A Review on Ecology of Interactions in Soybean Vein Necrosis Orthotospovirus (SVNV): Plants, Vectors, Virus Dispersal and Management Perspectives

Abstract

Soybean vein necrosis orthotospovirus (SVNV, Genus: Orthotospovirus, Family: Tospoviridae, Order Bunyavirales) is a vector and seed transmitted virus that infects soybean in different countries around the world. The purpose of this review paper was to provide information about SVNV, its geographic dispersal, vectors, disease transmission mode, alternative host plants, diagnostic tools and management. SVNV is a negative-sense single-stranded RNA virus reported in all soybean growing states in the USA, Egypt and Canada. SVNV can replicate in plants belonging to six different families, including the Leguminosae member mung bean, which is a major component of the diet of poor people of Asia. The most efficient and abundant SVNV vector species is Neohydatothrips variabilis (Beach.) (Sericothripinae: Thripidae). Five other insect species have the potential to transmit the virus, but their rate of transmission is very low. In addition to leaf necrosis, this virus can decrease seed oil content by 0.1% that may lead to a decrease in quality of SVNV infected seed in oilseed markets. In fact, in the infected seeds the quantity of the undesirable linolenic acid, a polyunsaturated fatty acid is increased. Broad presence of SVNV in all soybean growing regions points to the need to manage vector and virus. However, research is needed to determine various management options for the virus and vector including breeding for genetic resistance.

1. Introduction

Soybean is one of the most valuable oil seed, food, forage, biodiesel, feed, and leguminous nitrogen fixer crop which improves soil structure through nodule formation, nitrogen fixation and enhances farmer income along with multiple other benefits [1, 2, 3, 4]. Soybean is the second most important broad acre agricultural crop in the US providing high cash benefits to farmers [5]. Soybean was first introduced in the US for agricultural usage as a forage crop in 1804 [6], probably as part of an interchange of seeds between France and US. However, there is some evidence from Georgia which documents soybean cultivation in 1765. Since 1940, the area under soybean cultivation increased so much that it is now mainly used as an oil seed crop. The expansion of soybean cultivation increased from about 2.7 billion bushels in 2000 to 4.39 billion bushels in 2017 in the US [7]. Brazil, US, and Argentina dominate soybean production around the world [8]. Soybean production has doubled during the last decade because of the increased income benefits to farmers and also because of the availability and diffusion of transgenic soybeans which are glyphosate resistant (first developed in 1998) [9, 10].

Soybean is affected by a plethora of diseases caused by bacteria, fungi and viruses as well as by pests such as insects and mites [11, 12]. The effect of diseases and pests on plants results in the reduction in soybean yield. For example, during 2014, the estimated loss due to diseases was 113 million bushels in 28 states in the US. Of this, losses caused by viruses were 11.6 million bushel [13, 14]. Forty-six viruses are known to infect soybeans [14], and among them eight are economically important viz., alfalfa mosaic virus (AMV), bean pod mottle virus (BPMV), peanut mottle virus (PeMoV), peanut stunt virus (PSV), soybean dwarf virus (SbDV), soybean mosaic virus (SMV), soybean vein necrosis virus (SVNV) and tobacco ringspot virus (TRSV) [13, 15].

2. Species soybean vein necrosis virus (tospoviridae: bunyavirales), history and dispersal in different continents of world

In 2008, soybean vein necrosis orthotospovirus (SVNV) was first reported in Tennessee (US). To date, 22 US states have reported the virus presence [16, 17, 18, 19, 20], and the incidence of soybean vein necrosis disease in some states has been very high. For instance, in a 3-year survey conducted in the mid-west and mid-south US, it was reported that SVNV was present in 49/50 fields [21]. While this survey highlighted one of the most extreme cases of SVNV presence, in the United States the percent incidence ranged between 10 and 80 depending upon the plant stage and geographic areas. In 2012, the virus was also reported in Canada [22]. The genetic diversity of SVNV was studied from samples taken from different states and showed low variability. In 2013, a comparison of the nucleocapsid protein (NP) coding sequence of SVNV isolates collected from different states was done and it was found that it had 98–100% similarity [16]. At that time, it was proposed that the virus was new and might have been introduced into the US or recently might have been moved to soybeans from other plant hosts [16]. The spread of SVNV is not limited to North America, in fact in 2017, it was reported in Egypt (Middle East) [23] where its incidence was about 67%.

Interestingly, SVNV can spread through seed, an unusual feature for a tospovirus [24], and the US is one of the largest soybean exporters, making seed transmission a concern to importing countries. Until now it is speculated that due to transmission by seed and global soybean trade, seed may be a major source of virus transmission to the entire world [24]. This is because Neohydatothrips variabilis (Beach) and other secondary vectors, although dominant in Middle East and North America, are not abundant in other parts of the world such as Asia ([23, 24, 25, 26]; Figure 1). Furthermore, it is unknown whether the virus is indigenous in importer countries because soybean has an Asian origin, so the disease may already be present in those countries but may have never been reported. Soybean vein necrosis disease symptoms are similar to many others caused by pathogens such as Cercospora and by other plant stresses, making its identification a challenge. A comprehensive survey of SVNV and its vectors in different countries is also missing. Until now Frankliniella fusca (Hinds), N. variabilis (Beach) and Frankliniella tritici (Fitch) have been found to be vectors of SVNV in the US [16, 32, 33, 34] but in Egypt Megalurothrips sjostedii (Trybom), N. variabilis (Beach), F. occidentalis (Pergande) and Caliothrips phaseoli (Hood) transmitted SVNV under experimental conditions [23].

