Advancing Vineyard Health: Insights and Innovations for Combating Grapevine Red Blotch and Leafroll Diseases (2025)
Chapter: 2 Current Knowledge on Grapevine Red Blotch Disease
2
Current Knowledge on Grapevine Red Blotch Disease
Grapevine red blotch disease (GRBD) was initially referred to as “red-leaf disease” on Vitis vinifera cv. Cabernet Sauvignon in California. It was first described in a master’s thesis (Calvi, 2011), which reported that grapevines were observed to have foliar symptoms similar to leafroll disease—with documented effects of a suspected viral pathogen both on leaves and fruit—but tested negative for all known viruses. Around the same time, a circular DNA virus was discovered in a declining V. vinifera cv. Cabernet franc vineyard in New York and tentatively named grapevine cabernet franc-associated virus (Krenz et al., 2012). A similar virus was then discovered in California, found to be associated with red blotch (i.e., red leaf disease) symptoms, and tentatively named grapevine red blotch-associated virus (Al Rwahnih et al., 2013), while a similar virus was also found in Washington State and tentatively named grapevine red leaf-associated virus (Poojari et al., 2013).
Since the aforementioned viruses were determined to have similar genomes, the scientific community agreed to refer to them as grapevine red blotch-associated virus until Koch’s postulates were satisfied demonstrating a causal relationship between the virus and the disease (Yepes et al., 2018; see Box 2-1), at which point the name was changed to grapevine red blotch virus (GRBV).
SYMPTOMS
GRBD foliar symptoms manifest differently in V. vinifera depending on the cultivar. Foliar symptoms vary among red or black-fruited cultivars
BOX 2-1
Establishing a Causal Relationship of GRBV in Red Blotch Disease
Demonstrating virus disease etiology is important to ensure that management efforts are focused on the correct agent. Using Koch’s postulates, researchers demonstrated that GRBV is the causal agent for GRBD.
Koch’s Postulates
In the late 19th century, microbiologist and physician Robert Koch formulated guidelines for establishing the causal agents of diseases (referred to as Koch’s postulates). Koch’s foundational studies on disease etiology established the causal agents of diseases for anthrax, cholera, and tuberculosis and were later applied to numerous other diseases of animals and eventually plant diseases. Briefly, the postulates state that:
- The agent must be consistently found associated with a disease phenotype, i.e., present with disease and absent from healthy phenotypes.
- The agent can be isolated from the diseased organism and identified for its intrinsic properties.
- When the agent is inoculated from the pure culture into a healthy organism, the same disease phenotype is observed.
- The same agent (identified for its intrinsic properties) can be re-isolated from the diseased organism.
It is challenging to fulfill Koch’s postulates to infer virus disease etiology because viruses cannot be isolated in pure cultures. Virus disease etiology has been further complicated by the advent of high-throughput sequencing and discoveries that mixed virus infections are common in nature. One tactic to isolate a plant virus in pure culture is to propagate a complementary DNA (cDNA) copy of the viral genome in Agrobacterium tumefaciens as an infectious clone.
Fulfilling Koch’s Postulates for GRBD a
To resolve the GRBD etiology, clones of the GRBV genome were generated by amplification of partial tandem repeats using clade 1 and clade 2 isolates as references. The GRBV clones were mobilized in A. tumefaciens and then agroinoculated in virus-negative grapevine (Vitis vinifera and rootstocks) plantlets in tissue culture via vacuum-assisted agroinoculation and agropricking, i.e., using a sterile metal pin to deliver cultures directly to the phloem. This study demonstrated that GRBD symptoms develop in the GRBV-inoculated V. vinifera cultivars in tandem with detection of actively replicating GRBV, in some cases requiring a dormancy period before symptom development. Although most of the rootstocks did not show GRBD symptoms, GRBV was detected in the agroinoculated rootstocks, thus fulfilling Koch’s postulates.
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(e.g., Pinot noir, Cabernet Sauvignon, Cabernet franc, Syrah), but typically appear on older leaves as red blotches on the leaf blade, which eventually may coalesce to cover the whole leaf, and the leaf may prematurely senesce (see Figure 2-1). In white-fruited cultivars (e.g., Chardonnay and Sauvignon blanc) symptoms are less conspicuous, but chlorosis and leaf curling may occur (see Figure 2-1). Foliar symptoms are similar to those resulting from other problems such as nutritional disorders, mite damage, and leafroll disease. Thus, using symptomatology for diagnosis is not reliable.
The economic impact of GRBV is dependent on several factors, including geographic location, initial infection incidence, cultivar, and price penalty for low-quality fruit. The impact of GRBV was estimated to range from $2,213 ($2,810.61 in 2024 dollars1) per hectare in eastern Washington (with a low infection rate and low price penalty) to $68,548 ($87,059.20 in 2024 dollars) per hectare in California’s Napa Valley, where infection rates and price penalties are high (Ricketts et al., 2017). These findings and subsequent studies underscore the importance of reducing GRBV inoculum sources (Ricketts et al., 2017; Cieniewicz et al., 2020a; Fuchs et al., 2021; Hobbs et al., 2022).
CAUSAL VIRUS
GRBV (species Grablovirus vitis, genus Grablovirus, family Geminiviridae) was the first geminivirus discovered in grapevine. Grapevine red blotch virus was the first member of a new genus in the Geminiviridae, called Grablovirus (Varsani et al., 2017), in which there are now three ratified members: grapevine red blotch virus, wild Vitis latent virus, and prunus latent virus (Varsani et al., 2017; Al Rwahnih et al., 2018; Perry et al., 2018). Evolutionary analyses of the available full genomes of GRBV demonstrate two major clades of GRBV (see Figure 2-2); of these, clade 1 has higher genomic variability but clade 2 contains more isolates (Krenz et al., 2014; Cieniewicz et al., 2020a; Thompson, 2022). More genetic variation was discovered when additional GRBV isolates were collected from a recently released interspecific hybrid cultivar ‘Blanc du Soleil’ and formed a distinct sub-clade within clade 2 (Ouro-Djobo et al., 2023), a finding suggesting that much remains to be discovered about GRBV diversity. In addition to genomic variability introduced by the accumulation of mutations, recombination among GRBV isolates has been reported in several studies (Krenz et al., 2014; Perry et al., 2016; Cieniewicz et al., 2018a; Thompson, 2022; Ouro-Djobo et al., 2023). Currently, no biological significance regarding different clades of GRBV has been described, though
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1 Adjusted for inflation using the Consumer Price Index from the U.S. Bureau of Labor Statistics.
SOURCES: Marc Fuchs, Cornell University (A, B, C) and Elizabeth Cieniewicz, Clemson University (D, E).
