Phylogeny of Geminivirus Coat Protein Sequences and Digital PCR Aid in Identifying Spissistilus festinus as a Vector of Grapevine red blotch-associated virus

Functional role of geminivirus encoded proteins in the host: Past and present

During plant-pathogen interaction, plant exhibits a strong defense system utilizing diverse groups of proteins to suppress the infection and subsequent establishment of the pathogen. However, in response, pathogens trigger an anti-silencing mechanism to overcome the host defense machinery. Among plant viruses, geminiviruses are the second largest virus family with a worldwide distribution and continue to be production constraints to food, feed, and fiber crops. These viruses are spread by a diverse group of insects, predominantly by whiteflies, and are characterized by a single-stranded DNA (ssDNA) genome coding for four to eight proteins that facilitate viral infection. The most effective means to managing these viruses is through an integrated disease management strategy that includes virus-resistant cultivars, vector management, and cultural practices. Dynamic changes in this virus family enable the species to manipulate their genome organization to respond to external changes in the environment. Therefore, the evolutionary nature of geminiviruses leads to new and novel approaches for developing virus-resistant cultivars and it is essential to study molecular ecology and evolution of geminiviruses. This review summarizes the multifunctionality of each geminivirus-encoded protein. These protein-based interactions trigger the abrupt changes in the host methyl cycle and signaling pathways that turn over protein normal production and impair the plant antiviral defense system. Studying these geminivirus interactions localized at cytoplasm-nucleus could reveal a more clear picture of host-pathogen relation. Data collected from this antagonistic relationship among geminivirus, vector, and its host, will provide extensive knowledge on their virulence mode and diversity with climate change.

Longer cluster hanging time decreases the impact of grapevine red blotch disease in Vitis vinifera L. Merlot across two seasons

BACKGROUND

Grapevine red blotch virus (GRBV) is a recently discovered virus and a major concern for the wine industry. Prior research indicated that GRBV delays grape ripening by reducing °Brix and anthocyanin concentrations in grapes from infected vines, resulting in higher ethanol concentrations in wines made from healthy fruit compared to diseased vines, which have an impact on sensory properties. In this study, infected fruit (Vitis vinifera L. Merlot) was sequentially harvested (in 2016 and 2017) and chaptalized (in 2017) to ameliorate the impact of GRBV on grape and final wine composition.

RESULTS

Chemical parameters including phenolic and volatile profiles of grapes and their subsequent wines were measured. Sensory properties were determined by descriptive analyses. Results demonstrated that GRBV decreased sugar accumulation and anthocyanin synthesis in grapes. Wines from GRBV grapes harvested at later ripening stage produced wines that were more similar chemically and sensorially to wines made from healthy fruit than to wines made from GRBV fruit harvested earlier.

CONCLUSION

A longer hang time of GRBV grapes is a potential strategy to mitigate the impacts of GRBV. However, chaptalization of diseased fruit must was inefficient at increasing similarities to wines made from healthy fruit. © 2023 Society of Chemical Industry.

Monitoring the Spread of Grapevine Viruses in Vineyards of Contrasting Agronomic Practices: A Metagenomic Investigation

This study investigated the transmission of grapevine viruses, specifically grapevine red blotch virus (GRBV) and grapevine Pinot gris virus (GPGV), in vineyards in Niagara Region, Ontario, Canada. Forty sentinel vines that were confirmed free of GRBV and GPGV by both high-throughput sequencing (HTS) and endpoint polymerase chain reaction (PCR) were introduced to two vineyards (one organic and one conventional) that were heavily infected with both GRBV and GPGV. Four months post-introduction, the sentinel vines were relocated to a phytotron. The HTS results from 15 months post-introduction revealed a widespread infection of GPGV among the sentinel vines but did not detect any GRBV. The GPGV infection rate of sentinel vines in the organic vineyard (13/18) was higher than in the conventional vineyard (1/19). The possibility of an alternative viral reservoir was assessed by testing the most abundant plants in between rows (Medicago sativa, Trifolium repens, Cirsium arvense and Taraxacum officinale), perennial plants in border areas (Fraxinus americana, Ulmus americana, Rhamnus cathartica) and wild grape (unknown Vitis sp.). The HTS result showed that cover crops and perennial plants did not harbor any grapevine viruses, while 4/5 wild grapes tested positive for GPGV but not GRBV. A pairwise sequence identity analysis revealed high similarities between the GPGV isolates found in the established vines on the vineyard and the newly contracted GPGV isolates in the sentinel vines, implicating a recent transmission event. This work provides novel insights into the spread of grapevine viruses in Niagara Region and is also the first direct proof of the spread of GPGV in natural vineyard conditions in North America.

Distinct Red Blotch Disease Epidemiological Dynamics in Two Nearby Vineyards

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 alters grape skin cell-wall composition impacting phenolic extractability during winemaking

BACKGROUND

Grapevine red blotch virus (GRBV) is the causal agent of grapevine red blotch disease and is known to delay grape ripening. However, grape cell-wall modifications during GRBV infection are largely unknown, even though the cell wall plays a large role in pathogenicity, viral interactions with host plants, and phenolic extractability during winemaking. Understanding the impact of GRBV infection on cell-wall metabolism is important for the development of potential mitigations strategies. In this study, high-throughput transcriptome sequencing was conducted on Vitis vinifera L. 'Merlot' grapes during ripening. The cell-wall composition, phenolic content, and phenolic extractability at two different commercial harvest points were also determined.

RESULTS

Log fold changes indicated a strong induction in diseased grapes at harvest of several transcripts involved in cell-wall solubilization and degradation. However, these observations did not translate to changes in cell-wall composition at either harvest point in diseased grapes, potentially suggesting post-transcriptional regulation. Moderate induction of pectin methylesterase inhibitor transcripts and transcripts associated with pathogenesis-related proteins coincided with increases in pectin and soluble proteins in cell walls of diseased grapes at harvest. Both pectin and pathogenesis-related proteins are known to retain phenolic compounds during winemaking.

CONCLUSION

Our study corroborates this finding when the percentage extractability of flavonols in wines was significantly lower when made from GRBV-infected fruit. These results suggest GRBV alters the grape cell walls, consequently decreasing phenolic extraction during winemaking. © 2023 Society of Chemical Industry.

The Three-Cornered Alfalfa Hopper, Spissistilus festinus, Is a Vector of Grapevine Red Blotch Virus in Vineyards

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.

Evaluation of Alfalfa Fields and Pastures as Sources of Spissistilus festinus (Hemiptera: Membracidae): Quantification of Reproductive and Nutritional Parameters

The threecornered alfalfa hopper (Spissistilus festinus) is a pest of grapevine, with damage caused by transmission of grapevine red blotch virus. Because grapevine is not a preferred host of the threecornered alfalfa hopper, abundance in vineyards depends on proximity to source habitats and presence of preferred hosts in vineyard understories. The potential for alfalfa fields and pastures in the Central Valley of California to serve as sources of threecornered alfalfa hopper was evaluated by quantifying parameters associated with threecornered alfalfa hopper reproductive and nutritional status. Laboratory studies determined that the threecornered alfalfa hopper is synovigenic, emerging as an adult prior to initiation of oogenesis and that females have multiple rounds of egg production. Alfalfa fields, irrigated pastures, and vineyards were sampled monthly. Adults were observed year-round in alfalfa fields and pastures, with populations peaking in fall. Gravid females were observed from February through November. While rare, adult threecornered alfalfa hoppers were collected from 2 of 4 sampled vineyards. In spring, adults were observed in samples collected from vineyard ground cover. In fall, adults were observed in samples collected from vineyard ground cover and foliage samples. Samples collected from pastures and vineyards were male biased, whereas equal numbers of males and females were observed in alfalfa fields. Adults collected from alfalfa fields were larger, heavier, and had greater estimated energetic reserves than adults collected from pastures. Adults collected from vineyards were of above average size and had relatively high estimated energetic reserves. Results suggest that alfalfa fields are more likely to serve as sources of threecornered alfalfa hoppers than irrigated pastures and that differences in male and female behavior may affect rates of pathogen transmission.

