Detection of Cucurbit leaf crumple virus in Florida Cucurbits

Whitefly-Transmitted Viruses of Cucurbits in the Southern United States

Cucurbits are economically important crops that are widely cultivated in many parts of the world, including the southern US. In recent years, higher temperatures have favored the rapid build-up of whiteflies in the fall-grown cucurbits in this region. As a result, whitefly-transmitted viruses (WTVs) have severely impacted the marketable yield of cucurbits. In this review, we discuss three major groups of WTVs negatively impacting cucurbit cultivation in the southern US, including begomoviruses, criniviruses, and ipomoviruses. Here, we discuss the available information on the biology, epidemiology and advances made toward detecting and managing these viruses, including sources of resistance and cultural practices.

Development of a multiplex RT-RPA assay for simultaneous detection of three viruses in cucurbits

Begomoviruses and criniviruses, vectored by whiteflies (Bemisia tabaci), are important threats to crops worldwide. In recent years, the spread of cucurbit leaf crumple virus (CuLCrV), cucurbit yellow stunting disorder virus (CYSDV) and cucurbit chlorotic yellows virus (CCYV) on cucurbit crops has been reported to cause devastating crop losses in many regions of the world. In this study, a multiplex recombinase polymerase amplification (RPA) assay, an isothermal technique for rapid and simultaneous detection of DNA and RNA viruses CuLCrV, CYSDV and CCYV was developed. Highly specific and sensitive multiplex RPA primers for the coat protein region of these viruses were created and evaluated. The sensitivity of the multiplex RPA assay was examined using serially diluted plasmid containing the target regions. The results demonstrated that multiplex RPA primers have high sensitivity with a detection limit of a single copy of the viruses. The multiplex RPA primers were specific to the target as indicated by testing against other begomoviruses, potyviruses and an ilarvirus, and no nonspecific amplifications were noted. The primers simultaneously detected mixed infection of CCYV, CYSDV and CuLCrV in watermelon and squash crude extracts. This study is the first report of a multiplex RPA assay for simultaneous detection of mixed infection of DNA and RNA plant viruses.

Effects of Host Plants and Their Infection Status on Acquisition and Inoculation of A Plant Virus by Its Hemipteran Vector

Whitefly, Bemisia tabaci Gennadius (B cryptic species), transmits cucurbit leaf crumple virus (CuLCrV) in a persistent fashion. CuLCrV affects several crops such as squash and snap bean in the southeastern United States. CuLCrV is often found as a mixed infection with whitefly transmitted criniviruses, such as cucurbit yellow stunting disorder virus (CYSDV) in hosts such as squash, or as a single infection in hosts such as snap bean. The implications of different host plants (inoculum sources) with varying infection status on CuLCrV transmission/epidemics is not clear. This study conducted a series of whitefly mediated CuLCrV transmission experiments. In the first experiment, three plants species: squash, snap bean, and tobacco were inoculated by whiteflies feeding on field-collected mixed-infected squash plants. In the second experiment, three plant species, namely squash, snap bean, and tobacco with varying infection status (squash infected with CuLCrV and CYSDV and snap bean and tobacco infected with CuLCrV), were used as inoculum sources. In the third experiment, squash plants with differential CuLCrV accumulation levels and infection status (either singly infected with CuLCrV or mixed infected with CuLCrV and CYSDV) were used as inoculum sources. Irrespective of plant species and its infection status, CuLCrV accumulation in whiteflies was dependent upon the CuLCrV accumulation in the inoculum source plants. Furthermore, differential CuLCrV accumulation in whiteflies resulted in differential transmission, CuLCrV accumulation, and disease phenotype in the recipient squash plants. Overall, results demonstrate that whitefly mediated CuLCrV transmission between host plants follows a virus density dependent phenomenon with implications for epidemics.

