Distribution, epidemiology and molecular variability of the begomovirus complexes associated with yellow vein mosaic disease of mesta in India

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.

Direct Foliar Application of dsRNA Derived From the Full-Length Gene of NSs of Groundnut Bud Necrosis Virus Limits Virus Accumulation and Symptom Expression

Groundnut bud necrosis virus (GBNV) is the most significant member of the genus Orthotospovirus occurring in the Indian subcontinent. There is hardly any effective measure to prevent GBNV in crop plants. In order to develop GBNV infection prevention procedure, we examined the effect of the direct foliar application of double-stranded RNA (dsRNA) derived from the full-length NSs gene (1,320 nucleotides) of GBNV. The bacterially expressed dsRNA to the non-structural (dsNSs) gene of GBNV was purified and delivered to plants as an aqueous suspension containing 0.01% Celite for evaluating its efficacy in preventing GBNV infection in systemic host, Nicotiana benthamiana as well as in local lesion and systemic host, cowpea cv. Pusa Komal (Vigna unguiculata). The dsNSs application and challenge-inoculation were conducted in three different combinations, where plants were challenge-inoculated with GBNV a day after, immediately, and a day before the application of dsNSs. N. benthamiana plants, which were not treated with dsRNA showed severe systemic wilting and death by 9-16 days post-inoculation (dpi). The non-treated cowpea plants exhibited many chlorotic and necrotic lesions on the cotyledonary leaves followed by systemic necrosis and death of the plants by 14-16 dpi. The dsNSs treated plants in all the combinations showed significant reduction of disease severity index in both N. benthamiana and cowpea. The treatment combination where the GBNV inoculation was conducted immediately after the dsNSs treatment was found to be the most effective treatment in preventing symptom expression. The viral RNA analysis by real time PCR also showed 20 and 12.5 fold reduction of GBNV in cowpea and N. benthamiana, respectively. Our results suggest that the foliar application of dsRNA derived from the full-length NSs gene of GBNV through Celite is successful in delivering long dsRNA leading to effective prevention of GBNV infection.

Natural occurrence of mesta yellow vein mosaic virus and DNA-satellites in ornamental sunflower (Helianthus spp.) in Pakistan

Weeds and ornamental plants serve as a reservoir for geminiviruses and contribute to their dissemination, genome recombination and/or satellite capture. Ornamental sunflower (Helianthus spp.) plants exhibiting mild leaf curl symptoms were subjected to begomovirus and DNA-satellites isolation. The full-length genome of the isolated begomovirus clone (Od1-A) showed 96.8% nucleotide (nt) sequence identity with mesta yellow vein mosaic virus (MeYVMV; accession no. FR772081) whereas, alphasatellite (Od1-a) and betasatellite (Od1-b) clones showed their highest nt sequence identities at 97.4% and 98.2% with ageratum enation alphasatellite (AEA; accession no. FR772085) and papaya leaf curl betasatellite (PaLCuB; accession. no. LN878112), respectively. The evolutionary relationships, average evolutionary divergence and the recombination events were also inferred. The MeYVMV exhibited 9.5% average evolutionary divergence and its CP and Rep had 9.3% and 12.2%, concomitantly; the alphasatellite and the betasatellite had 8.3% and 5.2%, respectively. The nt substitution rates (site^-1 year⁻¹) were found to be 6.983 × 10^-04 and 5.702 × 10^-05 in the CP and Rep of MeYVMV, respectively. The dN/dS ratio and the Tajima D value of MeYVMV CP demonstrated its possible role in host switching. The absolute quantification of the begomovirus demonstrated that mild symptoms might have a correlation with low virus titer. This is the first identification of MeYVMV and associated DNA-satellites from ornamental sunflower in Pakistan. The role of sequence divergence, recombination and importance of MeYVMV along with DNA-satellites in extending its host range is discussed.

Occurrence and variability of begomoviruses associated with bhendi yellow vein mosaic and okra enation leaf curl diseases in south-western India

Bhendi yellow vein mosaic disease (BYVMD) and Okra enation leaf curl disease (OELCuD) are common diseases of okra/bhendi [Abelmoschus esculentus (L.) Moench] affecting both pod yield and quality in the Indian subcontinent. BYVMD is caused by the infection of a begomovirus and associated betasatellite. In this study, we have made an attempt to investigate the diversity of begomoviral and the satellite sequences in okra samples showing BYVMD and OELCuD, by using a rapid PCR-based approach on 46 samples collected from 23 locations of Southern and Western India. We have also analyzed nine RCA-generated full-length begomoviral clones, some generated from the above samples displaying BYVMD and some OELCuD. By the PCR approach, we find the presence of begomovirus okra enation leaf curl virus (OELCuV) in most samples, irrespective of the disease being displayed (BYVMD or OELCuD). The nine apparently full-length sequences also show high identities with OELCuV and show instances of both intra-specific as well as intra-strainal recombination. We have also analyzed the begomoviral sequences associated with BYVMD and OELCuD from publicly available nucleotide sequence databases and show much higher sequence diversity amongst BYVMV, as compared to OELCuV. This is the first study which comprehensively demonstrates the presence of OELCuV in okra samples showing BYVMD and those showing OELCuD.