Potential Implications and Management of Grapevine Viruses in Mexico: A Review

Worldwide, virus infections in grapevines are of concern due to the potential for economic loss. Although the grape industry in Mexico is relatively small and focused mainly on the local market, production dates back to the time of the Spanish colonization. This manuscript discusses the findings on grapevine viruses in Mexico. Nine viruses have been identified in the last fifty years, including grapevine red blotch virus (GRBV), grapevine leafroll-associated virus 3 (GLRaV-3), grapevine fanleaf virus (GFLV), and grapevine virus A (GVA). Important information is provided about these viruses and viral pathogens that have not yet been reported in Mexico, but represent an ongoing threat to plant health and grapevine production in other viticultural regions of the world. Strategies for virus control in vineyards are described. The information discussed here should be shared with growers and stakeholders to prevent future negative impacts on the Mexican grapevine industry and to save ancient grapevine accessions.

Analysis of the genome of grapevine red blotch virus and related grabloviruses indicates diversification prior to the arrival of Vitis vinifera in North America

In this study 163 complete whole-genome sequences of the emerging pathogen grapevine red blotch virus (GRBV; genus Grablovirus, family Geminiviridae) were used to reconstruct phylogenies using Bayesian analyses on time-tipped (heterochronous) data. Using different combinations of priors, Bayes factors identified heterochronous datasets (3×200 million chains) generated from strict clock and exponential tree priors as being the most robust. Substitution rates of 3.2×10^-5 subsitutions per site per year (95% HPD 4.3-2.1×10^-5) across the whole of the GRBV genome were estimated, suggesting ancestral GRBV diverged from ancestral wild Vitis latent virus 1 around 9 000 years ago, well before the first documented arrival of Vitis vinifera in North America. Whole-genome analysis of GRBV isolates in a single infected field-grown grapevine across 12 years identified 12 single nucleotide polymorphisms none of which were fixed substitutions: an observation not discordant with the in silico estimate. The substitution rate estimated here is lower than those estimated for other geminiviruses and is the first for a woody-host-infecting geminivirus.

Transmission of Grapevine Red Blotch Virus by Spissistilus festinus [Say, 1830] (Hemiptera: Membracidae) between Free-Living Vines and Vitis vinifera 'Cabernet Franc'

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.

Current commercial dPCR platforms: technology and market review

Digital polymerase chain reaction (dPCR) technology has provided a new technique for molecular diagnostics, with superior advantages, such as higher sensitivity, precision, and specificity over quantitative real-time PCRs (qPCR). Eight companies have offered commercial dPCR instruments: Fluidigm Corporation, Bio-Rad, RainDance Technologies, Life Technologies, Qiagen, JN MedSys Clarity, Optolane, and Stilla Technologies Naica. This paper discusses the working principle of each offered dPCR device and compares the associated: technical aspects, usability, costs, and current applications of each dPCR device. Lastly, up-and-coming dPCR technologies are also presented, as anticipation of how the dPCR device landscape may likely morph in the next few years.

The Identification of Tautoneura mori as the Vector of Mulberry Crinkle Leaf Virus and the Infectivity of Infectious Clones in Mulberry

Mulberry crinkle leaf virus (MCLV) is a novel geminivirus identified from mulberry. The pathogenicity and natural vector transmission of MCLV remain unknown. Here, infectious clones consisting of the complete tandem dimeric genome of MCLV in a binary vector were constructed and agroinoculated into young mulberry plants. The results showed that the infectious clones of MCLV were systemically infectious in mulberry, but the infected mulberry plants did not show any virus infection-like symptoms. The natural transmission vectors of MCLV were also identified from possible vector insects occurring on the MCLV-infected mulberry plants. The vector ability of Tautoneura mori was identified through an inoculation assay. Three of 21 (14.3%) plants inoculated with T. mori collected from MCLV-infected mulberry plants grown naturally were found to be MCLV-positive 50 days postinoculation. These MCLV-positive mulberry plants did not show any virus infection-like symptoms. Collectively, these results suggest that MCLV is infectious to mulberry plants but, by itself, does not induce infection symptoms. The leafhopper T. mori was experimentally determined to be a transmission vector of MCLV for the first time.

Grapevine Red Blotch Disease Etiology and Its Impact on Grapevine Physiology and Berry and Wine Composition

Grapevine red blotch virus (GRBV) has become widespread in the United States since its identification in 2012. GRBV is the causative agent of grapevine red blotch disease (GRBD), which has caused detrimental economic impacts to the grape and wine industry. Understanding viral function, plant–pathogen interactions, and the effects of GRBV on grapevine performance remains essential to developing potential mitigation strategies. This comprehensive review examines the current body of knowledge regarding GRBV, to highlight gaps in the knowledge and potential mitigation strategies for grape growers and winemakers.

Grapevine Red Blotch Virus Is Transmitted by the Three-Cornered Alfalfa Hopper in a Circulative, Nonpropagative Mode with Unique Attributes

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.

Co-Occurrence of Viruses, Plant Pathogens, and Symbionts in an Underexplored Hemipteran Clade

Interactions between insect symbionts and plant pathogens are dynamic and complex, sometimes involving direct antagonism or synergy and sometimes involving ecological and evolutionary leaps, as insect symbionts transmit through plant tissues or plant pathogens transition to become insect symbionts. Hemipterans such as aphids, whiteflies, psyllids, leafhoppers, and planthoppers are well-studied plant pests that host diverse symbionts and vector plant pathogens. The related hemipteran treehoppers (family Membracidae) are less well-studied but offer a potentially new and diverse array of symbionts and plant pathogenic interactions through their distinct woody plant hosts and ecological interactions with diverse tending hymenopteran taxa. To explore membracid symbiont–pathogen diversity and co-occurrence, this study performed shotgun metagenomic sequencing on 20 samples (16 species) of treehopper, and characterized putative symbionts and pathogens using a combination of rapid blast database searches and phylogenetic analysis of assembled scaffolds and correlation analysis. Among the 8.7 billion base pairs of scaffolds assembled were matches to 9 potential plant pathogens, 12 potential primary and secondary insect endosymbionts, numerous bacteriophages, and other viruses, entomopathogens, and fungi. Notable discoveries include a divergent Brenneria plant pathogen-like organism, several bee-like Bombella and Asaia strains, novel strains of Arsenophonus-like and Sodalis-like symbionts, Ralstonia sp. and Ralstonia-type phages, Serratia sp., and APSE-type phages and bracoviruses. There were several short Phytoplasma and Spiroplasma matches, but there was no indication of plant viruses in these data. Clusters of positively correlated microbes such as yeast-like symbionts and Ralstonia, viruses and Serratia, and APSE phage with parasitoid-type bracoviruses suggest directions for future analyses. Together, results indicate membracids offer a rich palette for future study of symbiont–plant pathogen interactions.