Combining Cultural Tactics and Insecticides for the Management of the Sweetpotato Whitefly, Bemisia tabaci MEAM1, and Viruses in Yellow Squash

The sweet potato whitefly, Bemisia tabaci MEAM1 Gennadius (Hemiptera: Aleyrodidae), and the complex of viruses it transmits are major limiting factors to squash production in the southeastern United States. At this time, insecticides are extensively relied upon for the management of whiteflies and, indirectly, whitefly-transmitted viruses. The development of a multi-faceted, integrated pest management (IPM) program is needed to increase the sustainability and profitability of squash production. Experiments in 2018 and 2019 evaluated the effects of insect exclusion netting (IEN) in combination with selected pesticides on whitefly population dynamics and virus incidence in greenhouse-grown squash seedlings. Field experiments from 2018 to 2021 evaluated the effects of mulch type (UV-reflective mulch, live mulch, and white plastic mulch), row covers, and insecticides on whitefly population dynamics, silver leaf disorder (SSL) intensity, virus symptom severity, and marketable yield. IEN significantly reduced whiteflies and virus incidence on squash seedlings in the greenhouse study. In the field mulch study, lower whitefly abundance and SSL intensity, as well as reduced virus symptom severity, were observed in plots with reflective mulch compared with white plastic or live mulch. In the insecticide/row cover study, whitefly abundance, SSL intensity, and virus symptom severity were lowest in the row cover and cyantraniliprole- and flupyradifurone-treated plots. Field plots with row covers and those with UV-reflective mulch consistently produced the greatest marketable yields. These findings demonstrate that growers can reduce whitefly and virus pressure and preserve yields in squash production in the southeastern United States by combining cultural and chemical tactics, including row covers, UV-reflective mulch, and select insecticides.

The Begomovirus Species Melon Chlorotic Leaf Curl Virus is Composed of Two Highly Divergent Strains that Differ in Their Genetic and Biological Properties

Since the early 1990s, squash production in Costa Rica has been affected by a whitefly-transmitted disease characterized by stunting and yellow mottling of leaves. The squash yellow mottle disease (SYMoD) was shown to be associated with a bipartite begomovirus, originally named squash yellow mild mottle virus (SYMMoV). It was subsequently established that SYMMoV is a strain of melon chlorotic leaf curl virus (MCLCuV), a bipartite begomovirus that causes a chlorotic leaf curl disease of melons in Guatemala. In the present study, the complete sequences of the DNA-A and DNA-B components of a new isolate of the strain MCLCuV-Costa Rica (MCLCuV-CR) were determined. Comparisons of full-length DNA-A sequences revealed 97% identity with a previously characterized isolate of MCLCuV-CR and identities of 90 to 91% with those of isolates of the strain MCLCuV-Guatemala (MCLCuV-GT), which is below or at the current begomovirus species demarcation threshold of 91%. A more extensive analysis of the MCLCuV-CR and -GT sequences revealed substantial divergence in both components and different histories of recombination for the DNA-A components. The cloned full-length DNA-A and DNA-B components of this new MCLCuV-CR isolate were infectious and induced SYMoD in a range of squashes and in pumpkin, thereby fulfilling Koch's postulates for this disease. However, in contrast to MCLCuV-GT, MCLCuV-CR induced mild symptoms in watermelon and no symptoms in melon and cucumber. Taken together, our results indicate that MCLCuV-CR and -GT have substantially diverged, genetically and biologically, and have evolved to cause distinct diseases of different cucurbit crops. Taxonomically, these viruses are at the strain/species boundary, but retain the designation as strains of Melon chlorotic leaf curl virus under current International Committee on Taxonomy guidelines.