Figure 1.

World map showing thrips species distribution and soybean vein necrosis virus (SVNV) presence in different countries [23, 27, 28, 29, 30, 31].

2.1 Symptoms related to infection

Infection by SVNV in soybean is characterized by necrosis of the veins as well as interveinal necrosis, followed by chlorosis of nearby leaf parenchyma [16, 35] (Figure 2). In 2013, a clear link between symptomology and virus association was described in soybean, which was confirmed later in various studies [16, 35], but some authors also found non-symptomatic SVNV positive soybeans plants [24], as well as an Asteraceae member, Dendranthema grandiflorum, which was virus positive using PCR [21, 35].

Figure 2.

Symptoms related to infection. a) Uninfected plant leaf. b) Symptomatic plant inoculated with SVNV through mechanical inoculation performed with a syringe. c) SVNV symptomatic plants infected via thripsN. variabilis transmission.

SVNV infection in soybean significantly reduces the oil content and may reduce the germination percentage, 100 seed weight (g), protein content percentage, and fiber content percentage [17]. An experiment was conducted to determine the seed transmission in discolored and damaged seeds, It showed that the virus was seed transmitted [24]. Another study conducted on mixed infection of SVNV and BPMV showed that both viruses can be present together as a mixed infection [25]. The seeds of BPMV infected soybean plants were also discolored. Interestingly BPMV is also seed transmitted [36]. It may be possible that both viruses used the same path to invade the seeds either through the developing embryo or any other route; however, research is needed in this context.

However, other studies conducted on the effect of SVNV on soybean yield determined that SVNV does not decrease the yield, but seed quality was affected [37]. Oil concentration was decreased by 0.1% with SVNV infection and linolenic acid, linoleic acid and stearic acid were increased [37]. This means that SVNV infection may result in lower marketability of soybean in high premium markets. In the oil market, a higher price is paid for seed which has lower linolenic acid and higher oleic acid. Bad quality seeds receive lower prices [17].

2.2 Alternative host range plants and their role as inoculum reservoirs

Weeds provide a valuable natural means of virus survival when the soybean is not present. Alternative host plant studies of SVNV showed that the virus can infect chrysanthemum D. grandiflorum (Asteraceae), ivy-leaved morning glory Ipomea hederacea Jacq (Convolvulaceae), field pumpkin Cucurbita pepo (Cucurbitaceae), soybean Glycine max (Leguminosae), cowpea Vigna unguiculata (Leguminosae), mung bean Vigna radiata (Leguminosae), benthamiana Nicotiana benthamiana (Solanaceae), wild tobacco Nicotiana tabacum (Solanaceae), tobacco Nicotiana glutinosa (Solanaceae) in the US [16]. However, in Egypt, ivy morning glory Convolvulus arvensis L. Ipomea hederacea Jacq (Convolvulaceae), soybean G. max. (Leguminosae) pulses Lupinus sativum (Leguminosae), mung beans Vigna radiate (Leguminosae), cheeseweed Malva parviflora L. Portulaca oleraceae (Portulaceae), benthamiana N. benthamiana (Solanaceae), tobacco N. tabacum (Solanaceae) are reported to serve as alternative hosts of SVNV [23]. Kudzu in the southern US States is a known overwintering host plant for the vector and virus [38].

2.3 Seed transmission

Seed transmission of viruses is a very complex phenomenon and is dependent upon the ability of a virus to penetrate the developing embryo as well as various factors including the type of host plant, time of infection of virus, amount of virus and mixed infection (compatibility of two viruses to propagate in the host plant cells at the same time) [39, 40, 41, 42, 43]. More than one hundred plant viruses are transmitted through seed [39, 44, 45]. Viruses often become difficult to control when they are transmitted through seed as well [39]. Virus transfer to the seed embryo can take place through different routes such as direct transfer, transfer through pollen, and indirect embryo invasion [39, 46]. Losses due to seed borne viruses increase when a stock of seed harboring virus is planted in a field [47].

There are contrary reports on the transmission of SVNV through seeds. One study conducted by Hajimurad [35] reported that like other orthotopsoviruses SVNV cannot be transmitted through seed but later in a study by Groves [24] found seed transmission and confirmed it through nested PCR and RNAseq. Hajimurad [35] did not find seed transmissibility and found only 1/1955 seeds were positive via ELISA. Hajimurad [35] considered that this observation was an anomaly and that SVNV is not seed transmitted. Another observation in the study by Hajimurad [35] was that all the seeds from the infected mother plants were non-symptomatic (not discolored or mottled, instead the seeds looked normal). However, Groves [24] used mottled and discolored seeds. Recently, a Zhou and Tzanetakis [25] study pointed that the mixed infection of SVNV and BPMV may lead to systemic infection of SVNV in the soybean seedlings. It may be that mixed infection of SVNV with BPMV results in the ability of SVNV to be seed transmitted. This is because it is hypothesized that SVNV uses the movement protein of the BPMV for systemic infection [25]. Although Zhou and Tzanetakis [48] also documented non-seed transmissibility of SVNV in 600 seedlings of field grown SVNV, most of the hybrid soybean seeds commercially available are not seed borne disease free. In SVNV, the seed transmission rate reported by Groves [24] is 6% which is considerable [24]. Until now, no virus belonging to Bunyavirales and Tosopoviridae has been regarded as a seed transmitted virus except SVNV, which gives SVNV a unique position among Tospoviridae [24, 49]. If the seed-transmission of SVNV is real, it would create a big challenge in the commercialization of soybean seeds for planting, especially in countries where SVNV is not present yet.