Use of Ground Covers to Control Three-Cornered Alfalfa Hopper, Spissistilus festinus (Hemiptera: Membracidae), and Other Suspected Vectors of Grapevine Red Blotch Virus

Grapevine red blotch virus (GRBV) is the causal agent of grapevine red blotch disease, which affects wine grapes and leads to reduced crop yield and quality. The pathogen-plant-vector relationship of GRBV is not well understood; however, some possible vectors have been identified: Caladonus coquilletti (Van Duzee; Hemiptera: Cicadellidae), Colladonus reductus (Van Duzee; Hemiptera: Cicadellidae), Erythroneura spp., Melanoliarus sp. (Hemiptera: Cixiidae), Osbornellus borealis DeLong. & Mohr (Hemiptera: Cicadellidae), Scaphytopius granticus (Ball; Hemiptera: Cicadellidae), Spissistilus festinus (Say). Of these species, S. festinus has been shown to transmit the virus to uninfected grapevines, making it of particular interest. Since the pathogen-plant-vector relationship of GRBV is not yet completely understood, pesticide use is not necessarily the best way to manage these possible vectors. Here we test if ground cover removal, by discing in spring, could reduce the activity of potential GRBV vectors. We show that S. festinus presence in the canopy was reduced in disc rows compared to just mowing the ground vegetation, whereas there were no differences in presence in the canopy between disc and mow rows of the other possible insect vectors. Erythroneura elegantula (Osborn; Hemiptera: Cicadellidae), a common pest of grapevines but not a candidate GRBV vector, was found to have higher densities in the canopy in disc rows compared to mow rows, an effect possibly mediated by changes in vine vigor associated with ground covers. We conclude that if S. festinus is a primary vector of GRBV, discing ground covers in early spring may be a viable way to reduce their presence in the vine canopy.

Impact of Rootstock and Season on Red Blotch Disease Expression in Cabernet Sauvignon (V. vinifera)

Grapevine red blotch virus (GRBV), the causative agent of grapevine red blotch disease, is widespread across the United States and causes a delay in ripening events in grapes. This study evaluates the effects of GRBV on Cabernet Sauvignon grape berry composition, grafted on two different rootstocks (110R and 420A) in two seasons (2016 and 2017). Total soluble solids, acidity, and anthocyanin concentrations were monitored through ripening and at harvest. Phenolic and volatile compounds were also analyzed at harvest to determine genotypic and environmental influences on disease outcome. Sugar accumulation through ripening was lower in diseased fruit (RB (+)) than healthy fruit across rootstock and season. GRBV impact was larger in 2016 than 2017, indicating a seasonal effect on disease expression. In general, anthocyanin levels and volatile compound accumulation was lower in RB (+) fruit than healthy fruit. Total phenolic composition and tannin content was higher in RB (+) fruit than healthy fruit in only 110R rootstock. Overall, GRBV impacted Cabernet Sauvignon grape composition crafted on rootstock 110R more than those crafted on rootstock 420A.

Economic Studies Reinforce Efforts to Safeguard Specialty Crops in the United States

Pathogen-tested foundation plant stocks are the cornerstone of sustainable specialty crop production. They provide the propagative units that are used to produce clean planting materials, which are essential as the first-line management option of diseases caused by graft-transmissible pathogens such as viruses, viroids, bacteria, and phytoplasmas. In the United States, efforts to produce, maintain, and distribute pathogen-tested propagative material of specialty crops are spearheaded by centers of the National Clean Plant Network (NCPN). Agricultural economists collaborated with plant pathologists, extension educators, specialty crop growers, and regulators to investigate the impacts of select diseases caused by graft-transmissible pathogens and to estimate the return on investments in NCPN centers. Economic studies have proven valuable to the NCPN in (i) incentivizing the use of clean planting material derived from pathogen-tested foundation plant stocks; (ii) documenting benefits of clean plant centers, which can outweigh operating costs by 10:1 to 150:1; (iii) aiding the development of disease management solutions that are not only ecologically driven but also profit maximizing; and (iv) disseminating integrated disease management recommendations that resonate with growers. Together, economic studies have reinforced efforts to safeguard specialty crops in the United States through the production and use of clean planting material.

Spissistilus festinus (Hemiptera: Membracidae) susceptibility to six generalist predators

Spissistilus festinus (Say) (Hemiptera: Membracidae) was shown to transmit Grapevine red blotch virus (GRBV) in a greenhouse study. Grapevines infected with GRBV exhibit reduced sugar accumulation, altered secondary metabolite production and delayed berry maturation that negatively impacts wine quality and economics. Augmentative biocontrol may be a useful integrated pest management (IPM) tool for suppressing S. festinus populations in vineyards, but minimal research has been conducted on testing potential predators against the different life stages of S. festinus. The susceptibility of S. festinus adults and nymphs (1^st through 5^th instar) to predation by six commercially available biocontrol agents in petri dish and bell bean plant arenas was determined under greenhouse conditions. No significant mortality of S. festinus nymphs or adults occurred when exposed to Cryptolaemus montrouzieri adults, C. montrouzieri larvae and Sympherobius barberi adults in petri dish or bell bean plant arenas. Significant mortality of 1^st and 2^nd instar nymphs of S. festinus in the presence of Zelus renardii nymphs was observed in petri dish but not in bell bean arenas. Hippodamia convergens adults and Chrysoperla rufilabris larvae both consumed a significant number of S. festinus nymphs in petri dish and bell bean arenas. No significant predation of S. festinus adults was documented in this experiment. Results of this study aid in identifying predators that may be suitable candidates for additional field testing to determine their potential efficacy as biocontrol agents of S. festinus in a vineyard setting.

Influence of Riparian Habitat and Ground Covers on Threecornered Alfalfa Hopper (Hemiptera: Membracidae) Populations in Vineyards

Grapevine red blotch virus (GRBV) is the causal agent of grapevine red blotch disease, which affects wine grapes and leads to reduced crop yield and quality. While some virus spread can be attributed to the propagation of infected plant material, a greenhouse assay recently demonstrated that the threecornered alfalfa hopper (Membracidae: Spissistilus festinus Say) can transmit GRBV between grapevines. While S. festinus is not considered an economic pest of wine grapes, this species is present in California vineyards and their feeding can cause petiole girdling. Recent surveys have noted a correlation between S. festinus populations and GRBV-positive vines in vineyard areas adjacent to riparian habitat. Here, S. festinus populations were monitored over a 2-yr period at multiple vineyard sites adjacent to riparian habitats. At each site, insects were sampled from ground covers and the vine canopy at the vineyard edge and interior, and vines in both locations were evaluated for petiole girdling. Results indicate that there was no difference in abundance of S. festinus at the vineyard edge and interior. Populations in the vine canopy were highest in the late spring and early summer, and this was followed by the appearance of petiole girdling, indicating a key period of potential GRBV transmission. Furthermore, activity in the vine canopy appears to be amplified when the quality of ground covers is reduced as the season progresses. That said, overall populations of S. festinus were relatively low and additional work is needed to characterize the timing and efficiency of transmission under field conditions.

Timing the Implementation of Cultural Practices for Spissistilus festinus (Hemiptera: Membracidae) in California Vineyards Using a Stage-Structured Degree-Day Model

The three-cornered alfalfa hopper, Spissistilus festinus (Say), was shown to transmit Grapevine red blotch virus (GRBV), the causative agent for Grapevine red blotch disease, in a greenhouse study on grapes. GRBV is a major concern of wine grape growers due to its economic impact on wine quality. Plants in the family Fabaceae are preferred hosts of S. festinus and are commonly planted as cover crops or present in a vineyard's native vegetation. In late winter, during grapevine dormancy, S. festinus migrate into vineyards to feed and reproduce on these cover crop and weed hosts. Tilling vineyard floor vegetation provides growers an opportunity to disrupt the life cycle of early instars that are relatively immobile, reducing the S. festinus first-generation population. Nymphal presence is difficult to detect. First through third instars were not detected in sweep net samples in a 2-yr weekly sampling study, whereas fourth and fifth instars were first found on the same sample date as emerging adults. A degree-day model was developed and successfully predicted when early S. festinus instars are present in the vineyard to aid in exploiting the time period when S. festinus is most susceptible to cultural control measures.