SOURCE: Reprinted from Cieniewicz, E. J., W. Qiu, P. Saldarelli, and M. Fuchs, 2020, Believing is seeing: Lessons from emerging viruses in grapevine, Journal of Plant Pathology 102:619-632; and Ouro-Djobo, A., K. Stevens, J. J. Scheiner, V. M. Tsolova, F. M., Pontasch, S. A. McBride, D. N. Appel, M. Al Rwahnih, and O. J. Alabi, 2023, Molecular characterization of divergent isolates of grapevine red blotch virus from Blanc du Soleil, an interspecific hybrid white grapevine cultivar, PhytoFrontiers 3(2):290-295.
infectious clones based on isolates from both clades have been generated (Yepes et al., 2018).
EPIDEMIOLOGY AND VECTOR(S)
GRBV is distributed in nearly all viticultural regions of the United States (Krenz et al., 2014; Adiputra et al., 2019; Brannen et al., 2018; Yao et al., 2018; Jones and Nita, 2019; Schoelz et al., 2021; Thompson et al., 2019; Hoffmann et al., 2020; Hu et al., 2021; Hu, 2022; Soltani et al., 2020). In North America GRBV is also widespread in Canada (Poojari et al., 2017, 2020; Xiao et al., 2018; Kahl et al., 2022) and Mexico (Gasperin-Bulbarela et al., 2019). Outside of North America GRBV has been detected in South Korea (Lim et al., 2016), Switzerland (Reynard et al., 2018, 2022), India (Marwal et al., 2019), Argentina (Luna et al., 2019), and Italy (Bertazzon et al., 2021). Many of these GRBV detections have been isolated events, often in germplasm collections in which the material can be traced back to a U.S. origin. However, some detections, e.g., the recent detections of GRBV in Italy in the Italian accession ‘Incrocio Dalmasso VIII-5’, cannot be explained by a North American origin (Bertazzon et al., 2021). So far, it is not known if GRBV occurs in vineyards in Europe (Reynard et al., 2022).
Evidence compiled thus far suggests that GRBV may have a North American origin, potentially having originated in wild grapevines and diverged from the ancestral wild Vitis latent virus prior to the cultivation of V. vinifera in North America (Cieniewicz et al., 2020a; Reynard et al., 2022; Thompson, 2022). GRBV was detected in archival grapevine material maintained in a herbarium sample collected from Sonoma County in 1940, suggesting it has been in commercial wine grape production in this region for at least 80 years (Al Rwahnih et al., 2015).
Wild Vitis reservoirs include free-living Vitis spp. and hybrids of V. vinifera and V. californica, all of which are naturally occurring in vineyard ecosystems (Krenz et al., 2014; Bahder et al., 2016a; Perry et al., 2016; Cieniewicz et al., 2018a; Achala et al., 2022; Wilson et al., 2022). Bahder et al. (2016a) reported GRBV detection on wild Rubus spp., but only transiently, which suggested that wild Rubus is not a systemic host of GRBV. Some experimental hosts have been identified as a result of agroinoculation of GRBV infectious clones, including snap bean (Phaseolus vulgaris) and Nicotiana benthamiana (Flasco et al., 2021). GRBV has not been found in any of the field-collected herbaceous weed or cover crop species tested to date (Cieniewicz et al., 2019; Wilson et al., 2022). The only confirmed natural, systemic hosts of GRBV are Vitis spp., interspecific hybrids (Cieniewicz et al., 2020a), and muscadines (Soltani et al., 2020).
GRBV is efficiently transmitted through vegetative propagation (i.e., grafting and propagation of cuttings), and by treehopper insect vectors
(Hemiptera: Membracidae). As with all grapevine viruses, GRBV is not transmitted mechanically via equipment. In the United States, the dynamics of secondary spread of GRBV vary by region (Cieniewicz et al., 2017b, 2019; Dalton et al., 2019; Achala et al., 2022) and even between neighboring vineyards (Cieniewicz et al., 2019; Flasco et al., 2023a). Epidemiological studies provide support that GRBV is spread by a Hemipteran vector in northern California and Oregon (Cieniewicz et al., 2017b, 2018b; Dalton et al., 2019; Achala et al., 2022), whereas GRBV spread was not apparent in New York (Cieniewicz et al., 2019) nor in the Niagara region of Canada (Vu et al., 2023). Spatiotemporal spread of GRBV has not been explored in other regions.
The primary treehopper species implicated in the spread of GRBV in the western United States based on transmission studies and abundance in California vineyards is Spissistilus festinus Say, the three-cornered alfalfa hopper (TCAH) (see Figure 2-3). In Oregon, secondary spread of GRBV has been observed in some vineyards where the TCAH was present, but spread was also observed at sites where TCAH was not found (Dalton et al., 2019). These authors did find Tortistilus spp. at sites where S. festinus was absent but spread of GRBV was occurring, suggesting a need to determine whether Tortistilus spp. are also vectors of GRBV (Dalton et al., 2019). However, Tortistilus spp. are unlikely to be major vectors in California based on testing of specimens caught from 102 vineyards in the Napa Valley (Hoyle et al., 2024). Another study in Oregon vineyards demonstrated spread of GRBV in areas where the presence of S. festinus (and other potential vectors) was noted, but no sampling of vectors was performed (Achala et al., 2022). To date, the only vector with confirmed epidemiological relevance is S. festinus.
TCAH have been shown to start testing positive for GRBV in vineyards during June in the northern hemisphere (Cieniewicz et al., 2018b). The concentration of GRBV in grapevines increases over the course of the growing season (Setiono et al., 2018) and could increase acquisition of GRBV in the summer months when TCAH uses Vitis spp. as a feeding host, increasing the risk of spread.
TCAH may be absent at locations where GRBV spread is apparently occurring, suggesting the possibility of additional vector species (Dalton et al., 2019). Initial surveys have tested other Hemipteran species collected from vineyards in North America for acquisition of GRBV. To date, several species of treehoppers in the family Membracidae have been proposed to transmit GRBV, but few others have been tested for vector competence (Flasco et al., 2023d). Other Hemipteran species collected from California vineyards have tested positive for GRBV ingestion, including the leafhoppers (Cicadellidae) Erythroneura elegantula Osborn and Erythroneura variabilis Beamer, Caladonus coquilletti Van Duzee, Colladonus reductus
SOURCE: Elizabeth Cieniewicz, Clemson University.