First Report of Cucurbit Chlorotic Yellows Virus affecting Watermelon in USA

Watermelon (Citrullus lanatus) is a high nutrient crop, high in vitamins and very popular in the U.S and globally. The crop was harvested from 101,800 acres with a value of $560 million in the U.S (USDA-NASS, 2020). California, Florida, Georgia and Texas are the four-leading watermelon-producing states in the U.S. During the fall season of 2020, plants in two North Florida watermelon fields, one in Levy County (~20 acres) and one in Suwannee County (~80 acres) with varieties Talca and Troubadour, respectively, exhibited viral-like symptoms. The fields had 100% disease incidence that led to fruit quality issues and yield losses of 80% and above. Symptoms observed in the watermelon samples included leaf crumpling, yellowing and curling, and vein yellowing similar to that of single/and or mixed infection of cucurbit leaf crumple virus (CuLCrV; genus: Begomovirus, family: Geminiviridae), cucurbit yellow stunting disorder virus (CYSDV; genus: Crinivirus, family: Closteroviridae) and squash vein yellowing virus (SqVYV; genus: Ipomovirus, family: Potyviridae), although the vine decline symptoms often associated with SqVYV infection of watermelon were not observed. All three viruses are vectored by whiteflies and previously described in Florida (Akad et al., 2008; Polston et al., 2008; Adkins et al., 2009). To confirm the presence of these viruses, RNA was isolated from 20 symptomatic samples using the RNeasy Plant Mini Kit (Qiagen, USA) as per protocol. This was followed by RT-PCR (NEB, USA) using gene-specific primers described for CuLCrV, CYSDV and SqVYV (Adkins et al., 2009). Amplicons of expected sizes were obtained for all the viruses with the infection of CuLCrV in 17/20, CYSDV in 16/20, and SqVYV in 8/20 samples. In addition, the presence of cucurbit chlorotic yellows virus (CCYV; genus: Crinivirus, family: Closteroviridae) in mixed infection was confirmed in 4/20 samples (3 leaves and 1 fruit) by RT-PCR with primers specific to the CCYV coat protein (CP), heat shock protein 70 homolog (HSP70h) and RNA dependent RNA polymerase (RdRp) designed based on the available CCYV sequences (Sup Table. 1). The RT-PCR amplification was performed using a symptomatic watermelon sample and the amplicons of RdRp, HSP70h and CP were directly sequenced by Sanger method, and the sequences of the amplicons were deposited in GenBank under the accession number: MW527462 (RdRp, 952 bp), MW527461 (HSP70h, 583 bp) and MW527460 (CP, 852 bp). BLASTn analysis demonstrated that the sequences exhibited an identity of 99% to 100% (RdRp and HSP70h, 100%; and CP, 99%) with the corresponding regions of the CCYV isolate Shanghai from China (accession number: KY400636 and KY400633). The presence of CCYV was further confirmed in the watermelon samples by ELISA (Loewe, Germany) using crude sap extracted from the RT-PCR-positive, symptomatic watermelon samples. CCYV was first identified in Kumamoto, Japan in 2004 on melon plants (Gyoutoku et al. 2009). The CCYV was previously reported on melon from Imperial Valley, California (Wintermantel et al., 2019), and more recently on squash in Tifton, Georgia (Kavalappara et al., 2021) and cantaloupe in Cameron, Texas (Hernandez et al., 2021). To our knowledge, this is the first report of CCYV on field watermelon production in the U.S. Continued monitoring of the CCYV in spring and fall watermelon crop, and cucurbit volunteers and weeds will be critical toward understanding the spread of this virus and its potential risk to watermelon in Florida and other regions of the U.S.

Low Frequency of Horizontal and Vertical Transmission of Cucurbit Leaf Crumple Virus in Whitefly Bemisia tabaci Gennadius

Cucurbit leaf crumple virus (CuLCrV), a bipartite begomovirus, is transmitted by whiteflies in a persistent and circulative manner. Like other begomoviruses, CuLCrV transmission via feeding is well understood; however, whether and how CuLCrV is transmitted by horizontal and vertical modes in its vector, Bemisia tabaci, remains unexplored. We studied transovarial and mating transmission of CuLCrV, and comparatively analyzed virus accumulation in whiteflies through feeding and nonfeeding modes. Furthermore, we quantified CuLCrV DNA A accumulation at different time points to determine whether this virus propagates in whiteflies. CuLCrV DNA A was transmitted vertically and horizontally by B. tabaci, with low frequency in each case. Transovarial transmission of CuLCrV DNA A was only 3.93% in nymphs and 3.09% in adults. Similarly, only a single viruliferous male was able to transmit CuLCrV DNA A to its nonviruliferous female counterparts via mating. In contrast, viruliferous females were unable to transmit CuLCrV DNA A to nonviruliferous males. Additionally, the recipient adults that presumably acquired CuLCrV transovarially and via mating were not able to transmit the virus to squash plants. We further report that the CuLCrV DNA A viral copy numbers were significantly lower in nonfeeding modes of transmission than in feeding ones. The viral copy numbers significantly decreased at succeeding time points throughout adulthood, suggesting no CuLCrV propagation in B. tabaci. Altogether, the low frequency of nonfeeding transmission, reduced virus accumulation in whiteflies, and absence of plant infectivity through nonfeeding transmission suggest that transovarial and mating CuLCrV transmission might not substantially contribute to CuLCrV epidemics.