Detection of Mesta yellow vein mosaic virus (MeYVMV) in field samples by a loop-mediated isothermal amplification reaction

A loop-mediated isothermal amplification (LAMP) assay was optimized for the detection of Mesta yellow vein mosaic virus (MeYVMV) in diseased plants of mesta (Hibiscus sabdariffa L.& H. cannabinus L.). The LAMP assay was optimized using a set of six primers targeting the MeYVMV genome and could be completed in 30-60 min at 63 °C. The LAMP amplification results were visualized by adding 1 μl of hydroxy naphthol blue (HNB) dye in a 25 μl LAMP reaction mixture prior to amplification as well as by electrophoresis. The LAMP assay, which detected MeYVMV in a 10^-5-fold diluted total DNA, was more sensitive than the PCR assay (10^-4-fold dilution). The optimized LAMP assay was able to detect MeYVMV in different parts of the kenaf and roselle plants. Similarly, the optimized PCR assay was also capable of detecting MeYVMV in all the different parts of the kenaf plant but failed to detect the virus in the stem and flower buds of the roselle plant. Validation of the LAMP and LAMP with HNB dye assays revealed that the optimized reactions can be used successfully for the in-situ detection of MeYVMV in field samples and in virus quarantine programs. This is the first report of the detection of the begomovirus species, MeYVMV, in the mucilaginous plant species, kenaf and roselle, using a LAMP assay.

A recombinant Tobacco curly shoot virus causes leaf curl disease in tomato in a north-eastern state of India and has potentiality to trans-replicate a non-cognate betasatellite

Leaf curl disease is a serious constraint in tomato production throughout India. Several begomoviruses were reported from different parts of the country; however, identity of begomovirus associated with leaf curl disease in tomato in north-eastern states of India was obscured. In the present study, the complete genome of an isolate (To-Ag-1) of begomovirus was generated from a leaf curl sample collected from Tripura state. However, no DNA-B and betasatellite were detected in the field samples. The genome of To-Ag-1 isolate contained 2,755 nucleotides that shared 94.7 % sequence identity with Tobacco curly shoot virus (TbCSV) and 71.3-90.1 % sequence identity with the other tomato-infecting begomoviruses occurring in the Indian subcontinent. Several inter-specific recombination events among different tomato-infecting begomoviruses from India and intra-specific recombination among different isolates of TbCSV reported from China were observed in the genome of To-Ag-1 isolate. Agroinoculation of the virus alone produced leaf curl symptoms in tomato and Nicotiana benthamiana. However, co-inoculation with a non-cognate betasatellite, Croton yellow vein mosaic betasatellite (CroYVMB) with the TbCSV resulted in increased severity of the symptoms both in tomato and N. benthamiana. Systemic distribution of the TbCSV and CroYVMB was detected in the newly developed leaves of tobacco and tomato, which showed ability of TbCSV to trans-replicate CroYVMB. The present study for the first time provides evidence of occurrence of TbCSV in tomato in north-eastern region of India and showed increased virulence of TbCSV with a non-cognate betasatellite.

Natural association of two different betasatellites with Sweet potato leaf curl virus in wild morning glory (Ipomoea purpurea) in India

Wild morning glory (Ipomoea purpurea) was observed to be affected by leaf curl and yellow vein diseases during summer-rainy season of 2009 in New Delhi, India. The virus was experimentally transmitted through whitefly, Bemisia tabaci to I. purpurea that reproduced the two distinct symptoms. Sequence analysis of multiple full-length clones obtained through rolling circle amplification from the leaf curl and yellow vein samples showed 91.8-95.3% sequence identity with Sweet potato leaf curl virus (SPLCV) and the isolates were phylogenetically distinct from those reported from Brazil, China, Japan and USA. Interestingly, two different betasatellites, croton yellow vein mosaic betasatellite and papaya leaf curl betasatellite were found with SPLCV in leaf curl and yellow vein diseases of I. purpurea, respectively. This study is the first report of occurrence of SPLCV in wild morning glory in India. SPLCV was known to infect other species of morning glory; our study revealed that I. purpurea, a new species of morning glory was a natural host of SPLCV. To date, betasatellite associated with SPLCV in Ipomoea spp. is not known. Our study provides evidence of natural association of two different betasatellites with SPLCV in leaf curl and yellow vein diseases of I. purpurea.

Begomovirus research in India: a critical appraisal and the way ahead

Begomoviruses are a large group of whitefly-transmitted plant viruses containing single-stranded circular DNA encapsidated in geminate particles. They are responsible for significant yield losses in a wide variety of crops in India. Research on begomoviruses has focussed on the molecular characterization of the viruses, their phylogenetic analyses, infectivities on host plants, DNA replication, transgenic resistance, promoter analysis and development of virus-based gene silencing vectors. There have been a number of reports of satellite molecules associated with begomoviruses. This article aims to summarize the major developments in begomoviral research in India in the last approximately 15 years and identifies future areas that need more attention.

Production of nitric oxide in host-virus interaction: a case study with a compatible Begomovirus-Kenaf host-pathosystem

Nitric oxide (NO) plays a key role in plant diseases resistance. Here we have first time demonstrated that begomovirus infection in susceptible H. cannabinus plants, results in elevated NO and reactive nitrogen species production during early infection stage not only in infected leaf but also in root and shoot. Production of NO was further confirmed by oxyhemoglobin assay. Furthermore, we used Phenyl alanine ammonia lyase as marker of pathogenesis related enzyme. In addition evidence for protein tyrosine nitration during the early stage of viral infection clearly showed the involvement of nitrosative stress.