Impact of grapevine red blotch disease on primary and secondary metabolites in 'Cabernet Sauvignon' grape tissues

The grapevine red blotch disease (GRBD) was first noticed in 2008, impacting grape ripening. In general, GRBD reduces grape and wine quality resulting in significant economic losses. The purpose of the present study was to evaluate the effect of GRBD on agronomical parameters of 'Cabernet Sauvignon' vines at harvest. Using a metabolomics approach, the influence on primary and secondary metabolite profiling in skin + pulp/flesh and seeds were also determined. GRBD influenced °Brix and berry weight, as well as primary and secondary metabolites in both tissues. 1D ¹H NMR was effective in quantifying the main primary and secondary metabolites affected by GRBD. RP-HPLC was similarly able to quantify the main phenolics affected. Multivariate analysis showed the influence of the virus on grape metabolites using both tools in two berry tissues. The effectiveness of both tools to describe sample variability was compared and the most affected metabolites in each tissue could be identified.

Water Deficits Do Not Improve Fruit Quality in Grapevine Red Blotch Virus-Infected Grapevines (Vitis vinifera L.)

Although deficit irrigation is used to improve fruit quality in healthy grapevines, it can potentially amplify negative effects of viral disease and reduce fruit quality in Grapevine Red Blotch Virus (GRBV) infected grapevines. Therefore, a 2-year field experiment was conducted to understand the interaction between GRBV infection and water deficits on disease development and vine physiology. Well-watered (WW) vines were irrigated at 100% of estimated crop evapotranspiration (ETc), while water deficit (WD) vines received water at 66 and 50% ETc in 2017 and 2018, respectively. Healthy (GRBV-) and infected (GRBV+) vines were confirmed by PCR assays. There were no significant effects of water deficits on foliar symptom onset in either year, but more severe water deficits in 2018 resulted in a more rapid symptom progression. GRBV+ vines had a higher Ψstem compared to GRBV- vines, but the effects of virus only appeared post-veraison and corresponded to decreased leaf gas exchange. In general, vine vegetative and reproductive growth were not reduced in GRBV+ vines. Yields were highest in WW/GRBV+ vines due to larger clusters containing larger berries. Consistent treatment effects on berry primary chemistry were limited to sugars, with no interactions between factors. Water deficits were able to somewhat increase berry anthocyanin concentration in GRBV+ fruit, but the effects were dependent on year. By comparison, virus status and water deficits interacted on skin tannins concentration such that they were decreased in WD/GRBV+ vines, but increased in WD/GRBV- vines. Water deficits had no effect on seed phenolics, with only virus status having a significant diminution. Although keeping GRBV+ vines well-watered may mitigate some of the negative effects of GRBD, these results suggest that water deficits will not improve overall fruit quality in GRBV+ vines. Ultimately, the control of fruit ripening imparted by GRBV infection seems to be stronger than abiotic control imparted by water deficits.

Impact of Grapevine Red Blotch Disease on Cabernet Sauvignon and Merlot Wine Composition and Sensory Attributes

Grapevine red blotch disease (GRBD) is a recently identified viral disease that affects grapevines. GRBD has been shown to impact grapevine physiology and grape composition by altering specific ripening events. However, no studies have been reported on the impact of GRBD on wine composition and its sensory attributes. This study evaluated the impact of GRBD on wine primary and secondary metabolites, in addition to its sensory properties, when making wines from Cabernet Sauvignon and Merlot grapes during two seasons. Wines made with GRBD-impacted fruit were lower in ethanol content when compared to wines made with grapes from healthy grapevines. This was attributed to the lower total soluble sugar (TSS) levels of diseased grapes due to delayed ripening at harvest. GRBD impacted wine phenolic composition by decreasing anthocyanin concentrations and increasing flavonol concentrations in some instances. Additionally, proanthocyanidin concentrations were also consistently higher in GRBD wines compared to wines made from healthy fruit. Descriptive analysis demonstrated that GRBD can impact wine style by altering aroma, flavor, and mouthfeel attributes. However, the extent of GRBD impact on wine composition and sensory properties were site and season dependent.

CRISPR/Cas9-mediated targeted mutagenesis of TAS4 and MYBA7 loci in grapevine rootstock 101-14

Pierce’s disease (PD) of grapevine (Vitis vinifera) is caused by the bacterium Xylella fastidiosa and is vectored by xylem sap-sucking insects, whereas Grapevine Red Blotch Virus (GRBV) causes Red Blotch Disease and is transmitted in the laboratory by alfalfa leafhopper Spissistilus festinus. The significance of anthocyanin accumulations in distinct tissues of grapevine by these pathogens is unknown, but vector feeding preferences and olfactory cues from host anthocyanins may be important for these disease etiologies. Phosphate, sugar, and UV light are known to regulate anthocyanin accumulation via miR828 and Trans-Acting Small-interfering locus4 (TAS4), specifically in grape by production of phased TAS4a/b/c small-interfering RNAs that are differentially expressed and target MYBA5/6/7 transcription factor transcripts for post-transcriptional slicing and antisense-mediated silencing. To generate materials that can critically test these genes’ functions in PD and GRBV disease symptoms, we produced transgenic grape plants targeting TAS4b and MYBA7 using CRISPR/Cas9 technology. We obtained five MYBA7 lines all with bi-allelic editing events and no off-targets detected at genomic loci with homology to the guide sequence. We obtained two independent edited TAS4b lines; one bi-allelic, the other heterozygous while both had fortuitous evidences of bi-allelic TAS4a off-target editing events at the paralogous locus. No visible anthocyanin accumulation phenotypes were observed in regenerated plants, possibly due to the presence of genetically redundant TAS4c and MYBA5/6 loci or absence of inductive environmental stress conditions. The editing events encompass single base insertions and di/trinucleotide deletions of Vvi-TAS4a/b and Vvi-MYBA7 at expected positions 3 nt upstream from the guideRNA proximal adjacent motifs NGG. We also identified evidences of homologous recombinations of TAS4a with TAS4b at the TAS4a off-target in one of the TAS4b lines, resulting in a chimeric locus with a bi-allelic polymorphism, supporting independent recombination events in transgenic plants associated with apparent high Cas9 activities. The lack of obvious visible pigment phenotypes in edited plants precluded pathogen challenge tests of the role of anthocyanins in host PD and GRBV resistance/tolerance mechanisms. Nonetheless, we demonstrate successful genome-editing of non-coding RNA and MYB transcription factor loci which can serve future characterizations of the functions of TAS4a/b/c and MYBA7 in developmental, physiological, and environmental biotic/abiotic stress response pathways important for value-added nutraceutical synthesis and pathogen responses of winegrape.