Van Duzee, Osbornellus borealis DeLong & Mohr, Scaphytopius graneticus Ball, Aceratagallia spp., Acinopterus angulatus Lawson, Colladonus sp., and Empoasca spp., planthoppers Melanoliarus sp. (Cixiidae), an unknown species from the family Delphacidae, and unknown species from the family Aphididae (Bahder et al., 2016b; Cieniewicz et al., 2018b; Wilson et al., 2022). Outside of California, Entylia carinata Forster, Enchenopa bionata Say, Stictocephala basalis Walker, and S. bisonia Kopp and Yonke were shown to ingest GRBV (Kahl et al., 2021; LaFond et al., 2022). Of all these species shown to ingest GRBV, only six have been tested for vector competence. In California Erythroneura ziczac Walsh, the Virginia creeper leafhopper, has been found to ingest GRBV (Bahder et al., 2016b) but vector status remains unclear because transmission was reported in only one (Poojari et al., 2013) of two studies (Bahder et al., 2016b). Treehoppers collected in the Okanagan and Similkameen valleys of British Columbia, S. basalis and S. bisonia (Membracidae), were shown to transmit GRBV to artificial diet in the laboratory (Kahl et al., 2021). In Missouri, treehoppers E. carinata and E. bionata were shown to acquire and transmit GRBV to grapevines; E. carinata was the second most collected species in four vineyards sampled, whereas E. bionata was rarely observed (LaFond et al., 2022). E. elegantula and E. variabilis were not found to transmit GRBV, but the acquisition and inoculation times used in the study were much shorter than those characterized as necessary for TCAH to acquire and transmit GRBV (Flasco et al., 2021).
Many species shown to ingest GRBV have not been tested for vector competence to transmit GRBV, and many are not good candidates for testing due to their overall low abundance or low detection of GRBV in individuals tested. While studies have detected GRBV in Hemipterans from multiple families, reports of vector transmission of plant viruses to date have shown a high degree of specificity for species of viruses being transmitted by one or few species of insects from a single insect family (Nault, 1997), and so far, vector competence has been confirmed in multiple studies for members of Membracidae. According to Flasco et al. (2023d), this does not rule out the possibility that closely related species (i.e., plant hoppers) are GRBV vectors, but experiments testing transmission efficiency using long-duration acquisition access periods and inoculation access periods are required to confirm vector status of all species. Ensuring that appropriate controls are used and that experiments are not confounded by virions present in honeydew excreted by insects in transmission assays can help avoid false classification of insects as vectors of GRBV (Flasco et al., 2023d).
The seasonal dynamics and ecology of the potential Hemipteran vectors are not well understood. Few studies have included data on the seasonal dynamics, distributions, or acquisition of GRBV for E. elegantula, C. coquilletti, Colladonus spp., Scaphytopius graneticus, Scaphytopius spp.,
Melanoliarus sp., Colladonus reductus, and Osbornellus borealis (Cieniewicz et al., 2018b; Wilson et al., 2020, 2022; Billings et al., 2021). These potential vector species may differ from TCAH in important aspects such as abundance in vineyards, timing of GRBV acquisition, seasonal population dynamics, and transmission abilities and efficiencies. The differences in the ecology and vector competence of different species will need to be understood if additional vectors are identified because these factors will influence the development of vector management strategies.
PATHOGEN-VECTOR INTERACTIONS
The TCAH transmits GRBV in a circulative and non-propagative manner and requires an extended acquisition access period before transmission occurs; 10 days of feeding on infected grapevines is required for GRBV to be acquired and circulate through the insect (Flasco et al., 2021). TCAH can transmit isolates of GRBV from the two primary phylogenetic clades reported in the United States, and vineyards containing both GRBV clade 1-infected vines and clade 2-infected vines have been reported (Flasco et al., 2023a). Efficiency of GRBV transmission by the TCAH is generally low (Bahder et al., 2016b; Flasco et al., 2021; Hoyle et al., 2022) but transmission to grapevines in the vineyard has been demonstrated (Flasco et al., 2023b) and GRBV spread in a California vineyard was positively associated with viruliferous TCAH (Cieniewicz et al., 2018b, 2019).
When feeding on leaf petioles and green shoots of grapevines, TCAH and other treehoppers cause girdles on grapevine tissue (see Figure 2-4) that act as nutrient sinks benefiting the insect (Smith, 2013; Preto et al., 2018a). These girdles are also believed to negatively influence infection and localization of the virus after transmission (Flasco et al., 2023b). Transmission experiments conducted in vineyards were more successful when two vector individuals were used to transmit GRBV to individual leaves than when 10-12 individuals were used to transmit GRBV to half- or whole shoots (Flasco et al., 2023b).
VECTOR-HOST INTERACTIONS
Research has revealed two distinct genotypes of TCAH in the United States that are differentiated by geography and not host plant, with samples from California and Arizona comprising one genetic group and individuals collected from Alabama, Mississippi, Georgia, North Carolina, and Virginia comprising the other (Cieniewicz et al., 2020b). Populations of each genotype were reared separately and shown to be reproductively compatible as a result of reciprocal male-female crosses from each genotype, with subtle morphological differences in the resulting progeny (Flasco
SOURCES: Elizabeth Cieniewicz, Clemson University (A, B) and reproduced from Cieniewicz et al. (2017a, Figure 14.5) (C).
and Fuchs, 2023). The mitochondrial cytochrome oxidase subunit 1 gene sequence in the resulting progeny was consistent with the maternal parent genotype in each cross, as expected for a mitochondrial gene (Flasco and Fuchs, 2023). Notably, this study also revealed that TCAH of the southeastern U.S. genotype transmit GRBV at a higher efficiency than the California genotype, highlighting the need to study GRBV ecology in the southeastern United States (Flasco and Fuchs, 2023).