The avenue of seed transmission opens points for discussion. For example, if SVNV cannot be transmitted through seeds then how did the virus reach to the Middle East? It must be either human movement or thrips long distance migration. Further research is needed to confirm the seed transmissibility or the migration routes.

2.4 Disease diagnostics

SVNV can be diagnosed with commercially available ELISA kits (for instance, Agdia, USA; & Life Technologies India). A Commercially available ELISA kits use synthesized antibodies. SVNV can also be diagnosed using PCR. Various authors have published PCR primers to amplify the different regions of the SVNV genome [16, 21, 50]. The variation in whole genome of SVNV can be measured through sequencing [21].

2.5 Molecular characterization of SVNV

SVNV is a spherical virus with a tri-segmented, negative-sense and ambisense, single-stranded RNA genome, containing 5 open reading frames [21, 51]. A schematic model of the SVNV virion based upon the literature [21, 24] is described in Figure 3. The diameter of the SVNV particles ranges between 80 and 100 nm [24]. The 3 genomic segments encode for putative proteins involved in virus replication, in plant defense evasion, virus movement in the plant, virus coating, and vector attachment [21]. The large segment (9010 nt) encodes for the putative RNA-dependent RNA polymerase which is necessary for virus replication [21, 52]. The method of replication has been described in detail for tomato spotted wilt orthotospovirus (TSWV), the type species of this genus [52]. The middle segment (M) is 4955 nt long, ambisense and has two ORFs. ORF 1 encodes for a putative non-structural movement protein (NSm). In TSWV infections, it is assumed that NSm makes tubular structures and is associated with plasmodesmata [53]. ORF 2 encodes for two putative glycoproteins, Gn and Gc, and their role in vector attachment has been well documented for TSWV [54]. Gn-Gc’s role in the F. occidentalis and TSWV interaction showed that membrane mediated endocytosis takes place through interaction of Gc glycoproteins with a 50 kda thrips protein, while the Gn glycoprotein interacts with a 94 kda thrips protein [55]. As a result of this process virions move from the point of attachment in the midgut to the hemocoel and eventually to muscle cells, and from there to the salivary glands. The putative role of Gn-Gc glycoprotein in TSWV attachment was corroborated when antibodies raised against these proteins stopped virus acquisition and transmission [51]. Research on SVNV and N. variabilis interaction showed that the virus was present in the principal salivary gland, tubular salivary gland and the efferent duct of infected thrips [34].

Figure 3.

Model of soybean vein necrosis virus particles showing different RNA segments (small, medium and large) coated by N proteins. Glycoprotein (Gn, Gc) spikes decorating the lipid bilayer. Molecules of RNA dependent RNA polymerase (RdRP) are enclosed in the virus particles.

The small segment (S) is ambisense, 2603 nt long, and contains two ORFs in opposite orientation [21]. ORF 1 encodes for the nonstructural silencing suppressor protein (NSs) [21]. This protein in TSWV binds dsRNA including miRNAs and siRNAs [52]. The role of NSs in SVNV and vector interaction still needs to be determined. ORF 2 encodes for the structural nucleocapsid protein (N) (31 kda) [21].

2.6 SVNV and vector association

Viruses belonging to Orthotospoviridae are persistent and propagative, which means that after entry into the vector insect, the virus multiplies in the insects and insects remain viruliferous for their entire life [54]. Studies conducted on the virus-vector relationship confirmed that N. variabilis (Beach.) is the primary vector of SVNV [16, 48]. The vector can acquire SVNV in the larval stages (L1 and L2) while only adults can transmit the virus [33], as for TSWV and other orthotopsoviruses. In addition, other thrips spp., F. fusca, F. tritici, F. occidentalis, C. phaseoli and Megalurothrips sjostedti can also transmit the virus [23]. In various experiments, transmission efficiency of vector thrips was evaluated. Keough, Han [32] reported that F. tritici, and F. fusca transmission percentage ranges between 5% and 35% respectively. Han, Nalam [34] proved that SVNV-NP was present in the principal salivary gland, efferent duct, tubular salivary gland, and midgut region in the adult viruliferous thrips F. tritici, F. fusca and N. variabilis through immuno-labeling against SVNV NP. The virus was not observed in uninfected thrips species. Acquisition of orthotospoviruses in thrips and further transmission to the salivary gland and dispersal to uninfected plants is a complex process and involves the virus’ ability to pass through the epithelial layer of the gut and then penetrate in the muscles and move through the tubular salivary gland to the efferent duct and the principal salivary glands [56, 57]. In F. occidentalis the contact between the salivary glands and the gut is closer in the first and second instar stage and later on when the insect grows to the pre-pupal and pupal stage the lack of contact is hypothesized to impede TSWV movement [57]. Although adult thrips can ingest the virus through feeding they cannot acquire the virus because the shift of virus to the salivary gland is not likely at the pupae and adult stages [57]. Also the tropism of virus replication shifts from the larval stages in the midgut epithelium to the salivary gland replication in the adult stages [57]. Moreover, the acquisition access time affects transmission of SVNV viz., transmission was higher after the 12 and 24 hrs acquisition access period (AAP) compared to 6 and 48 hrs AAP (Han, Nalam [34]).