Droplet Digital PCR Technology for Detection of Ilyonectria liriodendri from Grapevine Environmental Samples

Black-foot disease is one of the most important soilborne diseases affecting planting material in grapevine nurseries and young vineyards. Accurate, early, and specific detection and quantification of black-foot disease causing fungi are essential to alert growers and nurseries to the presence of the pathogens in soil, and to prevent the spread of these pathogens through grapevines using certified pathogen-free planting material and development of resistance. We comparatively assessed the accuracy, efficiency, and specificity of droplet digital PCR (ddPCR) and real-time PCR (qPCR) techniques for the detection and quantification of Ilyonectria liriodendri in bulk and rhizosphere soils, as well as grapevine endorhizosphere. Fungal abundance was not affected by soil-plant fractions. Both techniques showed a high degree of correlation across the samples assessed (R² = 0.95) with ddPCR being more sensitive to lower target concentrations. Roots of asymptomatic vines were found to be a microbial niche that is inhabited by black-foot disease fungi.

The impact of grapevine red blotch disease on Vitis vinifera L. Chardonnay grape and wine composition and sensory attributes over three seasons

BACKGROUND

Grapevine red blotch virus (GRBV) is a recently discovered DNA virus, which was demonstrated to be responsible for grapevine red blotch disease (GRBD). Its presence has been confirmed in the United States, Canada, Mexico, and South Korea in white and red Vitis vinifera cultivars, including Chardonnay. It has been shown that the three-cornered alfalfa treehopper (Spissistilus festinus) was able to both acquire the GRBV from a grapevine infected and transmit it to healthy grapevines in glasshouse conditions. Studies found that GRBD impacts fruit price, grapevine physiology, and grape berry composition and metabolism in red cultivars. This study evaluated the impact of GRBD on V. vinifera L. Chardonnay grape and wine composition and sensory properties from one vineyard during the 2014, 2015 and 2016 seasons.

RESULTS

Grapes from symptomatic red blotch diseased grapevines were lower in total soluble solids, flavan-3-ol, and total phenolic content, and higher in flavonol content when compared to grapes from healthy grapevines. Wines made with grapes from symptomatic grapevines resulted mostly in lower ethanol content and higher pH when compared to wines made from healthy grapevines. Analysis of volatile compounds and descriptive analysis demonstrated that GRBD can impact wine style by altering aroma, flavor, and mouthfeel attributes.

CONCLUSIONS

The impacts of GRBD on grape composition directly influenced wine chemistry. The decreased ethanol content impacted not only the levels of volatile compounds but the sensory perception during descriptive analysis. The extent of GRBD impact on the grape composition and wine composition and sensory attributes varied between seasons. © 2019 Society of Chemical Industry.

Two Distinct Genotypes of Spissistilus festinus (Say, 1830) (Hemiptera, Membracidae) in the United States Revealed by Phylogenetic and Morphological Analyses

Spissistilus festinus (Say, 1830) (Hemiptera: Membracidae) is a frequent pest of leguminous crops in the Southern United States, and a vector of grapevine red blotch virus. There is currently no information on the genetic diversity of S. festinus. In this study, populations of S. festinus were collected in 2015-2017 from various crops and geographic locations in the United States, and fragments of the mitochondrial cytochrome C oxidase 1 (mt-COI) gene and the nuclear internal transcribed spacer 2 (ITS2) region were characterized by polymerase chain reaction and sequencing. Maximum-likelihood and Bayesian analyses of the mt-COI and ITS2 sequences yielded similar phylogenetic tree topologies, revealing two distinct genetic S. festinus lineages with all of the specimens from California comprising one phylogenetic clade, alongside a single GenBank entry from Arizona, and all specimens from the Southeastern United States comprising a statistically-supported distinct clade, regardless of host and year of collection. The mt-COI gene fragment showed up to 10.8% genetic distance between the two phylogenetic clades. These results suggest the existence of two genotypes within S. festinus in the United States. The only distinct morphological trait between the two genotypes was a less elevated pronotum in the representative specimens from California, compared to the representative specimens from the Southeastern United States. Since this phenotypic feature is inconspicuous, a diagnostic polymerase chain reaction targeting a variable region of the mt-COI fragment was developed to reliably distinguish between the specimens of the two genotypes of S. festinus and to facilitate their specific identification.

Geminiviruses: Taxonomic Structure and Diversity in Genomic Organization

BACKGROUND

Geminiviridae is one of the best-characterized and hence, one of the largest plant-virus families with the highest economic importance. Its members characteristically have a circular ssDNA genome within the encapsidation of twinned quasi-icosaheadral virions (18-38 nm size-range).

OBJECTIVE

Construction of a narrative review on geminiviruses, to have a clearer picture of their genomic structure and taxonomic status.

METHODS

A thorough search was conducted for papers and patents regarding geminiviruses, where relevant information was used to study their genomic organization, diversity and taxonomic structure.

RESULTS

Geminiviruses have been classified into nine genera (viz., genus Begomovirus, Mastrevirus, Curtovirus, Topocuvirus, Becurtovirus, Turncurtovirus, Capulavirus, Eragrovirus and Grablovirus) having distinct genomic organizations, host ranges and insect vectors. Genomic organization of all genera generally shows the presence of 4-6 ORFs encoding for various proteins. For now, Citrus chlorotic dwarf-associated virus (CCDaV), Camellia chlorotic dwarf-associated virus (CaCDaV) and few other geminiviruses are still unassigned to any genera. The monopartite begomoviruses (and few mastreviruses) have been found associated with aplhasatellites and betasatellites (viz., ~1.3 kb circular ssDNA satellites). Recent reports suggest that deltasatellites potentially reduce the accumulation of helper-Begomovirus species in host plants. Some patents have revealed the methods to generate transgenic plants resistant to geminiviruses.

CONCLUSION

Geminiviruses rapidly evolve and are a highly diverse group of plant-viruses. However, research has shown new horizons in tackling the acute begomoviral diseases in plants by generating a novel bio-control methodology in which deltasatellites can be used as bio-control agents and generate transgenic plants resistant to geminiviruses.

Naked-Eye Detection of Grapevine Red-Blotch Viral Infection Using a Plasmonic CRISPR Cas12a Assay

Herein, we described a novel plasmonic CRISPR Cas12a assay for the visual, colorimetric detection of grapevine viral infections. Our assay generates rapid and specific colorimetric signals for nucleic acid amplicons by combining the unique target-induced incriminate single-stranded DNase activity of Cas12a with plasmon coupling of DNA functionalized gold nanoparticles. The practical applicability of our plasmonic assay was successfully demonstrated through the detection of emerging red-blotch viral infections in grapevine samples collected from commercial vineyards.