Research on hosts of GRBV, feeding and reproductive hosts of TCAH, seasonal dynamics, and dispersal of hemipterans in vineyards has revealed important information about factors influencing the spread of GRBV. TCAH has been found to feed on and girdle leaf petioles and green shoots of grapevines (Smith, 2013; Preto et al., 2018b) and lay eggs in V. vinifera, but nymphs cannot complete development on grapevines and TCAH does not survive on dormant wood (see Figure 2-5) (Preto et al., 2018b). The plant host range of the TCAH vector is wide; species in the family Asteraceae are preferred feeding hosts and species in the family Fabaceae are primary breeding hosts (Newsom et al., 1983; Preto et al., 2018a; Hoyle et al., 2023). This means that TCAH are utilizing Vitis spp. as occasional feeding hosts as they move through the environment, or when other preferred hosts are limited or absent, and that acquisition and transmission of GRBV likely occurs during periods in which primary feeding hosts are scarce. TCAH adults from overwintering generations have been collected before budbreak from ground-cover of vineyards. They are believed to complete one to two generations in California vineyards. TCAH and GRBV tend to be aggregated along field edges, with spread appearing localized and extending into vineyards over time in most study locations (Cieniewicz et al., 2017b; Dalton et al., 2019; Preto et al., 2019; Flasco et al., 2023a), but TCAH abundance and girdling are not always greater at field edges or locations adjacent to riparian habitats (Wilson et al., 2020). Populations of TCAH are believed to move into vineyards from riparian habitats or other natural habitats near vineyards, whereas fabaceous cover crops and weeds support TCAH populations and may facilitate TCAH spread throughout vineyards (Cieniewicz et al., 2017a; Preto et al., 2019; Kron and Sisterson, 2020a; Wilson et al., 2020; Sisterson et al., 2023). The first in-field generation of adults and immatures collected on grapevines was observed to coincide with anthesis, when the flower is fully open and ready to be pollinated (Preto et al., 2019). In California, girdling of grapevines was first observed in June or July, when TCAH are relatively abundant; however, after vegetation on the vineyard floor dried in August, populations of TCAH captured decreased, but girdling increased (Cieniewicz et al., 2018b; Preto et al., 2019; Wilson et al., 2020) and continued until early November (Preto et al., 2019). In a laboratory flight mill study, males were observed to fly longer and farther than females, with an average flight distance of 570.2 m compared to an average of 239.6 m for
SOURCE: Elizabeth Cieniewicz, Clemson University.
females (Antolínez et al., 2023). Age also influenced flight duration and distance of TCAH; males aged 8-21 days old and females aged 15-21 days old flew longer and farther than individuals aged 2-7 days old. There were no differences in the number of flights, time to first flight, or percentage of individuals engaging in flight between the different sexes or age groups. Under natural conditions TCAH may be influenced by temperature, wind, barometric pressure, plant-host associated cues, and biological factors, but findings from these laboratory studies can help to guide future field studies on insect dispersal among available host plants in the landscape.
HOST-PATHOGEN INTERACTIONS AND HOST DEFENSE MECHANISMS
Since geminiviruses rely on host DNA replication machinery, geminiviruses reprogram the cell cycle in order to make the DNA replication machinery available (Hanley-Bowdoin et al., 2013). RNA silencing is an antiviral strategy that is highly conserved among plants; thus, many viruses have evolved mechanisms to suppress RNA silencing by interfering with one or more parts of the RNA silencing pathway (Incarbone and Dunoyer, 2013; Pumplin and Voinnet, 2013; Csorba and Burgyan, 2016). Geminiviruses are subject to both transcriptional gene silencing and post-transcriptional gene silencing. Therefore, many geminiviruses have viral suppressors of RNA silencing that overcome methylation (transcriptional gene silencing) and also suppessors that interfere with small RNA signaling (post-transcriptional gene silencing) (Bisaro, 2006; Hanley-Bowdoin et al., 2013). So far, for GRBV, a single study has reported open reading frames (ORFs) C2 and V2 as having silencing suppressor activity, but the specific functions of these genes in terms of their interference with silencing are not yet known (Weligodage et al., 2023).
The GRBV genome is composed of a single molecule of circular, single-stranded DNA (see Figure 2-6), approximately 3.2 Kb (Krenz et al., 2012). There are seven putative ORFs, four in the viral orientation and three in the complementary orientation, for which expression is temporally regulated based on the virus infection cycle (Krenz et al., 2012; Al Rwahnih et al., 2013; Vargas-Ascencio et al., 2019). The complementary sense (c-sense) ORFs encode the early proteins, i.e., those involved in genome replication, and the viral sense (v-sense) ORFs encode the late proteins, i.e., the structural proteins such as the putative coat protein (CP) and movement protein. The v-sense and c-sense ORFs are separated by a short intergenic region and a long intergenic region but overlap within their respective groups. The first six ORFs (three overlapping ORFs each in v-sense and c-sense) were initially predicted by in silico analyses, analogous to other geminiviruses (Al Rwahnih et al., 2013; Krenz et al., 2014). Expression of these six ORFs was
SOURCE: Adapted from Thompson (2022). Reprinted from the Journal of General Virology, Microbiology Society.
later confirmed with additional evidence for a small seventh ORF, named ORF V0, upstream of V2, discovered by RNA sequencing (Vargas-Ascencio et al., 2019) (see Figure 2-6). The Rep protein is translated as a result of a messenger RNA (mRNA) splicing event that fuses ORFs C1 and C2, which has been observed in GRBV and in other geminiviruses (Nash et al., 2011). Splicing in the v-sense ORFs has only been observed in the capulaviruses,
mastreviruses, and now the grabloviruses in the Geminiviridae (Vargas-Ascencio et al., 2019). The V2 and V3 ORFs may have a role in movement (Guo et al., 2015). Vargas-Ascencio et al. (2019) propose the v-sense splicing event, which was confirmed by RNA sequencing, to result in the V2 ORF being out of frame, thus resulting in higher downstream expression of V1 (encoding the CP), which would be consistent with mechanisms proposed for mastreviruses. The putative function of the V0 is still unknown, but the sequence is highly conserved among at least 74 grablovirus sequences, and therefore V0 is likely to have an important biological function. Expression of the v-sense ORFs, including the CP (V1), is still not well understood and attempts to visualize viral particles have failed repeatedly (Vargas-Ascencio et al., 2019), suggesting the importance of an amenable model host in order to more effectively study GRBV gene expression.
To date, it has not been possible to visualize GRBV particles, although a twinned icosahedral virion structure is predicted based on homology to other geminiviruses (Zhang et al., 2001; Hipp et al., 2017; Hesketh et al., 2018). However, GRBV protein products from ORFs V1 (putative CP) and V2 (putatively involved in movement) were detected in grapevine petioles and leaves using mass spectrometry, with CP detection six times higher in petioles compared to leaves (Buchs et al., 2018). The same study noted upregulation of flavonoid biosynthesis proteins in GRBV-infected grapevines, suggesting the activation of plant defense against GRBV (Buchs et al., 2018). Wallis and Sudarshana (2016) also observed upregulation of amino acids involved in plant defense in GRBV-infected Cabernet Sauvignon and Cabernet franc both before and after symptom development. They also suggested that shifts in vine physiology responses to GRBV could be related to defenses activated against other stresses (Wallis and Sudarshana, 2016).