Development of Loop-Mediated Isothermal Amplification Assay for Rapid Detection of Cucurbit Leaf Crumple Virus

A loop-mediated isothermal amplification (LAMP) assay was developed for simple, rapid and efficient detection of Cucurbit leaf crumple virus (CuLCrV), one of the most important begomoviruses that infects cucurbits worldwide. A set of six specific primers targeting a total 240 nt sequence regions in the DNA A of CuLCrV were designed and synthesized for detection of CuLCrV from infected leaf tissues using real-time LAMP amplification with the Genie^® III system, which was further confirmed by gel electrophoresis and SYBR™ Green I DNA staining for visual observation. The optimum reaction temperature and time were determined, and no cross-reactivity was seen with other begomoviruses. The LAMP assay could amplify CuLCrV from a mixed virus assay. The sensitivity assay demonstrated that the LAMP reaction was more sensitive than conventional PCR, but less sensitive than qPCR. However, it was simpler and faster than the other assays evaluated. The LAMP assay also amplified CuLCrV-infected symptomatic and asymptomatic samples more efficiently than PCR. Successful LAMP amplification was observed in mixed virus-infected field samples. This simple, rapid, and sensitive method has the capacity to detect CuLCrV in samples collected in the field and is therefore suitable for early detection of the disease to reduce the risk of epidemics.

Genetics and Genomics of Cotton Leaf Curl Disease, Its Viral Causal Agents and Whitefly Vector: A Way Forward to Sustain Cotton Fiber Security

Cotton leaf curl disease (CLCuD) after its first epidemic in 1912 in Nigeria, has spread to different cotton growing countries including United States, Pakistan, India, and China. The disease is of viral origin-transmitted by the whitefly Bemisia tabaci, which is difficult to control because of the prevalence of multiple virulent viral strains or related species. The problem is further complicated as the CLCuD causing virus complex has a higher recombination rate. The availability of alternate host crops like tomato, okra, etc., and practicing mixed type farming system have further exaggerated the situation by adding synergy to the evolution of new viral strains and vectors. Efforts to control this disease using host plant resistance remained successful using two gene based-resistance that was broken by the evolution of new resistance breaking strain called Burewala virus. Development of transgenic cotton using both pathogen and non-pathogenic derived approaches are in progress. In future, screening for new forms of host resistance, use of DNA markers for the rapid incorporation of resistance into adapted cultivars overlaid with transgenics and using genome editing by CRISPR/Cas system would be instrumental in adding multiple layers of defense to control the disease-thus cotton fiber production will be sustained.

Transmission of Squash vein yellowing virus to and From Cucurbit Weeds and Effects on Sweetpotato Whitefly (Hemiptera: Aleyrodidae) Behavior

Since 2003, growers of Florida watermelon [Citrullus lanatus (Thunb.) Matsum. and Nakai] have periodically suffered large losses from a disease caused by Squash vein yellowing virus (SqVYV), which is transmitted by the whitefly Middle East-Asia Minor 1 (MEAM1), formerly Bemisia tabaci (Gennadius) biotype B. Common cucurbit weeds like balsam apple (Momordica charantia L.) and smellmelon [Cucumis melo var. dudaim (L.) Naud.] are natural hosts of SqVYV, and creeping cucumber (Melothria pendula L.) is an experimental host. Study objectives were to compare these weeds and 'Mickylee' watermelon as sources of inoculum for SqVYV via MEAM1 transmission, to determine weed susceptibility to SqVYV, and to evaluate whitefly settling and oviposition behaviors on infected vs. mock-inoculated (inoculated with buffer only) creeping cucumber leaves. We found that the lowest percentage of watermelon recipient plants was infected when balsam apple was used as a source of inoculum. Watermelon was more susceptible to infection than balsam apple or smellmelon. However, all weed species were equally susceptible to SqVYV when inoculated by whitefly. For the first 5 h after release, whiteflies had no preference to settle on infected vs. mock-inoculated creeping cucumber leaves. After 24 h, whiteflies preferred to settle on mock-inoculated leaves, and more eggs were laid on mock-inoculated creeping cucumber leaves than on SqVYV-infected leaves. The transmission experiments (source of inoculum and susceptibility) show these weed species as potential inoculum sources of the virus. The changing settling preference of whiteflies from infected to mock-inoculated plants could lead to rapid spread of virus in the agroecosystem.