Pest categorisation of non-EU viruses and viroids of Vitis L

Following a request from the EU Commission, the Panel on Plant Health addressed the pest categorisation of the viruses and viroids of Vitis L. determined as being either non-EU or of undetermined standing in a previous EFSA opinion. These infectious agents belong to different genera and are heterogeneous in their biology. With the exclusion of grapevine virus 101-14.N.23.9.1/South Africa/2009 for which very limited information exists, the pest categorisation was completed for 30 viruses or viroids having acknowledged identities and available detection methods. All these viruses are efficiently transmitted by vegetative propagation techniques, with plants for planting representing the major pathway for long-distance dispersal and thus considered as the major pathway for potential entry. Depending on the virus, additional pathway(s) can also be seeds, pollen and/or vector(s). Most of the viruses categorised here are known to infect only one or few plant genera, but some of them have a wide host range, thus extending the possible entry pathways. Grapevine yellow speckle viroid 2, blueberry leaf mottle virus, grapevine Ajinashika virus, grapevine Anatolian ringspot virus, grapevine berry inner necrosis virus, grapevine deformation virus, grapevine fabavirus, grapevine red blotch virus, grapevine stunt virus, grapevine Tunisian ringspot virus, grapevine vein-clearing virus, temperate fruit decay-associated virus, peach rosette mosaic virus, tobacco ringspot virus, tomato ringspot virus meet all the criteria evaluated by EFSA to qualify as potential Union quarantine pests (QPs). With the exception of impact for the EU territory, on which the Panel was unable to conclude, blackberry virus S, grapevine geminivirus A, grapevine leafroll-associated virus 7, grapevine leafroll-associated virus 13, grapevine satellite virus, grapevine virus E, grapevine virus I, grapevine virus J, grapevine virus S, summer grape enamovirus, summer grape latent virus satisfy all the other criteria to be considered as potential Union QPs. Australian grapevine viroid, grapevine cryptic virus 1, grapevine endophyte endornavirus and wild vitis virus 1 do not meet all the criteria evaluated by EFSA to be regarded as potential Union QPs because they are not known to cause an impact on Vitis. For several viruses, especially those recently discovered, the categorisation is associated with high uncertainties mainly because of the absence of data on their biology, distribution and impact. Since this opinion addresses specifically non-EU viruses, in general these viruses do not meet the criteria assessed by EFSA to qualify as a potential Union regulated non-quarantine pests.

Spatial Associations of Vines Infected With Grapevine Red Blotch Virus in Oregon Vineyards

Spread and in-field spatial patterns of vines infected with grapevine red blotch virus (GRBV) were documented in Oregon vineyards using field sampling, molecular diagnostics, and spatial analysis. Grapevine petiole tissue collected from 2013 to 2016 was tested using quantitative polymerase chain reaction for GRBV. At Jacksonville in southern Oregon, 3.1% of vines were infected with GRBV in 2014, and GRBV incidence reached 58.5% of study vines by 2016. GRBV-infected plants and GRBV-uninfected plants were spatially aggregated at this site in 2015, and infected plants were spatially associated between years 2015 and 2016. In a southern Oregon vineyard near Talent, 10.4% of vines were infected with GRBV in 2014, and infection increased annually to 21.5% in 2016. At Talent, distribution of the infected vines was spatially associated across all years. GRBV infection was highest at Yamhill, in the Willamette Valley, where 31.7% of the tested vines had GRBV infection in 2014. By 2016, 59.2% of the vines tested positive for GRBV. Areas of aggregation increased and were spatially associated across all years. From 2013 to 2015, GRBV was not detected at Milton-Freewater in eastern Oregon. Spatial patterns of GRBV infection support evidence of spread by a mobile insect vector. GRBV is a significant threat to Oregon wine grape production because of its drastic year-over-year spread in affected vineyards.

Seasonal Dynamics of Spissistilus festinus (Hemiptera: Membracidae) in a Californian Vineyard

The three-cornered alfalfa hopper, Spissistilus festinus (Say) was shown to transmit Grapevine red blotch virus (GRBV) in a greenhouse study. GRBV is the causal agent of Grapevine Red Blotch Disease, which reduces the quality of wine produced from infected grapes. Due to the general lack of prior concern regarding S. festinus on grapevines, the biology of this species in vineyards has been largely unknown. A 2-yr study with weekly sampling was conducted in a Californian vineyard to increase the knowledge of S. festinus seasonal dynamics and distribution. The overwintering S. festinus adults were first captured in the vineyard before bud break. Detection of late-instar S. festinus nymphs, the first in-field adult generation, and grape anthesis occurred concurrently in 2016 and 2017. Two in-field S. festinus generations were documented by peaks in sweep net sampling of vineyard groundcover in 2016, whereas only one generation was observed in 2017. There appears to be an inverse relationship between the number of S. festinus adults sampled on ground cover and the number of girdles in the grapevine canopy. Spissistilus festinus exhibited an aggregated distribution in the vineyard and a significant edge effect. Results from this study will contribute to the development of sampling and management guidelines and determine timing of control measures to reduce populations of S. festinus within vineyards to minimize the virus spread.

Impact of Grapevine Red Blotch Disease on Grape Composition of Vitis vinifera Cabernet Sauvignon, Merlot, and Chardonnay

Grapevine red blotch disease (GRBD) is a recently recognized viral disease that affects grapevines ( Vitis vinifera L.). Currently little is known about its impact on grape composition. This study focused on the impact of GRBD on grape primary and secondary metabolites (mainly phenolic compounds) of three Vitis vinifera L. cultivars during two seasons. Grapes from symptomatic red blotch diseased vines (RB (+)) mostly had lower concentration of total soluble solids (TSS) and higher titratable acidity (TA) levels when compared to grapes from healthy vines (RB (-)) at harvest. GRBD impacted grape phenolic composition by mostly decreasing anthocyanin and increasing flavonol and proanthocyanidin (PA) contents in berry skins. No major impacts were observed on seed phenolics. RB (+) grapes contained more amino and carboxylic acids, while RB (-) grapes contained more oligosaccharides, polyols, and some specific monosaccharides at harvest. The impact of GRBD on grape composition was variable and dependent on the cultivar, site, and season.

Grapevine Red Blotch Virus May Reduce Carbon Translocation Leading to Impaired Grape Berry Ripening

Grapevine red blotch virus (GRBV) is suspected to alter berry ripening and chemistry. This study performed a physiological characterization of GRBV infected grapevines with attention to the factors leading to chemical changes during ripening of Cabernet Sauvignon in two rootstocks, 110R and 420A. RB(+) grapevines had transiently lower net photosynthesis; however, berry total soluble solids (TSS) accumulation was consistently reduced in the two years of study. Accumulation of anthocyanins and loss of titratable acidity and proanthocyanins were also delayed in RB(+) plants. However, the comparison of samples with the same TSS led to lower pH and anthocyanins content. The reduction in carbon import into berries under mild and transient reductions in carbon fixation suggested an impairment of translocation mechanisms with RB(+), leading into a desynchronization of ripening-related processes.

Evidence for the splicing of grablovirus transcripts reveals a putative novel open reading frame

Grapevine red blotch virus (GRBV) is type member of the newly identified genus Grablovirus. It possesses a single-stranded circular DNA genome of around 3200 nucleotides encoding three open reading frames (ORFs) in both the virion sense, the V1 (CP), V2 and V3, and complementary sense, C1 (RepA), C2 and C3. As shown for members of the genus Mastrevirus, the C1 and C2 ORFs are predicted to fuse through splicing to form a replication-associated protein (Rep). Data obtained using high-throughput sequencing (RNA-Seq) of three RNA-enriched populations, extracted from GRBV-infected grapevine (Vitis vinifera), confirmed the presence of the predicted C1-C2 intron (nts 2288-2450), but in addition identified a larger virion-sense intron (nts 251-589) spanning the V2 ORF. Evidence for both introns in a number of isolates was supported by bioinformatic analysis of publicly available datasets (n=20). These observations were further supported by RT-PCR analyses in both GRBV-infected grapevine and transient expression assays where GRBV genome segments were agro-inoculated onto Nicotiana benthamiana. The donor site of the virion-sense intron is located within two small ORFs, V0 and V02, while the acceptor site is two-thirds along the V2 ORF. Splicing at these positions is predicted to delete the N terminus of the encoded V2 protein. Comparative analyses of full-length GRBV sequences and the related tentative grabloviruses Prunus geminivirus A and wild Vitis virus 1 support the existence of both introns and V0. The probable regulatory role of these introns in the GRBV infection cycle is discussed.