GRBD effects on vine physiology, fruit characteristics, and wine attributes vary by cultivar (Rumbaugh et al., 2021a) and also by vintage and rootstock (Wallis, 2022). GRBV interferes with foliar metabolism and metabolite translocation (Wallis and Sudarshana, 2016; Martínez-Lüscher et al., 2019; Levin and Achala, 2020), reduces pruning weight (Reynard et al., 2018; Bowen et al., 2020), reduces total soluble solids and anthocyanin accumulation (Calvi, 2011; Girardello et al., 2019; Lee et al., 2021), and alters grape ripening (see Figure 2-7) by interfering with hormone pathways (Blanco-Ulate et al., 2017; Rumbaugh et al., 2022). As effects on fruit directly impact wine attributes, some studies have described GRBV effects on wine such as reduced ethanol in Chardonnay (Girardello et al., 2020a) and Merlot (Girardello et al., 2020b). GRBV also alters grape skin cell wall composition, reducing the extractability of phenols in infected vines (Rumbaugh et al., 2023). Changes in the chemical profiles of wines made from GRBV-infected fruit reflect sensory attributes (e.g., mouthfeel and
SOURCE: Alice Wise, Cornell Cooperative Extension, Suffolk County, NY.
astringency), as well (Girardello et al., 2020a,b). GRBV effects on fruit and wine composition are especially problematic for a value-added fruit crop like grapevine. Understanding the mechanisms underlying the impacts of GRBV on fruit qualities could aid in developing potential mitigation strategies.
DIAGNOSTICS
Effective management of GRBD would involve the implementation of a comprehensive strategy that includes the use of certified disease-free planting material, regular monitoring by early detection, prompt removal of infected vines, strict quarantine measures, control of vectors (once identified), and the best viticultural practices (Sudarshana et al., 2015; Cieniewicz et al., 2017a; Meng et al., 2017). Early, sensitive, and reliable detection methods are paramount for disease management (Sudarshana et al., 2015). Various diagnostic techniques have been developed for GRBV detection,
each with distinct attributes in terms of sensitivity, specificity, cost, and applicability (Krenz et al., 2014; Li et al., 2019; Romero Romero et al., 2019). This section discusses these available diagnostic techniques and strategies for their use.
Detection of GRBV by PCR and qPCR
Since the advent of polymerase chain reaction (PCR) in the late 1980s, PCR has been widely used in the detection of grapevine viruses and evolved into the gold standard for nucleic acid amplification techniques (Mullis et al., 1986; Rowhani et al., 1993; Gambino, 2015). PCR was the first method developed for detecting GRBV (Krenz et al., 2014). Two sets of primers were designed in the viral genome encoding the CP and replicase genes, and a pair of primers was also designed to amplify a fragment of the 16S ribosomal RNA (rRNA) gene as an internal PCR control, resulting in a triplex PCR assay used for a GRBV survey in the United States (Krenz et al., 2014). Later, Setiono et al. (2018) developed a quantitative PCR (qPCR) using a set of primers targeting the replicase gene in GRBV and the grapevine actin gene to find the best time and tissues for collecting samples for detection. Although the quantity of GRBV is measured by qPCR, conventional PCR and qPCR have comparable sensitivity in detecting GRBV (Krenz et al., 2014; Setiono et al., 2018).
Detection of GRBV by LAMP
Loop-mediated isothermal amplification (LAMP) has been used to detect many plant viruses (Bhat et al., 2022). LAMP amplifies a target DNA fragment to detectable levels by using Bst DNA polymerase from Bacillus stearothermophilus and a set of four or six primers within a short time. The amplified DNA fragments can be detected directly by the color change of the reaction or indirectly by gel electrophoresis or lateral flow assay. An improved “pin-prick” LAMP method was developed to detect GRBV (Romero Romero et al., 2019). In this method, no DNA extraction is required; instead, the template for the LAMP is made by stabbing grapevine leaves or petioles three times with a 10-µl pipette tip and mixing trace amounts of tissues with sterile water. After the LAMP reagents and primers are added to the template, the reaction is performed at 65°C for 35 minutes, and the color change from pink to yellow indicates the presence of GRBV in the sample. The pin-prick GRBV LAMP method is 10,000 times more sensitive, costs less, and takes less time than conventional PCR. Leaves, petioles, and dormant budwood tissues can be pricked by the pipette tips for preparing the template. Although it is complicated to design the three sets of primers, online programs are now available for designing
the primers, such as PrimerExplorer,2 and primer sets are available for the detection of GRBV by LAMP (Romero Romero et al., 2019). In a recent study, pin-prick GRBV LAMP was shown to be a reliable, cost-effective, and rapid assay for detecting GRBV in the late developmental stages of grapevines (DeShields and Achala, 2023).
Detection of GRBV by RPA
Recombinase polymerase amplification (RPA) is another isothermal procedure for amplifying a target DNA fragment under a constant temperature in a short time. RPA has been commercially developed by TwistDx. It requires recombinase, DNA polymerase, single-strand binding protein, and a pair of primers specific to the target DNA sequences. A modified RPA assay, AmplifyRP Acceler8, has been developed for detecting GRBV (Li et al., 2017). In this assay, the primers and probes were designed in the CP and replicase region of the GRBV genome, and crude leaf extract was used directly in the reaction. The AmplifyRP Acceler8 was demonstrated to be consistent with PCR in detecting GRBV and is 100 times more sensitive than conventional PCR. Since RPA reagents are delivered in a pellet form under normal temperatures, crude tissue extracts are used, and the reaction is performed at 37°C for 20 minutes, RPA is considered to be a practical method for the on-site detection of GRBV in vineyards.
Detection of GRBV by RCA
Rolling circle amplification (RCA) is an isothermal enzymatic process to amplify circular DNA molecules. It relies on a DNA or an RNA polymerase, with Φ29 DNA polymerase being one of the commonly used enzymes in this technique. RCA is particularly useful for various molecular biology and diagnostics applications because it generates long single-stranded DNA (ssDNA) or RNA molecules from a circular template (Gu et al., 2018). Previously, RCA was used to obtain the complete GRBV genome (Al Rwahnih et al., 2013; Krenz et al. 2014; Thompson, 2022) and to create GRBV infectious clones (Yepes et al., 2018). Although RCA has been used to amplify the whole genome of GRBV, it has not been developed for diagnosing GRBV. The application of RCA in diagnostics could be further studied to potentially provide an additional method to detect GRBV.
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Detection of GRBV by CRISPR/Cas12a-Based Assay
Clustered regularly interspaced short palindromic repeats (CRISPR)associated 12a (CRISPR/Cas12a) was first developed for detecting human viruses (Chen et al., 2018) and is now used in the on-site detection of plant DNA viruses (Bhat et al., 2022). This assay is performed on a DNA fragment of a virus that is initially amplified by PCR, LAMP, or RPA. In this method, a single guide RNA is designed to bind to the amplified viral DNA fragments where it guides Cas12a to cut the target DNA and the single-stranded probe DNA molecules by the indiscriminate DNAase of Cas12a. The degraded products can be visually detected by a color change or lateral flow assay. A plasmonic CRISPR/Cas12a assay has been developed to detect GRBV visually by observing the color change in the reaction tubes (Li et al., 2019). This method still requires the extraction of total nucleic acids from grapevine tissues, but it can be improved by using RPA to amplify the target DNA fragments of GRBV first and then applying the plasmonic CRISPR/Cas12a assay.