Shazly [23] reported F. occidentalis, C. phaseoli and M. sjostdi can transmit SVNV with transmission efficiencies of 3.4, 6.7 and 3.3% respectively. However, major transmission of SVNV may be attributed mainly to N. variabilis as it was abundant in soybean crop in the US and Egypt compared to other species and due to higher transmission efficiency (70%) [23, 32, 33].

The host plant has a role in virus transmission. Shazly [23] stated that N. variabilis collected from cowpea can transfer virus 15% less efficiently than thrips collected from soybean. However, soybean thrips collected from mung bean had a transmission efficiency of 12.5%, while thrips collected from weeds such as Melilotus indicus and Melochia corchiforia can transfer virus with a transmission efficiency of 7.6 and 2.8% respectively [22].

There are complex theories regarding the thrips arrival, migration pattern, oviposition, hibernation and dispersion in the soybean fields (Figures 1–4) [21, 37]. According to Mueller, Higley [58] soybean thrips overwinter in southern states and annually migrate to northern US States (Figure 4). However, Anderson, Irizarry [17], and Zhou and Tzanetakis [48] postulated that due to the high number of thrips in soybean growing season in northern US states, soybean thrips may overwinter on perennial weeds and then during the early summer propagate on cover crops. Cover crops such as buckwheat and vegetables such as melon and winter pea can sustain SVNV and its vectors so they can act as reservoir to maintain inoculum from the overwintered insects and increase their number on the soybean crop [37, 59]. Irizarry, Elmore [59] proposed that alfalfa and other cover crops may act as the host of vectors before soybean planting in Wisconsin and Iowa. Zhou, Aboughanem-Sabanadzovic [38] suspected that Kudzu is a natural reservoir of SVNV and may be a natural shelter for the thrips during south to north movement every year because Kudzu is extensively present in the soybean growing region and interstate regions in the south.

Figure 4.

Migration, dispersal and winter diapause of soybean theories, hypothesis and results. Here the yellow colored states are north eastern states. Light blue states = southern US states, purple = mid-west states, green = western states. This schematic diagram is based upon the Mueller, Higley [58] and Irizarry, Elmore [59] paper. Here the green leaf plant in southern states depict the weeds on which thrips overwinter in south and in the summer they migrate to the soybean crop in the north east and mid-west. However, according to Bloomingdale, Irizarry [60] the thrips do not migrate in the winter and they over winter on the weeds in the mid-west. However, in the northern states due to low temperature and snow the thrips cannot survive under the field conditions.

Soybean is not thought to be the original host of SVNV because SVNV isolates collected in various locations on soybeans had more than 98% similarity [16]. However, comparison of the various isolates was done on the basis of the NP gene [16]. It would be interesting to look at the similarity of SVNV isolates in other genomic segments.

The SVNV transmission is complex because different vector species feed on different wild plants, weeds, cover crops and then eventually transfer the virus to the target crop. Furthermore, the virus can also be transferred to other regions along with infected seeds (Table 1) [24].