Feeding and Reproductive Hosts of Spissistilus festinus (Say) (Hemiptera: Membracidae) Found in Californian Vineyards

Grapevine red blotch virus (GRBV) currently poses a serious threat to the wine industry. The three-cornered alfalfa hopper (3CAH), Spissistilus festinus (Say) (Hemiptera: Membracidae), was recently shown in a greenhouse study to be a vector of GRBV in grapes. Feeding and reproductive status of 10 weed and 10 cover crop species commonly found in grape vineyards were evaluated in no-choice tests. Species confirmed as reproductive hosts were subsequently evaluated in choice tests to determine S. festinus oviposition preference. Spanish clover, dandelion, birdsfoot trefoil, common groundsel, field bindweed, magnus peas, bell beans, blando brome, purple vetch, black medick, subterranean clover, crimson clover, and woollypod vetch were all found to be reproductive hosts. The plant species that exhibited the greatest nymph emergence in each of the three groups tested for preference were all in the family Fabaceae. Knowledge of plant species present in vineyards that serve as alternative hosts for S. festinus can contribute to developing management strategies to mitigate virus spread.

Absolute Quantification of Grapevine Red Blotch Virus in Grapevine Leaf and Petiole Tissues by Proteomics

Grapevine red blotch is a recently identified viral disease that was first recognized in the Napa Valley of California. Infected plants showed foliar symptoms similar to leafroll, another grapevine viral disease, on vines testing negative for known grapevine leafroll-associated virus. Later, the Grapevine red blotch virus (GRBV) was independently discovered in the US states of California and New York and was demonstrated to be the causal agent of red blotch disease. Due to its wide occurrence in the United States, vector transmission, and impacts on grape industry, this virus has the potential to cause serious economic losses. Despite numerous attempts, it has yet not been possible to isolate or visualize viral particles from GRBV-infected plants, thereby hampering the development of a serological assay that would facilitate GRBV detection in grapevine. In this work, mass spectrometry approaches were applied in order to quantify GRBV in infected plants and identify potential biomarkers for viral infection. We present for the first time the physical detection on the protein level of the two GRBV genes V1 (coat protein) and V2 in grapevine tissue lysates. The GRBV coat protein load in petioles was determined to be in the range of 100-900 million copies per milligram wet weight by using three heavy isotope labeled reference peptides as internal standards. In leaves on the other hand, the V1 copy number per unit wet tissue weight appeared to be about six times lower than in petioles, and about 300 times lower in terms of protein concentration in the extractable protein mass, albeit these estimations could only be made with one reference peptide detectable in leaf extracts. Moreover, we found in leaf and petiole extracts of GRBV-infected plants a consistent upregulation of several enzymes involved in flavonoid biosynthesis by label-free shotgun proteomics, indicating the activation of a defense mechanism against GRBV, a plant response already described for Grapevine leafroll-associated virus infection on the transcriptome level. Finally and importantly, we identified some other microorganisms belonging to the grapevine leaf microbiota, two bacterial species (Novosphingobium sp. Rr 2-17 and Methylobacterium) and one virus, Grapevine rupestris stem pitting-associated virus.

Vitis vinifera (Vitales: Vitaceae) as a Reproductive Host of Spissistilus festinus (Hemiptera: Membracidae)

Feeding by the three-cornered alfalfa hopper, Spissistilus festinus (Say) (Hemiptera: Membracidae) results in girdling of grapevine petioles and shoots. Its significance as an economic pest of grape has increased since it was shown to transmit Grapevine red blotch virus (GRBV) in a greenhouse study. However, the status of grapevines as a reproductive host for S. festinus remained undetermined. Adult S. festinus were caged onto three regions of the grapevines: apical shoot, green shoot, and dormant cane. Their ability to reproduce was determined by weekly destructive sampling for 7 wk. Successful oviposition and nymphal emergence were observed on apical and green shoots, but not on dormant canes. However, insect development beyond the second nymphal instar did not occur. Knowledge of S. festinus reproduction on grapevines will be an important consideration in designing management guidelines to minimize the spread of GRBV in vineyards.

The Distribution and Detection of Grapevine red blotch virus in its Host Depend on Time of Sampling and Tissue Type

Grapevine red blotch virus (GRBV) is the causal agent of grapevine red blotch, an emerging disease that affects cultivated grapevine such as Vitis vinifera. The ability to detect viruses in grapevine is often hindered by low virus titers compounded by a variable distribution in the plant and seasonal variations. In order to examine these two variables in relation to GRBV, we developed a quantitative polymerase chain reaction (qPCR) method that incorporates both internal and external references to enhance assay robustness. In greenhouse-grown vines infected with GRBV, qPCR identified highest virus titers in the petioles of fully expanded leaves and significantly reduced levels of virus in the shoot extremities. In vineyard-grown vines infected with GRBV, the virus titer in July and October 2016 followed a pattern similar to that found for the greenhouse-grown plants but, most strikingly, close to half (44%) of the samples analyzed in June 2015 tested negative for infection. The technique presented and results obtained highlight the variability of virus distribution in its host and provide a useful guide for selecting the best tissues for optimal GRBV diagnosis.

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.

Prevalence and Genetic Diversity of Grabloviruses in Free-Living Vitis spp

The distribution and diversity of grapevine red blotch virus (GRBV) and wild Vitis virus 1 (WVV1) (genus Grablovirus; family Geminiviridae) were determined in free-living Vitis spp. in northern California and New York from 2013 to 2017. Grabloviruses were detected by polymerase chain reaction in 28% (57 of 203) of samples from California but in none of the 163 samples from New York. The incidence of GRBV in free-living vines was significantly higher in samples from California counties with high compared with low grape production (χ² = 83.09; P < 0.001), and in samples near (<5 km) to compared with far (>5 km) from vineyards (χ² = 57.58; P < 0.001). These results suggested a directional spread of GRBV inoculum predominantly from vineyards to free-living Vitis spp. WVV1 incidence was also significantly higher in areas with higher grape production acreage (χ² = 16.02; P < 0.001). However, in contrast to GRBV, no differential distribution of WVV1 incidence was observed with regard to distance from vineyards (χ² = 0.88; P = 0.3513). Two distinct phylogenetic clades were identified for both GRBV and WVV1 isolates from free-living Vitis spp., although the nucleotide sequence variability of the genomic diversity fragment was higher for WWV1 (94.3 to 99.8% sequence identity within clade 1 isolates and 90.1 to 100% within clade 2 isolates) than GRBV (98.3% between clade 1 isolates and 96.9 to 100% within clade 2 isolates). Additionally, evidence for intraspecific recombination events was found in WVV1 isolates and confirmed in GRBV isolates. The prevalence of grabloviruses in California free-living vines highlights the need for vigilance regarding potential grablovirus inoculum sources in order to protect new vineyard plantings and foundation stock vineyards in California.

World Management of Geminiviruses

Management of geminiviruses is a worldwide challenge because of the widespread distribution of economically important diseases caused by these viruses. Regardless of the type of agriculture, management is most effective with an integrated pest management (IPM) approach that involves measures before, during, and after the growing season. This includes starting with resistant cultivars and virus- and vector-free transplants and propagative plants. For high value vegetables, protected culture (e.g., greenhouses and screenhouses) allows for effective management but is limited owing to high cost. Protection of young plants in open fields is provided by row covers, but other measures are typically required. Measures that are used for crops in open fields include roguing infected plants and insect vector management. Application of insecticide to manage vectors (whiteflies and leafhoppers) is the most widely used measure but can cause undesirable environmental and human health issues. For annual crops, these measures can be more effective when combined with host-free periods of two to three months. Finally, given the great diversity of the viruses, their insect vectors, and the crops affected, IPM approaches need to be based on the biology and ecology of the virus and vector and the crop production system. Here, we present the general measures that can be used in an IPM program for geminivirus diseases, specific case studies, and future challenges.