Preference of Bemisia tabaci biotype B on zucchini squash and buckwheat and the effect of Delphastus catalinae on whitefly populations

BACKGROUND

Zucchini squash, Cucurbita pepo L., is an important vegetable crop in Florida. Physiological disorders and insect-transmitted diseases are major problems for squash growers in semi-tropical regions around the world. Bemisia tabaci (Gennadius) biotype B is a significant whitefly pest and is largely responsible for transmitting viruses and causing physiological disorders in squash. Several studies have shown that whitefly populations are reduced when crops are interplanted with non-host cover crops or mulches. The aim of the present study was to determine how the presence of buckwheat, Fagopyrum esculentum Moench, and a key predator, Delphastus catalinae (Horn), affect whitefly colonization on squash.

RESULTS

Whitefly densities were higher on squash than on buckwheat. The introduction of D. catalinae on squash significantly reduced whitefly populations. Overall, there were higher densities of D. catalinae on squash where the whitefly pest was more concentrated compared with buckwheat.

CONCLUSION

The study provided preliminary evidence that D. catalinae, when used in conjunction with buckwheat as a living mulch, may aid in reducing whiteflies in squash. This greenhouse experiment highlights the need to investigate a multitactic approach of intercropping buckwheat with squash and the incorporation of D. catalinae in the field to manage populations of whiteflies and whitefly-transmitted diseases. © 2015 Society of Chemical Industry.

Spatial and Temporal Analysis of Squash vein yellowing virus Infections in Watermelon

Squash vein yellowing virus (SqVYV) is a whitefly-transmitted ipomovirus infecting watermelon and other cucurbits that was recently introduced to Florida. Effects on watermelon are devastating, with total vine collapse, often near harvest, and fruit rendered unmarketable by brown, discolored flesh. The epidemiology of SqVYV was studied in a 1-ha field of 'Fiesta' watermelon over six growing seasons (I to VI) to characterize the spatial patterning of disease and temporal rate of disease progress, as well as its association with Cucurbit leaf crumple virus (CuLCrV) and Cucurbit yellow stunting disorder virus (CYSDV), two additional whitefly-transmitted viruses that often occur with SqVYV. The field was scouted at regular intervals for the length of the season for incidence of virus and number of whiteflies. Incidence of SqVYV reached 100% during seasons I, II, and V and 20% during season III. SqVYV did not occur during seasons IV and VI. SqVYV progressed in a characteristic logistic fashion in seasons I, II, and V but less so in season III. The rate of disease progress was similar for the three seasons with high disease incidence, with an average value of 0.18. A positive correlation between the area under the disease progress curve and whitefly-days was found, where both progress curves were calculated as a function of thermal time (degree days, base 0°C). SqVYV displayed significant but variable levels of aggregation, as indicated by its fit to the β-binomial distribution, the binary power law, and ordinary runs analysis. Association analysis indicated that the viruses were largely transmitted independently. Results of this study provide epidemiological information that will be useful in the development of management strategies for SqVYV-induced vine decline, and provide new information for CuLCrV and CYSDV.

The complete genome sequence of New World jatropha mosaic virus

Full-length sequences of a bipartite begomovirus were obtained from a plant of Jatropha multifida in Florida showing symptoms of foliar mosaic, distortion and necrosis. Sequences of four clones each of a DNA-A and DNA-B were obtained, which showed very low sequence diversity among themselves. The clones were infectious when biolistically inoculated to J. multifida, Phaseolus vulgaris and Nicotiana tabacum, but not to J. curcas. The DNA-A sequences had less than 89 % pairwise identity scores with the DNA-A of other begomoviruses. The DNA-A appeared to be a recombinant in that 18 % of the DNA-A (470 nt) had a pairwise identity score of 91.98 % with RhRGMV, indicating that this portion most likely originated from a virus closely related to RhRGMV. The remaining 82 % of the DNA-A had lower identity scores with TbMoLCV (87.84 %) and RhRGMV (87.46 %), which suggests that this part of the component originated from an undescribed virus. There was no evidence for recombination in the DNA-B. Equivalent sequences of the DNA-A had the highest identity score (94.18 %) with a 533-nt sequence obtained from J. multifida from Puerto Rico in 2001 (GenBank accession no. AF058025). Pairwise comparison, recombination and phylogenetic analysis, and biology suggest that these clones are those of jatropha mosaic virus first reported from Puerto Rico. This is the first report of the complete genome sequence of jatropha mosaic virus.