Country	Crop	Productive region/state	Species	Experimental conditions (Field or greenhouse experiment)	Identification technique	Plant hosts	Reference
USA	Alfalfa, buckwheat, and crimson clover, red clover	Iowa	N. variabilis	Greenhouse	Insects were slide mounted and identified to the level of species on the plants. Progeny formation was also observed.	Alfalfa, buckwheat, crimson clover and red clover are intermediate host of vector but buckwheat is inoculum reservoir as well. Buckwheat is open end host.	Zhou and Tzanetakis [16]
USA	Smart weed, cucumber, Crab apple, Viburnum, Willow, and Jackson	Iowa, Illinois, Maryland, Virginia	N. variabilis	Field collections	Field Collection from the plant hosts and then taxonomic identification after slide mounting until the species level.	Dead end host alternate hosts of vector but virus cannot replicate.	Hood [61]
USA	Hackberry, Elm and clover	Iowa	N. variabilis	Field conditions	Taxonomic identification	Presence of virus in these host plants have not been studied yet.	Beach (1896) [62]
USA	Cotton	Alabama, Arkansas, Georgia, Louisinia, Missisippi, Tennsesse	N. variabilis	Field conditions	Field capture, slide mounting and identification until the species level.	Dead end host, thrips can feed but the virus can not replicate in cotton.	Cook, Allen [63]
USA	Lima beans and Snap beans	Arkansas	N. variabilis	Field sampling	Field capture of thrips from the spring planted crop and slide preparation for identification of thrips species	Replication of virus in Lima beans and snap beans has not been studied so far.	Sweeden and McLeod [64]
USA	Horse radish	Illinois	N. variabilis	Field sampling	Field capture of thrips from the spring planted crop and slide preparation for identification of thrips species	Replication of virus in horse radish has not been studied so far.	Gerdes [65]
USA	Tomato	Virginia	N. variabilis	Field sampling	Population sampling	Virus can not replicate and thrips feed on it.	Nault, Speese Iii [66]
USA	Cotton, Peanut and Soybeans	Virginia	N. variabilis	Yellow sticky cards	Yellow sticky cards were placed in the fields and thrips were counted after one-week interval. Insects were identified to the level of species.	Soybean is target crop. Whereas the virus cannot replicate in peanut and cotton. Peanut and cotton are dead end host.	Samler [67]
USA	Peach orchards	Georgia	N. variabilis	Field collection	Fields were sprayed with insecticide and killing thrips fell down in big sheets of aluminuim and were preserved in ethanol 70%.	Replication of SVNV in peach has not been studied yet.	Yonce, Payne [68]
Hungary (Europe)	Soybeans		N. variabilis	Monitoring	Slide preparation	Target crop.	Ábrahám [69]
Egypt	Groundnut, soybeans, cowpea, mung beans, Phaseolus vulgaris (Beans), Egyptian bean, Medic, Yellow sweet clover, Granny vine, ivy morning glory, and Chocolate weed	Cairo	N. variabilis	Monitoring	Slide preparation and identification of species.	Virus can replicate in cowpea, mung beans, and ivy morning glory. The other plants are dead end host, thrips can replicate but virus presence has either not been studied or virus do not replicate	Shazly [23]
USA	Mist flower Conocinium coelestinum (L.) DC	North Florida	Frankliniella tritici	Field collection	Slide preparation and identification of species.	SVNV infection status as the inoculum reservoir is not confirmed	Chellemi, Funderburk [70]
USA	Flowering dog wood; Cornus florida L.	North Florida	F. tritici, Frankliniella occidentalis	Field collection	Slide preparation and identification of species.	SVNV infection status as the inoculum reservoir is not confirmed	Chellemi, Funderburk [70]
USA	Daisy fleabane Erigeron annus	North Florida	F. tritici	Field collection	Slide preparation and identification of species.	SVNV infection status as the inoculum reservoir is not confirmed	Chellemi, Funderburk [70]
USA	Dog fennel; Eupatorium eapillifolium (Lam.)	North Florida	F. fusca	Field collection	Slide preparation and identification of species.	SVNV infection status as the inoculum reservoir is not confirmed	Chellemi, Funderburk [70]
USA	Ivy morning glory	North Florida	F. tritici	Field collection	Slide preparation and identification of species.	SVNV infection status as the inoculum reservoir is not confirmed	Chellemi, Funderburk [70]
USA	Dwarf dandelion Krigina virginica	North Florida	F. titici, F. occidentalis	Field collection	Slide preparation and identification of species.	SVNV infection status as the inoculum reservoir is not confirmed	Chellemi, Funderburk [70]
USA	Lantana	North Florida	F. tritici, F. occidentalis	Field collection	Slide preparation and identification of species.	SVNV infection status as the inoculum reservoir is not confirmed	Chellemi, Funderburk [70]
USA	Hedgeprivet	North Florida	F. tritici, F. occidentalis, F. fusca	Field collection	Slide preparation and identification of species.	SVNV infection status as the inoculum reservoir is not confirmed	Chellemi, Funderburk [70]
USA	Blue toadflax	North Florida	F. tritici, F. occidentalis, F. fusca	Field collection	Slide preparation and identification of species.	SVNV infection status as the inoculum reservoir is not confirmed	Chellemi, Funderburk [70]
USA	Japanese Honeysuckle	North Florida	F. tritici, F. occidentalis	Field collection	Slide preparation and identification of species.	SVNV infection status as the inoculum reservoir is not confirmed	Chellemi, Funderburk [70]
USA	Water primrose	North Florida	F. tritici	Field collection	Slide preparation and identification of species.	SVNV infection status as the inoculum reservoir is not confirmed	Chellemi, Funderburk [70]
USA	Crab apple	North Florida	F. tritici, F. occidentalis	Field collection	Slide preparation and identification of species.	SVNV infection status as the inoculum reservoir is not confirmed	Chellemi, Funderburk [70]
USA	Creeping wood sorrel	North Florida	F. tritici, F. occidentalis	Field collection	Slide preparation and identification of species.	SVNV infection status as the inoculum reservoir is not confirmed	Chellemi, Funderburk [70]
USA	Yellow wood sorrel	North Florida	F. tritici, F. occidentalis, F. fusca	Field collection	Slide preparation and identification of species.	SVNV infection status as the inoculum reservoir is not confirmed	Chellemi, Funderburk [70]
USA	Parthenium weed	North Florida	F. tritici	Field collection	Slide preparation and identification of species.	SVNV infection status as the inoculum reservoir is not confirmed	Chellemi, Funderburk [70]
USA	Chickasaw pulum	North Florida	F. fusca, F. occidentalis	Field collection	Slide preparation and identification of species.	SVNV infection status as the inoculum reservoir is not confirmed	Chellemi, Funderburk [70]
USA	Wild cherry	North Florida	F. tritici, F. occidentalis	Field collection	Slide preparation and identification of species.	SVNV infection status as the inoculum reservoir is not confirmed	Chellemi, Funderburk [70]
USA	False dandelion	North Florida	F. tritici, F. occidentalis, F. fusca	Field collection	Slide preparation and identification of species.	SVNV infection status as the inoculum reservoir is not confirmed	Chellemi, Funderburk [70]
USA	Wild radish	North Florida	F. tritici, F. occidentalis, F. fusca	Field collection	Slide preparation and identification of species.	SVNV infection status as the inoculum reservoir is not confirmed	Chellemi, Funderburk [70]
USA	Rose	North Florida	F. tritici, F. occidentalis	Field collection	Slide preparation and identification of species.	SVNV infection status as the inoculum reservoir is not confirmed	Chellemi, Funderburk [70]
USA	Sand black berry	North Florida	F. tritici, F. occidentalis, F. fusca	Field collection	Slide preparation and identification of species.	SVNV infection status as the inoculum reservoir is not confirmed	Chellemi, Funderburk [70]
USA	Sassafras	North Florida	F. tritici	Field collection	Slide preparation and identification of species.	SVNV infection status as the inoculum reservoir is not confirmed	Chellemi, Funderburk [70]
USA	Arrow leaf sida	North Florida	F. tritici, F. occidentalis	Field collection	Slide preparation and identification of species.	SVNV infection status as the inoculum reservoir is not confirmed	Chellemi, Funderburk [70]
USA	Large hop clover	North Florida	F. tritici, F. occidentalis, F. fusca	Field collection	Slide preparation and identification of species.	SVNV infection status as the inoculum reservoir is not confirmed	Chellemi, Funderburk [70]
USA	Crimson clover	North Florida	F. tritici, F. occidentalis, F. fusca	Field collection	Slide preparation and identification of species.	SVNV infection status as the inoculum reservoir is not confirmed	Chellemi, Funderburk [70]
USA	Venus looking glass	North Florida	F. tritici, F. occidentalis, F. fusca	Field collection	Slide preparation and identification of species.	SVNV infection status as the inoculum reservoir is not confirmed	Chellemi, Funderburk [70]
USA	Moss verbena	North Florida	F. occidentalis, F. fusca	Field collection	Slide preparation and identification of species.	SVNV infection status as the inoculum reservoir is not confirmed	Chellemi, Funderburk [70]
USA	Brazilian verbena	North Florida	F. tritici, F. occidentalis	Field collection	Slide preparation and identification of species.	SVNV infection status as the inoculum reservoir is not confirmed	Chellemi, Funderburk [70]
USA	Common vetch	North Florida	F. tritici, F. occidentalis	Field collection	Slide preparation and identification of species.	SVNV infection status as the inoculum reservoir is not confirmed	Chellemi, Funderburk [70]
USA	Chinese wisteria	North Florida	F. tritici, F. occidentalis	Field collection	Slide preparation and identification of species.	SVNV infection status as the inoculum reservoir is not confirmed	Chellemi, Funderburk [70]