Detection of GRBV by Hyperspectral Imaging
Hyperspectral imaging, which can be performed remotely by mounting hyperspectral cameras on unmanned aerial vehicles, represents a promising avenue for the advancement of virus disease scouting (Moghadam et al., 2017; Nguyen et al., 2021; Peng et al., 2022); however, the exploration of this technology in the context of virus-infected vines, specifically targeting GRBV, has been relatively limited (Reynolds et al., 2018). Within the visible region of the electromagnetic spectrum, the most discriminative wavelengths for predicting virus presence primarily reside in the red and orange regions, corresponding to anthocyanin presence and wavelengths associated with the absorption characteristics of chlorophyll and carotenoids (Sawyer et al., 2023). Further research could help enhance the precision of virus prediction methodologies with hyperspectral imaging. Particularly noteworthy is the consideration of extending the analysis to a broader electromagnetic spectrum range, a strategy for effectively evaluating and classifying vines that pose greater challenges regarding virus diagnosis.
Strategies for the Use of Available GRBV Diagnostic Techniques
The selection of the most suitable detection method depends on various factors, including available resources, required sensitivity, and the nature and number of the samples being analyzed. Overall, isothermal amplification methods such as LAMP and RPA provide accessible, cost-effective, and time-efficient options for routine GRBV testing (Li et al., 2017; Romero
Romero et al., 2019). These methods are enhanced by using pin-pricked DNA extraction and crude tissue extract to expedite the diagnostic process. However, the sensitivity and specificity of all diagnostic tests are significantly influenced by factors such as the timing of sample collection and the type of tissue examined. For example, qPCR, LAMP, and endpoint PCR achieve their highest sensitivity when used to test basal and middle leaf samples (DeShields and Achala, 2023). During specific phenological stages, such as fruit set and veraison, qPCR exhibits a sensitivity of 98 percent, while LAMP demonstrates sensitivity values of 49 percent and 78 percent from basal leaf samples during the same stages, respectively. At the harvest and dormancy stage, qPCR, LAMP, and PCR exhibit 100 percent sensitivity in basal and middle leaf or dormant cane samples (DeShields and Achala, 2023).
Determining the optimal sample number is an important consideration for comprehensive diagnostics. Research suggests that four tissue samples per vine is the optimal number for effectively discriminating between GRBV-positive and GRBV-negative vines (DeShields and Achala, 2023). Another sampling method for minimizing false negatives in diagnostic assays is a composite sampling of petiole tissue from older leaves at the base of the vine with three evenly distributed excisions (Reynard et al., 2018; Setiono et al., 2018). Considering that GRBV titer varies among cultivars and even from cordon to cordon in a single vine, it is critical to determine which tissues shall be sampled and the dates for sampling. A recent study reported that GRBV titer is consistently high in infected grapevines in different vineyards in three states in June (Flasco et al., 2023c), suggesting that sampling tissues for diagnostics in June will reduce incidences of false negative results. Kahl et al. (2022) also found a low rate of false negatives when basal leaves were sampled in summer months.
Visual determination of GRBD, as would be used by practitioners when scouting vineyards, is most effectively done when symptom expression peaks prior to leaf fall. However, symptom onset is variable between sites, cultivars, and vines of different ages, and can be confounded with other biotic and abiotic stressors (Adiputra et al., 2019; Rohrs et al., 2023), undermining the utility of visual inspection in facilitating the accurate and timely detection of GRBV.
Emerging technologies like plasmonic CRISPR Cas12a assays have shown improved sensitivity, offering promising options for GRBV detection (Li et al., 2019). Other methods that can be employed to detect low-titer plant viruses include serological methods, exemplified by immunocapture PCR (IC-PCR), which enhance sensitivity by combining immunocapture with PCR (Mulholland, 2009); digital LAMP (dLAMP), a digital variation of LAMP that enables highly sensitive quantification of viral nucleic acids (Panno et al., 2020); and high-throughput
sequencing (HTS), which can identify known and novel viruses at low titers with metagenomic sequencing (Boonham et al., 2014; Massart et al., 2014).
In general, the choice of a diagnostic technique should be guided by factors such as the number of samples requiring testing, the nature of the plants (i.e., foundation stock, nursery stock, germplasm, commercial vineyards, or a source of budwood), the phenological stage of grapevines, available resources, the urgency of diagnosis, and the desired level of sensitivity. Merging cost-effective methods with advancements in molecular diagnostics can enhance accessibility and reliability in GRBV detection and visual assessment, thereby contributing to more effective disease management.
MANAGEMENT
GRBD poses a significant threat to the viticulture industry with the potential for substantial economic losses (Al Rwahnih et al., 2013; Sudarshana et al., 2015; Cieniewicz et al., 2017a). The effective management of this disease would involve a comprehensive strategy (see Figure 2-8) that includes the use of certified virus-tested planting material, regular monitoring and early detection, prompt removal of infected vines, strict quarantine measures, control of vectors, and best viticultural practices (Sudarshana et al., 2015; Cieniewicz et al., 2017a; Meng et al., 2017). However, given that GRBV is a pathogen that has been recognized only recently, tactics for management of GRBV are still in the early stages of development and refinement.
Management of viral grapevine diseases relies mainly on prophylactic (preventive) measures because viral infections are impossible to cure once they are established in the vineyard. Although some management strategies aim to mitigate symptoms, there is limited evidence of the effectiveness of such methods and there is a risk that they could inadvertently promote virus inoculum accumulation in vineyards. Thus, prophylactic methods to prevent infection are considered to have the highest chance of success. These management strategies can be divided into those that may be employed before planting (pre-plant) and those that may be employed after planting (post-plant) and are described in the following sections.
