast, the RdRp-based phylogenetic tree indicated segregation of GVA isolates from California and Washington into six clades, independent of geographic origin or cultivar. Phylogenetic network coupled with recombination analyses showed putative recombination events in both RdRp and CP sequence data sets, with more of these events located in the CP sequence. The preponderance of divergent variants of GVA co-replicating within individual grapevines could increase viral genotypic complexity with implications for phylogenetic analysis and evolutionary history of the virus. The knowledge of genetic diversity of GVA generated in this study will provide a foundation for elucidating the epidemiological characteristics of virus populations at different scales and implementing appropriate management strategies for minimizing the spread of genetic variants of the virus by vectors and via planting materials supplied to nurseries and grape growers.

Genetic diversity and tissue and host specificity of Grapevine vein clearing virus

Grapevine vein clearing virus (GVCV) is a new badnavirus in the family Caulimoviridae that is closely associated with an emerging vein-clearing and vine decline disease in the Midwest region of the United States. It has a circular, double-stranded DNA genome of 7,753 bp that is predicted to encode three open reading frames (ORFs) on the plus-strand DNA. The largest ORF encodes a polyprotein that contains domains for a reverse transcriptase (RT), an RNase H, and a DNA-binding zinc-finger protein (ZF). In this study, two genomic regions, a 570-bp region of the RT domain and a 540-bp region of the ZF domain were used for an analysis of the genetic diversity of GVCV populations. In total, 39 recombinant plasmids were sequenced. These plasmids consisted of three individual clones from each of 13 isolates sampled from five grape varieties in three states. The sequence variants of GVCV could not be phylogenetically grouped into clades according to geographical location and grape variety. Codons of RT or ZF regions are subject to purifying selection pressure. Quantitative polymerase chain reaction assays indicated that GVCV accumulates abundantly in the petioles and least in the root tip tissue. Upon grafting of GVCV-infected buds onto four major grape cultivars, GVCV was not detected in the grafted 'Chambourcin' vine but was present in the grafted 'Vidal Blanc', 'Cayuga White', and 'Traminette' vines, suggesting that Chambourcin is resistant to GVCV. Furthermore, seven nucleotides were changed in the sequenced RT and ZF regions of GVCV from a grafted Traminette vine and one in the sequenced regions of GVCV from grafted Cayuga White but no changes were found in the sequenced regions of GVCV in the grafted Vidal Blanc. The results provide a genetic snapshot of GVCV populations, which will yield knowledge important for monitoring GVCV epidemics and for preventing the loss of grape production that is associated with GVCV.

Molecular characterization and impacts of a strain of Grapevine leafroll-associated virus 2 causing asymptomatic infection in a wine grape cultivar

BACKGROUND

Grapevine leafroll (GLD) is considered as the most economically important virus disease affecting wine grapes (Vitis vinifera L.) in many grapevine-growing regions. GLD produces distinct symptoms in red- and white-berried cultivars. In this study, we determined the complete genome sequence of an asymptomatic strain of Grapevine leafroll-associated virus 2 (GLRaV-2) and studied its impacts on fruit yield and berry quality attributes in an own-rooted, red-berried wine grape cultivar.

FINDINGS

The complete genome of GLRaV-2 obtained from a red-berried wine grape cultivar Sangiovese, designated as GLRaV-2-SG, was determined to be 16,474 nucleotides in length. In pairwise comparisons, using complete genome sequences of GLRaV-2 strains available in GenBank, GLRaV-2-SG was more closely related to GLRaV-2-OR1 from Oregon, USA, and GLRaV-2-93/955 from South Africa, and distantly related to GLRaV-2-BD from Italy and GLRaV-2-RG from USA. Fruit yield estimates and berry quality analysis at the time of commercial harvest indicated that GLRaV-2-SG had little impact on fruit yield and total soluble solids, juice pH and total anthocyanins of berry skin.

CONCLUSIONS

Because so little is known about the effects of asymptomatic virus infections in wine grapes, this study expanded our knowledge of the occurrence and impacts of GLRaV-2 causing asymptomatic infections. Our results indicated that an asymptomatic strain of GLRaV-2 may not cause significant effects to overall fruit yield and berry quality in own-rooted vines, but can affect its host in more subtle ways. Since disease symptoms are not apparent, relying on visual symptoms during disease surveys may result in the escape of asymptomatic strains of GLRaV-2. Thus, it is necessary to use appropriate diagnostic assays for reliable detection of viruses causing asymptomatic infections.