Table 1.

Alternative host plants of vector species in different parts of the world.

Seasonally, many plants can support the thrips vector species and virus in various parts of the world until the principal crop is planted. A detailed study is needed in the spring and winter to examine the alternative host plants of vector and virus reservoirs. The detailed list of possible alternative host plants of the vector and their confirmation as the virus reservoir in different parts of the world is described in Table 1.

2.7 Life cycle of N. variabilis (Beach)

Soybean thrips lay eggs inside the leaf parenchymatous tissues near the leaf vein using a barbed ovipositor (Figure 5). A female lays about 70–90 eggs in her lifetime. Eggs hatch into first instar larvae having red eyes. These first instar larvae are transparent and feed on the leaf. The second instar larvae are pale yellow. The first instar duration is 3–4 days. Second and third instar duration is 2–3 days each. Fourth 4th instar duration is 2–4 days. Total adult male duration is 17–19 days and female duration is 20–23 days. Virus infection increased female survival [62]. Males are haploid. The mode of asexual reproduction is Arrehenotoky unlike T. tabaci L. where the mode of reproduction is deuterotoky.

Figure 5.

Life cycle of N. variabilis. The colored photographs were taken through the Olympus microscope 5RTV with colored CCD camera attached.

2.8 Management of SVNV and vector

The importance of SVNV seems to be increasing. Several years ago, it was largely unknown, but recent studies have raised concerns about its severity. Management of seed and vector borne viruses requires complex knowledge of vector ecology, type of virus transmission (circulative, semi persistent, persistent), mode of virus introduction in the field (primary or secondary spread), the method of perception of the volatile compounds by insect sensillae, insect response to the plant released stressed volatile compounds, complex interaction between herbivores occupying same niche and threshold level of disease and vector as well [71]. Management considerations include:

1. The first step is always to start with clean seed. Planting damaged and discolored seeds may increase the chance of virus. Planting with mycorrhizae will increase plant vigor, canopy establishment, plant height, number and weight of nodules, number and weight of pods, total grain yield [72] and plant would be able to combat viruses and vector [72].