Physiological Effects of Squash vein yellowing virus Infection on Watermelon

Squash vein yellowing virus (SqVYV) is the cause of viral watermelon vine decline. The virus is whitefly-transmitted, induces a systemic wilt of watermelon plants, and causes necrosis and discoloration of the fruit rind. In the field, SqVYV is often detected in watermelon in mixed infections with other viruses including the aphid-transmitted Papaya ringspot virus type W (PRSV-W). In this study, watermelon plants of different ages were inoculated with SqVYV or SqVYV+PRSV-W in the greenhouse or SqVYV in the field to characterize the physiological response to infection. Symptoms of vine decline appeared about 12 to 16 days after inoculation with SqVYV regardless of plant age at time of inoculation, plant growth habit (trellised or nontrellised), and location (greenhouse or field). However, the presence of PRSV-W delayed the appearance of vine decline symptoms by 2 to 4 days, and vine decline did not develop on plants with no fruit. For all inoculation treatments, more severe symptoms were observed in younger watermelon plants. Physiological responses to SqVYV infection included reduction in plant and fruit weights, alterations in fruit rind and flesh color, reduction in fruit sucrose content, increase in fruit acid content, and changes in plant nutrient composition, particularly increases in Ca, Mg, B, Mn, and Zn and decreases in K and N. These results demonstrate wide-ranging physiological effects of SqVYV infection and provide new insights into watermelon vine decline.

Semipersistent Whitefly Transmission of Squash vein yellowing virus, Causal Agent of Viral Watermelon Vine Decline

Squash vein yellowing virus (SqVYV), a recently described Ipomovirus sp. in the family Potyviridae, is the cause of viral watermelon vine decline, a devastating disease in Florida. SqVYV is known to be transmitted by the whitefly, Bemisia tabaci (Gennadius) B strain, but details of the transmission process have not previously been investigated. We completed a series of experiments to determine efficiency of transmission, effects of different acquisition and inoculation access periods, the length of time that whiteflies retained transmissible virus, and the minimum time needed to complete a cycle of acquisition and inoculation. Efficiency was low, with at least 30 whiteflies per plant needed for consistent transmission. Acquisition leading to later transmission peaked at 4 h, and inoculation access periods longer than 4 to 8 h led to no increase in infection rates. Whiteflies retained virus only a short time, with no transmission by 24 h after removal from infected plants. A minimum of 3 h was needed to complete a cycle of transmission under laboratory conditions. These results demonstrate semipersistent transmission of SqVYV and will help refine models of the epidemiology of this virus and the disease it causes.

Ecology and management of whitefly-transmitted viruses of vegetable crops in Florida

A variety of fresh market vegetables, including watermelon and tomato are economically important crops in Florida. Whitefly-transmitted Squash vein yellowing virus (SqVYV) was first identified in squash and watermelon in Florida in 2005 and shown to cause a severe decline of watermelon vines as crops approach harvest. Florida is most economically impacted by SqVYV, although the virus has been detected more recently in Indiana and South Carolina. The origin and evolutionary history of SqVYV, one of the few members of the genus Ipomovirus within the family Potyviridae, are not known. Sequence diversity of SqVYV isolates collected at different times, from different locations and from different plant species is being analyzed for insights into the origin of the virus. More recently, Cucurbit leaf crumple virus (CuLCrV) and Cucurbit yellow stunting disorder virus (CYSDV), also whitefly-transmitted, have been detected in watermelon in Florida. Tomato yellow leaf curl virus (TYLCV) was first detected in south Florida tomato crops in 1997. Several surveys have been conducted in the region to identify alternative hosts for these four viruses. Cucurbit weeds including Balsam-apple (Momordica charantia), creeping cucumber (Melothria pendula) and smellmelon (Cucumis melo var. dudaim) provide reservoirs for SqVYV, CuLCrV and/or CYSDV. Green bean (Phaseolus vulgaris) also can be a reservoir for CuLCrV. No wild hosts of TYLCV have been reported in Florida. The effectiveness of insecticides and silver plastic mulch to manage whiteflies and mitigate TYLCV has been demonstrated and is currently being evaluated for SqVYV, CuLCrV and CYSDV. In addition, potential sources of SqVYV resistance have been identified in greenhouse and field screening of watermelon germplasm. Further studies to refine these sources of resistance are underway. Lastly, a comprehensive map of 33,560 hectares (82,928 acres) of vegetable fields in the three counties comprising the majority of the southwest Florida vegetable production area has been developed to identify 'hot spots' and reservoir crops for viruses and whiteflies, and will be useful in evaluation of management strategies to decrease virus incidence in commercial fields.