Survey for major viruses in commercial Vitis vinifera wine grapes in Ontario

BACKGROUND

In recent years, the Ontario grape and wine industry has experienced outbreaks of viral diseases across the province. Little is known about the prevalence of viruses and viral diseases in Ontario. Since 2015, we have conducted large-scale surveys for major viruses in commercial wine grapes in order to obtain a comprehensive understanding of the prevalence and severity of viral diseases in Ontario.

METHODS

A total of 657 composite leaf samples representing 3285 vines collected from 137 vine blocks of 33 vineyards from three appellations: Niagara Peninsula, Lake Erie North Shore and Prince Edward County. These samples covered six major red cultivars and five major white grape cultivars. Using a multiplex RT-PCR format, we tested these samples for 17 viruses including those involved in all major viral diseases of the grapevine, such as five grapevine leafroll-associated viruses (GLRaV-1, 2, 3, 4, 7), grapevine red blotch virus (GRBV), grapevine Pinot gris virus (GPGV), grapevine rupestris stem sitting-associated virus (GRSPaV), grapevine virus A (GVA), grapevine virus B (GVB), grapevine fleck virus (GFkV), arabis mosaic virus (ArMV), tomato ringspot virus (ToRSV), trapevine fanleaf virus (GFLV), among others.

RESULTS

Fourteen of the 17 viruses were detected from these samples and the predominant viruses are GRSPaV, GLRaV-3, GFkV, GPGV and GRBaV with an incidence of 84.0, 47.9, 21.8, 21.6 and 18.3%, respectively. As expected, mixed infections with multiple viruses are common. 95.6% of the samples included in the survey were infected with at least one virus; 67% of the samples with 2-4 viruses and 4.7% of the samples with 5-6 viruses. The major grape cultivars all tested positive for these major viruses. The results also suggested that the use of infected planting material may have been one of the chief factors responsible for the recent outbreaks of viral diseases across the province.

CONCLUSIONS

This is the first such comprehensive survey for grapevine viruses in Ontario and one of the most extensive surveys ever conducted in Canada. The recent outbreaks of viral diseases in Ontario vineyards were likely caused by GLRaV-3, GRBV and GPGV. Findings from this survey provides a baseline for the grape and wine industry in developing strategies for managing grapevine viral diseases in Ontario vineyards.

Complete nucleotide sequence of a new geminivirus isolated from Vitis vinifera in India: a symptomless host of Grapevine red blotch virus

Symptomless grape plants (Vitis vinifera) cultivated in Jind, Punjab, have been found to carry a Grapevine red blotch virus (GRBV). Evaluation of full length DNA sequence (3204 bp) of the virus (KU522121) has revealed similarity with mastrevirus, begomovirus, and other Grapevine red blotch viruses reported in the US and Canada. Similar to naturally growing plants, agroinfiltrated model plants with infectious clone of GRBV do not show any visible disease warning sign. To the best of our knowledge, this is the first report of a symptomless host Vitis vinifera from Indian vineyards harbouring a Grapevine geminivirus.

Causative Role of Grapevine Red Blotch Virus in Red Blotch Disease

Grapevine red blotch virus (GRBV) has a monopartite single-stranded DNA genome and is the type species of the genus Grablovirus in the family Geminiviridae. To address the etiological role of GRBV in the recently recognized red blotch disease of grapevine, infectious GRBV clones were engineered from the genome of each of the two previously identified phylogenetic clades for Agrobacterium tumefaciens-mediated inoculations of tissue culture-grown Vitis spp. plants. Following agroinoculation and one or two dormancy cycles, systemic GRBV infection was detected by multiplex polymerase chain reaction (PCR) in Vitis vinifera exhibiting foliar disease symptoms but not in asymptomatic vines. Infected rootstock genotype SO4 (V. berlandieri × V. riparia) exhibited leaf chlorosis and cupping, while infection was asymptomatic in agroinoculated 110R (V. berlandieri × V. rupestris), 3309C (V. riparia × V. rupestris), and V. rupestris. Spliced GRBV transcripts of the replicase-associated protein coding region accumulated in leaves of agroinfected vines, as shown by reverse-transcription PCR; this was consistent with systemic infection resulting from virus replication. Additionally, a virus progeny identical in nucleotide sequence to the infectious GRBV clones was recovered from agroinfected vines by rolling circle amplification, cloning, and sequencing. Concomitantly, subjecting naturally infected grapevines to microshoot tip culture resulted in an asymptomatic plant progeny that tested negative for GRBV in multiplex PCR. Altogether, our agroinoculation and therapeutic experiments fulfilled Koch's postulates and revealed the causative role of GRBV in red blotch disease.

Prunus geminivirus A: A Novel Grablovirus Infecting Prunus spp

Increased use of metagenomics for routine virus diagnosis has led to the characterization of several genus level geminiviruses from tree fruit long thought to exclusively host RNA viruses. In this study, the identification and molecular characterization of a novel geminivirus is reported for the first time in Prunus spp. The virus, provisionally named Prunus geminivirus A (PrGVA), was identified by Illumina sequencing from an asymptomatic plum tree. PrGVA was subsequently confirmed by rolling cycle amplification, cloning, and Sanger sequencing of its complete genome (3,174 to 3,176 nucleotides) from an additional 18 (9 apricot and 9 plum) field isolates. Apart from the nonanucleotide motif TAATATT↓AC present in its virion strand origin of replication, other conserved motifs of PrGVA support its geminiviral origin. PrGVA shared highest complete genome (73 to 74%), coat protein amino acid (83 to 85%) and rep-associated amino acid (74%) identities with Grapevine red blotch virus (GRBV). PrGVA was graft but not mechanically transmissible. Quantitative polymerase chain reaction screening of Prunus spp. in the National Clonal Germplasm Repository collection using newly designed primers and probes revealed 69.4% (apricot), 55.8% (plum), and 8.3% (cherry) incidences of PrGVA. PrGVA is proposed as a novel member of the genus Grablovirus based on its close genome and phylogenetic relationship with GRBV.

Digital PCR Technology for Detection of Palm-Infecting Phytoplasmas Belonging to Group 16SrIV that Occur in Florida

Phytoplasmas are an economically important group of plant pathogens that negatively impact a wide variety of plants in agricultural and natural ecosystems. In Florida, palm trees are essential elements in the nursery and landscaping industries that suffer from diseases caused by phytoplasmas that are related to each other but are classified in two different subgroups, 16SrIV-A and 16SrIV-D. In this study, a TaqMan assay was developed for digital polymerase chain reaction (dPCR) to detect both palm-infecting phytoplasmas found in Florida. When compared with real-time PCR assays and nested PCR assays, dPCR was capable of detecting the phytoplasmas at much lower concentrations than was possible by using real-time PCR and nested PCR. Additionally, the assay was capable of detecting 16SrIV-B phytoplasma as well as isolates representing the 16SrI and 16SrIII phytoplasma groups. Due to sequence identity of primer annealing regions across diverse phytoplasmas, the assay is likely to be successful for detection of a wide variety of phytoplasmas. The increased sensitivity of this dPCR assay over real-time PCR will allow for earlier detection of phytoplasma infection in palm trees, as well as for screening of salivary glands of candidate insect vector species. These advantages should aid timely management decisions to reduce disease spread and rapid determination of phytoplasma transmission by vectors.