   Monitoring can provide an estimate of thrips types present on soybean and nearby crops. Monitoring can be done using the beating sheet method or counting the number of adult thrips on the upper most leaves and preserving the specimens in 70% ethanol. Irwin, Yeargan [73] demonstrated that N. variabilis were higher in number at uppermost leaves. So, for estimation of ETL (Economic Threshold Level) of thrips population, thrips should be sampled from upper most leaves. The suspected infected leaves can be sent to a disease diagnostic lab which can confirm the presence of SVNV. However, presence of thrips on plants does not mean that they are causing enough damage to justify the application of insecticides. Yellow sticky traps/blue sticky traps, yellow or black water traps can also help to determine the kind of thrips species present in soybean fields. Insect samples can be sent to taxonomists at USDA for species identification.

2. Irizarry [37], Shazly [23] and Zhou and Tzanetakis [16] found that soybean vein necrosis virus can propagate in crimson clover, tobacco, mung beans, alfalfa, chrysanthemum, ivy morning glory, squash, black eyed pea, blind weed, peas, cheese weed, common purslane and melon. Plantation of soybeans near weeds and alternative host of soybean vein necrosis virus may increase the inoculum of SVNV in soybean plants. Control of weeds may decrease the virus prevalence. Planting of glyphosate resistant seeds may suppress the weeds and hence can increase the yield through reduction in competition between soybean and weeds. However, weeds or host plants during the overwintering season should be rogued. Culling and removal of the infected reservoir plants and weeds may suppress the SVNV inoculum.

3. Moreover, the winter pea, red clover and ivy morning glory can sustain adults of thrips and immature. Since winter pea, red clover and ivy morning glory can sustain the virus and vector, avoiding plantation of these crops near soybean at least 15 m apart may help to reduce pest numbers.

4. Nature is rich with biocontrol agents which suppress the thrips population. Chrysopa larvae, Geocorus, Orius, predatory thrips, parasitic nematodes and predatory mites can suppress pest numbers [74, 75]. In our insect rearing facility, we observed high reductions in pest numbers, when Cucumeris mites were present. Cucumeris mites can be exploited to control vector numbers in field conditions.

5. Unlike other plant pathogens, orthotospoviruses are not spread by shearing or pruning. Hence pruning or cutting the infected parts of plants would not help to reduce inoculum.

6. Pesticides can be used against vectors for management of the vector population. However, increased application of insecticides may lead to insecticide resistance, as it has been already reported in F. occidentalis populations. Cyantriniliprole (Minceto Pro or any formulation) is quite effective in reducing thrips number [67].

7. In the case of N. variabilis we did not observe pupation in soil for P2 (pupae) and P1 (pre-pupae) stage. Vance [76] reported that soybean thrips under experimental conditions can pupate on leaves but in nature they pupate in the soil if it is available. We grew soybean thrips on plants and always found pre-pupae and pupal stage on leaves [62]. Some other thrips species do not move into soil and hence they can pupate on leaves, so we assume from our studies that fumigation of soil would not help reducing pest numbers however, this may help in greenhouse conditions to reduce F. tritici and F. fusca numbers.

8. For thrips control insecticide treated seeds, provide protection for about 40 days. Also, in northern US states thrips arrive in the month of July and hence symptoms appear in August. But in southern states thrips colonized soybean in May and symptoms were observed in June. This may point to the movement of the vector from South to North [77]. Losses are higher in southern states as compared to Northern US states, however research is still needed to understand comparative losses in southern and northern states. Irizarry [37] estimated losses in between soybean growing states but their studies did not compare infected and uninfected plants, but only compared less symptomatic and higher symptomatic plants due to lack of control plants. Still more studies are needed in field conditions to determine the impact of virus on yield and quality. Application of thiamethoxam, imidacloprid, acetamiprid, lambda cyhalothrin, & chlorpyrifos can provide effective control of thrips populations. In northern US states, thrips populations do not reach to higher numbers because of low temperature, rainfall, and overwintering period but in the south the population grows rapidly and hence pesticide applications may be required.

9. In the US, a high SVNV incidence in soybean crops was reported, and yield losses on full-season crops were marginal but in double-cropped beans the losses were substantial [17, 37]. Since planting takes place later, thrips colonized on normal cultivated soybeans shift at flowering stage to the double-cropped beans when the plants are often very small, only about 12–24 inches tall. Populations of thrips are very high on double-cropped beans and yield is remarkably decreased [17]. On double-cropped beans insecticide application along with yellow sticky card placement, and Cucumeris release may help to reduce the pest losses.

3. Future research suggestions

• Acibenzolar S methyl, or other organic compounds that like salicylic acid induce plant resistance. Application of this product can reduce bacterial and fungal diseases. Also, this will induce salicylic acid in plants which may reduce SVNV incidence through promotion of plant resistance through a phyto hormone pathway. However, all research related to Acibenzolar S methyl has been conducted with Acibenzolar S methyl and TSWV interaction but has not been done with soybean plants and SVNV. Further research on time of Acibenzolar S methyl application before thrips attack through spray may determine if induction of salicyclic acid can reduce SVNV.

• In TSWV and thrips interaction, Gn-Gc glycoproteins have a specific role in the receptor-mediated endocytosis and movement of virions from insect gut to the salivary gland. Although Han, Nalam [78] showed the virus presence in salivary gland of N. variabilis, movement of virus within vector has not been determined. Future Research on specific thrips protein which bind with Gn and Gc glycoproteins may help to understand putative role of these viral proteins in thrips cells.