Exploring the diversity of plant DNA viruses and their satellites using vector-enabled metagenomics on whiteflies

Current knowledge of plant virus diversity is biased towards agents of visible and economically important diseases. Less is known about viruses that have not caused major diseases in crops, or viruses from native vegetation, which are a reservoir of biodiversity that can contribute to viral emergence. Discovery of these plant viruses is hindered by the traditional approach of sampling individual symptomatic plants. Since many damaging plant viruses are transmitted by insect vectors, we have developed "vector-enabled metagenomics" (VEM) to investigate the diversity of plant viruses. VEM involves sampling of insect vectors (in this case, whiteflies) from plants, followed by purification of viral particles and metagenomic sequencing. The VEM approach exploits the natural ability of highly mobile adult whiteflies to integrate viruses from many plants over time and space, and leverages the capability of metagenomics for discovering novel viruses. This study utilized VEM to describe the DNA viral community from whiteflies (Bemisia tabaci) collected from two important agricultural regions in Florida, USA. VEM successfully characterized the active and abundant viruses that produce disease symptoms in crops, as well as the less abundant viruses infecting adjacent native vegetation. PCR assays designed from the metagenomic sequences enabled the complete sequencing of four novel begomovirus genome components, as well as the first discovery of plant virus satellites in North America. One of the novel begomoviruses was subsequently identified in symptomatic Chenopodium ambrosiodes from the same field site, validating VEM as an effective method for proactive monitoring of plant viruses without a priori knowledge of the pathogens. This study demonstrates the power of VEM for describing the circulating viral community in a given region, which will enhance our understanding of plant viral diversity, and facilitate emerging plant virus surveillance and management of viral diseases.

Distribution of four viruses in single and mixed infections within infected watermelon plants in Florida

Whitefly-transmitted Squash vein yellowing virus (SqVYV) and Cucurbit leaf crumple virus (CuLCrV) and aphid-transmitted Papaya ringspot virus type W (PRSV-W) have had serious impact on watermelon production in southwest and west-central Florida in the past 5 years. Tissue-blot nucleic acid hybridization assays were developed for simple, high-throughput detection of these three viruses as well as Cucurbit yellow stunting disorder virus (CYSDV), which was first reported in Florida in 2008. To determine virus distribution within plants, we collected 80 entire plants just before or during the harvest period in a systematic sample, 20 each on 11 April, 18 April, 26 April, and 3 May 2007, from a fruiting commercial watermelon field near Immokalee, FL showing symptoms of infection by SqVYV, CuLCrV, and PRSV-W and, possibly, CYSDV. This was followed by a sampling of five plants collected at harvest showing symptoms of virus infection on 11 October 2007 in a different commercial planting located in Duette, FL. Tissue prints were made from cross sections of watermelon plants from the crowns through the tips at 0.6-m intervals on nylon membranes and nucleic acid hybridization assays were used for virus detection. Results from testing crown tissue showed that SqVYV, CuLCrV, and PRSV-W were present in ≈37, 44, and 54%, respectively, of the 80 plants collected over the four sampling dates from the first field. For individual vines diagnosed with SqVYV, the distribution of SqVYV in vine tissue decreased proportionately with distance from the crown. The probability of detecting SqVYV was 70% at the base of the vine compared with 23% at the tip of the vine. In contrast, CuLCrV tended to be more evenly distributed throughout the plant, with ≈10% higher probability of detection at the growing tip relative to the crown of the plant. The distribution of PRSV-W resembled that of SqVYV but with ≈20% higher probability of detection at the tip of the vine. Similar trends were detected in the smaller sampling; however, CYSDV was also detected in three of the plants. Overall, the results indicated that SqVYV and PRSV-W were distributed differently than CuLCrV in watermelon plants, and this difference has implications on how samples should be collected and may affect vector acquisition and transmission of these viruses.