Association of a DNA virus with grapevines affected by red blotch disease in California

In the Napa Valley of California, vineyards of 'Cabernet Franc' (CF) clone 214, 'Cabernet Sauvignon' clone 337, and 'Zinfandel' clone 1A (Z1A) with grapevines exhibiting foliar symptoms of red blotches, marginal reddening, and red veins that were accompanied by reduced sugar accumulation in fruit at harvest were initially suspected to be infected with leafroll-associated viruses. However, reverse-transcription polymerase chain reaction (PCR) tests were negative for all known leafroll-associated viruses, with the exception of Grapevine leafroll-associated virus 2 in Z1A. Metagenomic analysis of cDNA libraries obtained from double-stranded RNA enriched nucleic acid (NA) preparations from bark scrapings of dormant canes on an Illumina platform revealed sequences having a distant relationship with members of the family Geminiviridae. Sequencing of products obtained by PCR assays using overlapping primers and rolling circle amplification (RCA) confirmed the presence of a single circular genome of 3,206 nucleotides which was nearly identical to the genome of a recently reported Grapevine cabernet franc-associated virus found in declining grapevines in New York. We propose to call this virus "Grapevine red blotch-associated virus" (GRBaV) to describe its association with grapevine red blotch disease. Primers specific to GRBaV amplified a product of expected size (557 bp) from NA preparations obtained from petioles of several diseased source vines. Chip bud inoculations successfully transmitted GRBaV to test plants of CF, as confirmed by PCR analysis. This is the first report of a DNA virus associated with red blotch disease of grapevines in California.

Genetic variability and evolutionary dynamics of viruses of the family Closteroviridae

RNA viruses have a great potential for genetic variation, rapid evolution and adaptation. Characterization of the genetic variation of viral populations provides relevant information on the processes involved in virus evolution and epidemiology and it is crucial for designing reliable diagnostic tools and developing efficient and durable disease control strategies. Here we performed an updated analysis of sequences available in Genbank and reviewed present knowledge on the genetic variability and evolutionary processes of viruses of the family Closteroviridae. Several factors have shaped the genetic structure and diversity of closteroviruses. (I) A strong negative selection seems to be responsible for the high genetic stability in space and time for some viruses. (2) Long distance migration, probably by human transport of infected propagative plant material, have caused that genetically similar virus isolates are found in distant geographical regions. (3) Recombination between divergent sequence variants have generated new genotypes and plays an important role for the evolution of some viruses of the family Closteroviridae. (4) Interaction between virus strains or between different viruses in mixed infections may alter accumulation of certain strains. (5) Host change or virus transmission by insect vectors induced changes in the viral population structure due to positive selection of sequence variants with higher fitness for host-virus or vector-virus interaction (adaptation) or by genetic drift due to random selection of sequence variants during the population bottleneck associated to the transmission process.

Grapevine leafroll-associated virus 1 occurs as genetically diverse populations

The genetic diversity of 34 isolates of Grapevine leafroll-associated virus 1 (GLRaV-1) from different wine, table, and ornamental grape cultivars in California, New York, and Washington States in the United States was investigated. Segments of the heat-shock protein 70 homolog (HSP70h) gene, coat protein (CP) gene, coat protein duplicate 2 (CPd2) gene, and open reading frame 9 (p24) were amplified by reverse-transcription polymerase chain reaction, cloned, and sequenced. A pairwise comparison of nucleotide sequences revealed intra- and interisolate sequence diversity, with CPd2 and HSP70h being the most and the least divergent, respectively, among the four genomic regions studied. The normalized values for the ratio of nonsynonymous substitutions per nonsynonymous site to synonymous substitutions per synonymous site indicated different purifying selection pressures acting on each of the four genomic regions, with the CP and CPd2 being subjected to the strongest and weakest functional constraints, respectively. A global phylogenetic analysis of sequences from the four genomic regions revealed segregation of GLRaV-1 isolates into three major clades and a lack of clearly defined clustering by geographical origin. In contrast, only two lineages were apparent when the CP and CPd2 gene sequences were used in phylogenetic analyses. Putative recombination events were revealed among the HSP70h, CP, and p24 sequences. The genetic landscape of GLRaV-1 populations presented in this study provides a foundation for better understanding of the epidemiology of grapevine leafroll disease across grape-growing regions in the United States. In addition, this study will benefit grape clean plant programs across the country in improving the sanitary status of planting materials provided to nurseries and grape growers.