• Non-Structural silencing suppressor proteins (NSs) in TSWV and thrips interaction are hypothesized to overcome thrips inner immune processes. Elucidating the putative role of SVNV NSs protein in N. variabilis may help to understand wide range of adaptability of virus to multiple vectors and increase in fitness of the thrips.

• In our experiments we found plant cultivars responded differently to vector colonization and hence virus titer was variable on different cultivars [62], similar results have been reported by Zhou et al., 2019. Possibly in nature there are certain processes involved which govern host plant resistance against vector virus. These mechanisms in relation to SVNV isolates may decrease SVNV incidence in farmer’s fields. However, SVNV resistant varieties may also be developed through strategizing against virus and vector.

• In our work on SVNV in Pakistan we found that symptomatic SVNV infected plants were present within one month after plantation of seed [62]. In US we did not find symptomatic plants until August while crop is planted in May [62]. This may be due to insecticide treated seeds, Thrips cannot colonize plants early in the season in US but in Pakistan herbicide resistant and insecticide treated seed is not available. Hence farmers and scientists use untreated seeds which may be reason behind higher disease incidence in Pakistan as compared to Northeastern US but studies regarding global warming and its relation to viral epidemics and insects’ abundance may help to better understand and forecast the disease incidence in future.

• The work on virus evolution would provide information about the origin of the virus. Up to the present, we have the characterization of SVNV from US and Egypt [62]. More information on sequence comparison may help to resolve this mystery of evolution of this virus. This is because soybean is native to Asia but now US, Brazil and Argentina dominate the world production, but since the virus can be transmitted through seed, may be this virus could have arrived along with seeds from Asia to US and inhabited here generation after generations until sequenced for first time in 2008 in Tennessee [21].

• Management of SVNV requires a broad knowledge of thrips natural history as well as knowledge of the biology of the virus inside the plant host and the vector. Until now research has been done on virus characterization and the vector/virus relationship, but research is needed to understand the resistance mechanisms in plants against SVNV. According to our research experiments we did not find any cultivar which is resistant to the virus although some varieties were less preferred and some were highly preferred by thrips resulting in lower and higher incidence of SVNV [62]. But soybean (G. max) was derived from Glycine soja about 9000 years ago. Interestingly, G. soja is still cultivated in Russia, Korea and East Asia (including China, India and Pakistan) since ancient times. It may be possible that these wild ancestors possess resistance against SVNV, as the case of Solanum peruvianum against TSWV. In this case the dominant or recessive resistant genes may be identified and virus incidence can be reduced through genetic engineering and utilization of gene silencing in plants.

• Various kinds of microbes induce resistance in plants against orthotospoviruses. One example is Pseudomonas fluorescens. P. fluorescens application to tomato induce polyphenoloxidase, B-1,3 glucanase, and chitinase. Plants growth and performance is enhanced, TSWV concentration is reduced (reference). However, the role of these microbes and SVNV has not been studied. May be in southern US states where SVNV incidence is high, application of these microbes before sowing may increase crop productivity and decrease SVNV. Further research in this field may explore opportunities of ecofriendly way of reducing disease incidence through enhancing planting vigor and promoting induced resistance.

• The diet of poor people in developing countries mostly consists of proteins derived from legumes. Mung beans, mash beans, & tofu are the food sources of the poor. Soybean vein necrosis virus decreases the oil content of seeds which decreases the profit margin of oil seed firms and hence the product become more expensive as well. The cost of production can be lowered through introduction of virus resistant cultivars and hence more high-quality food can be provided to poor of the world.

• Disruption of the binding of the virus to its vector through transgenic cultivar development has been a pursuit of IPM specialists against viruses and vectors. In TSWV and F. occidentalis interaction, Gn glycoproteins promotes virus penetration of the thrips epithelial cells by membrane mediated endocytosis. Gn rich transgenic soybeans can be developed and their response to virus transmissibility by the thrips vector may be monitored under lab conditions and then it can be used in the field for vector and virus management.

4. Conclusion

Soybean vein necrosis virus is an important seed and vector transmitted virus present in middle East, US and Canada. This virus can decrease the oil content percentage. SVNV can be transmitted through different species of thrips. Among them N. variabilis is an important vector. SVNV has also been reported in various species of weeds where it can over winter. In the US, Kudzu is an important interstate virus reservoir for migrating thrips. Although various species of thrips can transmit SVNV, the rate of transmission of N. variabilis is considerably higher. SVNV is a negative sense single stranded RNA virus that can replicate in thrips and plants. Management of SVNV must be strategized as the vector and virus colonization on double beans can lower plant yield. Hence monitoring of thrips population using yellow sticky cards, and application of new chemistry insecticides should be done on late planted soybeans to reduce the pest pressure on double cropped soybeans. Future research is needed to understand the mechanism of propagation of SVNV in plant seeds, development of resistant varieties, exploring the role of Gn rich transgenic soybeans, and of gene silencing, a method that could be used to control SVNV.

Acknowledgments

The authors would like to acknowledge the Fulbright grant for PhD studies of the first author. We also wish to acknowledge the Pennsylvania Soybean Board (PSB) for providing funds for graduate student research (PSB #199751).

Conflict of interest

The authors declare that there are no conflicts of interest.