Pre-Plant Management
Clean Plant Programs
GRBV can be spread through infected propagation material (Sudarshana et al., 2015). This route of introduction, as opposed to vector transmission, is the predominant way by which GRBV is introduced into new areas and
points to the need for clean plant programs that reduce the risk of spread of the virus (Poojari et al., 2017; Fuchs et al., 2021). One such program is the National Clean Plant Network (NCPN; see Box 2-2), which includes six Clean Plant Centers across the United States that focus on the propagation of clean grapevines (i.e., those derived from stocks that have tested negative for certain identified viruses).3 Although activities vary across centers, as a whole the NCPN-Grapes centers import grapevine accessions under quarantine conditions, conduct diagnostics and virus elimination therapies, and maintain foundation collections to ultimately distribute clean plant materials to nurseries for further commercial propagation (see Box 2-2). Clean plants can also be certified4 by state departments of agriculture in some states; the largest of these certification programs for grapevine is the California Grapevine Registration and Certification program, which was established in the 1950s and recently added GRBV to its list of pathogens of concern in response to grower interest.5 The use of clean (i.e., derived from virus-negative stocks) plants is voluntary, but the high quality of nursery stock developed through clean plant programs and the resulting savings in management costs have provided an incentive for growers to make use of them (Arnold et al., 2019).
Studies demonstrate that certification programs are a cost-effective means of mitigating the impact of grapevine diseases. In one analysis, the value of using certified nursery stock was estimated at over $20 million annually for the mitigation of grapevine leafroll-associated virus 3 alone, substantially outweighing the costs of the certification program (Fuller et al., 2019). In light of estimates that GRBD could cost Napa County growers approximately $34,000 per acre over the 25-year lifetime of a vineyard (2023 dollars) (Ricketts et al., 2017)—potentially resulting in hundreds of millions of dollars in lost revenue and management costs for individual
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3 See https://www.nationalcleanplantnetwork.org/grapes-1.
4 It is important to distinguish between how the terms “clean” and “certified” are used in this report in reference to grapevine planting material. Throughout this report, both terms indicate that steps have been taken to minimize the likelihood that the material is infected with an economically important virus (e.g., GLRaV-3 or GRBV); both clean and certified planting materials are tested for target viruses and maintained under conditions that minimize the risk of infection. However, each term has a specific meaning reflecting the context in which these steps are taken. In the case of “clean” plants, Grape Clean Plant Centers within the National Clean Plant Network determine what viruses to test for, what testing methods to use, and what protocols to use to minimize the risk of infection. In the case of “certified” or “certified clean” plants, certification programs administered by state departments of agriculture set rules that determine what viruses to test for, what testing methods to use, and what protocols to use to minimize the risk of infection. Because requirements may vary among clean plant centers and different state certification programs, “clean” and “certified” plants may be subject to different standards and practices.
5 See https://www.cdfa.ca.gov/plant/pe/nsc/docs/regs/ccr_3024_grapevine.pdf.
BOX 2-2
Production and Distribution of Clean Grapevines in the United States
The National Clean Plant Network (NCPN), foundation collections, and state certification programs provide processes and materials that help to prevent the spread of GRBD and other grapevine diseases with the goal of ensuring rootstocks or scions are derived from virus-negative sources before they are introduced to a vineyard.
National Clean Plant Network and NCPN-Grapes
The mission of the NCPN is summarized as “healthy agriculture through clean plants” with a vision of “safeguarding and supporting specialty crops by providing a sustainable source of clean plant material through innovation, collaboration, translational science, and outreach.” There are currently 47 collaborating programs at 35 clean plant centers in 20 U.S. states. The NCPN was founded in 2008, originally supporting only fruit trees and grapevines, and has since grown to include seven major crop groups: fruit trees, grapevines, citrus, berries, hops, sweet potatoes, and roses. In general, the clean plant centers work with one or more NCPN crops and conduct diagnostics for graft-transmissible pathogens; pathogen elimination therapies; and maintenance, production, and distribution of clean plants in foundation collections.
NCPN-Grapes is a collaborative endeavor that includes clean plant center directors, industry members (growers and nurseries), extension associates, and federal and state regulators. NCPN-Grapes includes six centers as of 2024 and is headquartered at Foundation Plant Services at the University of California, Davis. Other clean plant centers supporting grapes include the Clean Plant Center Northwest at Washington State University, the Midwest Center of NCPN-Grapes at Missouri State University, the Eastern NCPN-Grapes Center at Cornell University, the Micropropagation and Repository Unit at North Carolina State University, and the Center for Viticulture and Small Fruit Research at Florida A&M University.
Grapevine Foundation Collections
Most clean plant centers maintain foundation (i.e., Generation 1 or G1) collections, which are collections of commercially relevant accessions of grapevine that have tested negative for known pathogens, are true-to-variety, and are maintained under conditions that minimize the risk of re-infection. Foundation collections are highly valued and vitally important to the preservation of clean plant material. Foundation Plant Services at the University of California, Davis recently moved its entire grapevine foundation collection indoors to a new $5.25 million greenhouse in response to GRBV pressure in its previous open-field foundation vineyard.
State Certification Programs
Clean plant centers produce clean plants and maintain foundation collections; however, they do not certify any plants. Certification is administered by state departments of agriculture. For example, in California, the California Department of Food and Agriculture administers the Grapevine Registration and Certification Program. Currently only five states have certification programs for grapevines: California, Missouri, New York, Oregon, and Washington. Since grape growers in many other states source material from nurseries in these states, the impact of these state certification programs extends beyond those particular states.
Foundation plants are maintained in G1 blocks at clean plant centers. Material that is propagated from foundation stock and established at nurseries can be certified by the states in registered increase blocks (G2 to G4). Plants from the increase blocks represent registered stock and are propagated and sold to growers as certified planting material. Each classification within the certification scheme is subject to specific regulations on pathogens to be tested for and how often testing needs to occur, as well as when and how material can be propagated. It is important to note that “certified” does not equate to “clean” or “virus-negative.”
growing areas—incorporating GRBV testing into certification programs is likely to pay off.
Genetic Resistance
Host plant resistance to insect vectors of plant viruses can dramatically affect the spread of disease within a crop (Kennedy, 1976), and plant tolerance to vectors or diseases can reduce the negative impacts of a vector or viral infection on plant health or yield. Routes of resistance can include non-preference (antixenosis), in which traits make the plant unattractive to the vector or do not provide appropriate stimuli to attract the vector; antibiosis, in which plant traits incapacitate or kill a vector; and tolerance, in which plant traits reduce the impact of a vector or infection. While resistance to a vector can be an important management tactic, there is also a risk that it can amplify disease spread depending on the form of the resistance and the dynamics of the pathosystem. No sources of genetic resistance to or tolerance of GRBV have been identified for grapevine, nor have traits conferring resistance to the insect vector TCAH been demonstrated in grapevine. Pre-plant options for GRBD management are therefore currently limited to the use of planting material derived from virus-negative sources,
and at this point that is ultimately the responsibility of the customer in California.