Genome organization, serology and phylogeny of Grapevine leafroll-associated viruses 4 and 6: taxonomic implications

Complete nucleotide sequences of the type isolate of Grapevine leafroll-associated virus 4 (GLRaV-4) and of an isolate of GLRaV-6 from cv 'Estellat' (GLRaV-6Est) were generated and compared mutually and with related viruses. The genome organization of both viruses resembled that of members of Subgroup I in the genus Ampelovirus (fam. Closteroviridae). The availability of these sequences, along with previously existing data on related GLRaVs, allowed critical review of the taxonomy and nomenclature of these viruses. In phylogenetic analyses, GLRaV-4 and -6Est consistently grouped with GLRaV-5, -9, and -Pr forming a poorly resolved sub-cluster ("GLRaV-4 group") within the genus Ampelovirus. In-depth study showed that genetic distances between these viruses do not exceed the intra-species diversity observed in other closteroviruses. In Western blots, partially purified preparations of GLRaVs -4, -5, -6 and -9 reacted only with homologous monoclonal antibodies, but were all recognized by polyclonal antisera to GLRaV-5 and GLRaV-9. Serological relatedness among these viruses was further confirmed in DAS-ELISA. In immuno-electron microscopy, GLRaV-6 particles appeared uniformly decorated with homologous monoclonal antibodies, whereas GLRaV-2, used as a control, showed "bipolar" morphology of the virion. Results of this study challenge taxonomy and nomenclature of several GLRaVs suggesting that they are divergent isolates of Grapevine leafroll-associated virus 4 and not, as has been assumed, distinct species (definitive and/or putative) in the genus Ampelovirus.

Vitis californica and Vitis californica × Vitis vinifera Hybrids are Hosts for Grapevine leafroll-associated virus-2 and -3 and Grapevine virus A and B

Vitis and non-Vitis spp. surrounding nine Napa Valley vineyards were surveyed for Grapevine leafroll-associated virus (GLRaV)-1 to -5 and -9, Grapevine virus A (GVA), Grapevine virus B (GVB), and Grapevine virus D (GVD). Vitis spp. from three riparian areas not adjacent to vineyards were also included. DNA fingerprinting and probability analyses indicated that the Vitis samples consisted primarily of Vitis californica followed by V. californica × V. vinifera hybrids. Single and mixed infections of GLRaV-2, -3, GVA, or GVB were detected by conventional or quantitative reverse-transcription polymerase chain reaction in 6 of the 66 V. californica and 11 of the 19 V. californica × V. vinifera hybrids. GLRaV-1, -4, -5, -9, and GVD were not detected. Phylogenetic analysis of GLRaV-2 and -3 partial coat protein gene nucleotide sequences indicated that the isolates from V. californica and V. californica × V. vinifera hybrids were closely related to isolates from V. vinifera.

Biological, molecular, and serological studies of a novel strain of grapevine leafroll-associated virus 2

In California, a novel closterovirus was detected in "Redglobe" grapevine, associated with graft incompatibility and given a trivial name "Grapevine rootstock stem lesion associated virus (GRSLaV)." The biological properties of the putative virus were ascertained when asymptomatic yet infected Redglobe scion buds were graft-inoculated onto test plants of Cabernet Sauvignon propagated on 18 different rootstocks. It proved lethal on test plants growing on rootstocks 1616C, 5BB, 5C, 3309C, and 1103 P, whereas latent infections occurred on the remaining scion-rootstock combinations. In contrast, GLRaV-2 type (type strain) produced only typical leafroll symptoms. In a different experiment, GLRaV-2 type was successfully sap-transmitted to N. benthamiana, whereas sap transmission of GRSLaV was unsuccessful. Double-stranded RNA was extracted from infected Redglobe grapevines, cloned, sequenced, and determined a genome length of 16,527 nucleotides. Computer-assisted analysis of open-reading frames (ORFs) revealed a genome organization typical of monopartite viruses in the genus Closterovirus with nine ORFs (range 71-79% identity) with GLRaV-2 type, the closest similar virus species within the family Closteroviridae. Also the 3'-UTR of GRSLaV consisted of 223 nucleotides with an extended oligo(A) tract similar to that of GLRaV-2 type, Beet yellow stunt virus, and Beet yellows virus. Recombinant GRSLaV coat protein was expressed in E. coli, purified, and immunized a rabbit to produce polyclonal antiserum. Serological data matched the molecular data, whereby exposed plant tissue extracts of grapevines infected by both viruses (GRSLaV and GLRaV-2) reacted positively with homologous and heterologous viral antisera but not with healthy grapevine extracts in ELISA and Western blot tests. Based on the comparative sequence data and shared antigens, GRSLaV is now considered a strain of GLRaV-2 and redesignated as Grapevine leafroll associated virus-2 Redglobe (GLRaV-2RG). Primers specific for GLRaV-2RG were developed, which did not amplify GLRaV-2 type strain. When both sets of specific primers were used in assays of different grapevine collections, the incidence of the respective viruses varied considerably, e.g., 1.7 and 13.5%, respectively, for GLRaV-2RG and GLRaV-2 type.