Post-Plant Management
Roguing
Once established in a geographic region, GRBV can spread to uninfected plants from infected plants within the same vineyard, from neighboring vineyards, or from wild Vitis (Bahder et al., 2016a; Cieniewicz et al., 2017b, 2019). The route of pathogen spread within vineyards is unclear, and there is mixed evidence regarding the prevalence and impact of this type of spread. Studies conducted in northern California and southern Oregon have shown evidence of secondary spread within vineyards from infected vines (Cieniewicz et al., 2017b; Dalton et al., 2019); however, secondary spread within vineyards has not been observed in other areas, such as New York (Cieniewicz et al., 2019). The lack of secondary spread in New York has been attributed in part to the absence of populations of TCAH in the site surveyed and underscores the role of infected propagation materials as a means of viral spread (Cieniewicz et al., 2019).
When GRBV is detected in a vineyard, current management guidelines recommend roguing (removing) infected vines. Roguing individual infected vines and replanting them with vines derived from virus-negative sources appears to be economically viable when relatively few vines are infected, defined as less than 30 percent of the vineyard (Ricketts et al., 2017). When more vines are infected, replanting the entire vineyard may be warranted and more cost-effective in the long term. Roguing infected vines significantly reduces the spread of the virus in vineyards where TCAH is known to occur (Achala et al., 2022). Frequent scouting for infected vines is important for roguing practices to be effective as a cultural management tactic. However, accurate and reliable molecular diagnostics are also critical as symptoms may not be readily apparent or may appear similar to those of other diseases, mite damage, or nutrient deficiencies. The likelihood of grower adoption of these practices increases with their knowledge regarding GRBD. Growers who have personally experienced losses on their farms are most likely to implement management programs (Hobbs et al., 2022).
Vector Management
Because of knowledge gaps regarding TCAH, other potential vectors, vector-virus interactions, and interactions between GRBV and grapevines, best practices for vector management are currently not well established.
Practices that have been considered include habitat management, insecticide application, and biological control.
Habitat management within vineyards may be important for disease management. Vineyards are diverse agroecosystems that are often populated with large numbers of plants beyond grapevines that may be reproductive hosts or adult feeding hosts for TCAH. These non-grapevine TCAH hosts may also be hosts of GRBV, or they may simply serve as green bridges that facilitate vector movement to grapevines. Discing ground-covers (the rows between vine rows) can reduce numbers of TCAH captured on yellow sticky cards placed in the vine canopy. This practice likely operates by spatially segregating more preferred TCAH hosts from grapevines, thus decreasing the likelihood of TCAH reaching grapevines (Billings et al., 2021).
Insecticide management of TCAH is not currently a recommended practice until more is known about virus transmission and seasonal population dynamics of the vector(s). The initial response of growers to the emergence of a novel insect-transmitted plant pathogen has often been intensive insecticide applications in efforts to control the vector (Cho et al., 1989; Culbreath et al., 2003; Alvarez et al., 2016; Wenninger and Rashed, 2024). However, reliance on intensive insecticide use rarely provides effective, sustainable disease management, and comprehensive integrated approaches are often ultimately more effective. TCAH is recognized as an occasional, minor pest of annual crops, such as soybeans and peanuts, and short-lived perennials, such as alfalfa, where feeding by late instar nymphs can cause girdling of stems resulting in stand loss (Andersen et al., 2002; Beyer et al., 2017). It is not known to be a vector for other plant pathogens besides GRBV. Significant knowledge gaps in the GRBV pathosystem, especially regarding the ecology and population dynamics of the vector(s), preclude definitive recommendations for the role of insecticides in management programs (Cieniewicz et al., 2017a).
Little is known about the potential for biological control of TCAH. A fungal pathogen, Erynia delphacis, has been identified infecting TCAH in the southeastern United States (Miller and Harper, 1987). Although the fungus was found to be highly virulent against TCAH, it is not host specific, and it may not be as virulent in drier climates with greater ultraviolet radiation (Quesada-Moraga et al., 2023). TCAH is susceptible to a number of insect predators and parasitoids. Kron and Sisterson (2020b) evaluated six commercially available predatory insects against nymphal and adult TCAH. They found only Hippodamia convergens adults (Coleoptera: Coccinellidae) and Chrysoperla rufilabris larvae (Neuroptera: Chrysopidae) were effective predators of TCAH. Medal et al. (1997) found that Geocoris punctipes and Nabis roseipennis favored preying on TCAH, even in the presence of alternative prey. These predatory insects
were most likely to prey on younger nymphs, so promoting their populations on reproductive hosts rather than directly on grapevines would be the most effective use.
Mitigation of GRBD Effects
GRBD is known to affect fruit and wine quality (Girardello et al., 2019; Pereira et al., 2021; Rumbaugh et al., 2021a). Virus infection reduces berry weight and alters the level of total soluble solids and profiles of primary and secondary phytochemicals, including phenolics. Research is ongoing to identify mechanisms to compensate for these effects. Supplemental irrigation can compensate for berry weight but does not provide consistent recovery of anthocyanins (Copp and Levin, 2021; Copp et al., 2022). At present, available crop management practices do not seem sufficient to overcome the adverse effects of GRBD (Copp et al., 2022; Kurtural et al., 2023). Delayed harvest of fruit from GRBV-infected vines can mitigate some of the effects on specific wine aroma compounds, but whether this practice is practical in a winemaking setting has not been determined (Rumbaugh et al., 2021b; Girardello et al., 2024). Ultimately, any tactics aimed at mitigating the effects of GRBD will not address the drivers of GRBV inoculum in vineyards, potentially exacerbating the problem of virus spread. For these reasons, Fuchs (2024) recommends focusing primarily on reducing virus inoculum in vineyards and reducing secondary spread of GRBV.
Factors Impacting Effective Management
GRBD management tactics at all levels rely on effective and affordable diagnostics, knowledge and adoption of strategies by growers, and credible evidence of the biological and ecological drivers of GRBV. Emphasizing grower knowledge acquisition and developing management strategies with economic feasibility in mind will improve management in practice (Hobbs et al., 2022). Fuchs (2020) highlights the importance of bridging the communication gap between researchers and growers (i.e., decision makers) in order to promote adoption of effective management strategies. Also, perceptions of wine grape crop quality may differ between winemakers and vineyard managers, which is another highlighted gap (Fuchs, 2020). Fuchs (2020) recommends applying a premium for clean, certified vines to increase confidence in the quality of certified nursery material and incentivize the use of this material. Economic feasibility is a major factor in the adoption of management strategies and is a message that resonates with growers (Fuchs, 2020; Hobbs et al., 2022, 2023). Most of the management strategies that have been demonstrated to be effective in managing GRBV and other grapevine viruses are relatively simple, but they are not widely
adopted, likely due to deficiencies in communication and knowledge dissemination that resonates with the decision makers (Fuchs, 2020).
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