Tactics for management of thrips (Thysanoptera: Thripidae) and tomato spotted wilt virus in tomato

Four studies were conducted in Georgia during spring 1999, 2000, 2001, and 2002 to evaluate various management tactics for reducing thrips and thrips-vectored tomato spotted wilt virus (TSWV) in tomato and their interactions relative to fruit yield. Populations of thrips vectors of TSWV, Frankliniella occidentalis (Pergande) and Frankliniella fusca (Hinds), were determined using flower and sticky trap samples. The management practices evaluated were host plant resistance, insecticide treatments, and silver or metallic reflective mulch. Averaged over all tests, the TSWV-resistant tomato 'BHN444' on silver mulch treatment had the largest effect in terms of reducing thrips and spotted wilt and increasing marketable yield. Of the insecticide treatments tested, the imidacloprid soil treatment followed by early applications of a thrips-effective foliar insecticide treatment provided significant increase in yield over other treatments. Tomato yield was negatively correlated with the number of F. fusca and percentage of TSWV incidence. F. occidentalis per blossom was positively correlated with percentage of TSWV incidence, but not with yield. No significant interactions were observed between cultivar reflective mulch main plot treatments and insecticide subplot treatments; thus, treatment seemed to be additive in reducing the economic impact of thrips-vectored TSWV. Control tactics that manage thrips early in the growing season significantly increased tomato yield in years when the incidence of TSWV was high (>17%).     

Genetic mapping of the Tsw locus for resistance to the Tospovirus Tomato spotted wilt virus in Capsicum spp. and its relationship to the Sw-5 gene for resistance to the same pathogen in tomato

The Tsw gene conferring dominant resistance to the Tospovirus Tomato spotted wilt virus (TSWV) in Capsicum spp. has been tagged with a random amplified polymorphic DNA marker and mapped to the distal portion of chromosome 10. No mapped homologues of Sw-5, a phenotypically similar dominant TSWV resistance gene in tomato, map to this region in C. annuum, although a number of Sw-5 homologues are found at corresponding positions in pepper and tomato. The relationship between Tsw and Sw-5 was also examined through genetic studies of TSWV. The capacity of TSWV-A to overcome the Tsw gene in pepper and the Sw-5 gene in tomato maps to different TSWV genome segments. Therefore, despite phenotypic and genetic similarities of resistance in tomato and pepper, we infer that distinct viral gene products control the outcome of infection in plants carrying Sw-5 and Tsw, and that these loci do not appear to share a recent common evolutionary ancestor.     

Transmission of Tomato spotted wilt virus isolates able and unable to overcome tomato or pepper resistance by its vector Frankliniella occidentalis

Tomato spotted wilt virus (TSWV) causes serious diseases of many economically important crops. Disease control has been achieved by breeding tomato and pepper cultivars with the resistance genes Sw‐5 and Tsw, respectively. However, TSWV isolates overcoming these genetic resistances have appeared in several countries. To evaluate the risk of spread of these resistance‐breaking isolates, we tested their ability of transmission by the main vector of TSWV, the thrips Frankliniella occidentalis. We compared the transmission rate by thrips of six TSWV isolates of different biotype (able or unable to overcome this resistance in pepper and tomato), and with divergent genotype (A and B). Our results indicate that the transmission rate was related to the amount of virus accumulated in thrips but not to virus accumulation in the source plants on which thrips acquired the virus. No correlation was found between transmission efficiency by thrips and the genotype or between transmission efficiency and the ability of overcoming both resistances. This result suggests that resistance‐breaking isolates have the same potential to be transmitted as the isolates unable to infect resistant tomato and pepper cultivars.     

Selection, biological properties and fitness of resistance-breaking strains of Tomato spotted wilt virus in pepper

In glasshouse tests, infective sap from plants infected with 17 different isolates of Tomato spotted wilt virus (TSWV) from four Australian states was inoculated to three Capsicum chinense accessions (PI 152225, PI 159236 and C00943) carrying single genes that confer hypersensitive resistance to TSWV. The normal response to inoculation was development of necrotic (hypersensitive) local lesions in inoculated leaves without systemic invasion, but 3/1386 infected plants also developed systemic susceptible reactions in addition to hypersensitive ones. Similarly when two isolates were inoculated to C. chinense backcross progeny plants, 1/72 developed systemic susceptible reactions in addition to localised hypersensitive ones. Using cultures from the four plants with susceptible reactions and following three to five further cycles of serial subculture in TSWV‐resistant C. chinense plants, four isolates were obtained that gave systemic susceptible type reactions in the three TSWV‐resistant accessions, and in TSWV‐resistant cultivated pepper (C. annuum). When three of these isolates were inoculated to tomato (Lycopersicon esculentum) breeding lines with single gene resistance to TSWV, resistance was not overcome. Similarly, none of the four isolates overcame partial resistance to TSWV in Lactuca virosa. When TSWV isolates were inoculated to tomato breeding lines carrying partial resistance from L. chilense, systemic infection developed which was sometimes followed by ‘recovery’. After four successive cycles of serial passage in susceptible cultivated pepper of a mixed culture of a resistance‐breaking isolate with the non resistance‐breaking isolate from which it came, the resistance‐breaking isolate remained competitive as both were still found. However, when the same resistance‐ breaking isolate was cultured alone, evidence of partial reversion to wild‐type behaviour was eventually obtained after five but not four cycles of long term serial subculture in susceptible pepper, as by then the culture had become a mixture of both types of strain. This work suggests that resistance‐breaking strains of TSWV that overcome single gene hypersensitive resistance in pepper are relatively stable. The findings have important implications for situations where resistant pepper cultivars are deployed widely in the field without taking other control measures as part of an integrated TSWV management strategy.     

Reflective mulch and acibenzolar-S-methyl treatments relative to thrips (Thysanoptera: Thripidae) and tomato spotted wilt virus incidence in tomato

Management of thrips-transmitted tomato spotted wilt (TSW) virus typically relies on tactics that either reduce the thrips vector numbers or change the plant's response to the virus to reduce economic loss. We attempted to quantify the interaction between two such tactics, reflective mulch and the plant activator acibenzolar-S-methyl (Actigard), respectively, on a TSW-susceptible tomato hybrid. A split plot experiment was conducted in 2009 and 2010 where main-plots were three types of plastic mulch (two metalized reflective vs. black) and subplots consisted of a range of plant defense activator applications. TSW pressure varied over year with 80% of untreated plants having TSW in 2009 where as <7% of plants was infected in 2010. No significant interaction between mulch and subplots was found relative to thrips and marketable yield in either year. In 2009, the seasonal average of Frankliniella fusca (Hinds) populations and incidence of TSW were significantly lower and yield significantly higher on both reflective mulches than on black mulch. Seasonal averages of thrips and fruit yield differed significantly among treatments of acibenzolar-S-methyl. However, there was a significant acibenzolar-S-methyl by mulch interaction relative to TSW incidence. In 2009, a minimum of acibenzolar-S-methyl at transplant plus foliar treatments at 10 and 20 d after transplant was required to significantly reduce TSW incidence compared with untreated plants before harvest. Under lower TSW pressure in 2010, average TSW incidence was significantly less in all plots treated with acibenzolar-S-methyl treated plots compared with the check. Acibenzolar-S-methyl treatments functioned better with the thrips reducing tactic, ultraviolet-reflective mulch. We propose that acibenzolar-S-methyl is less effective than metalized reflective mulch in reducing the incidence of TSW in tomato.     

Variation in thrips species composition in field crops and implications for tomato spotted wilt epidemiology in North Carolina

Thrips were surveyed in tomato spotted wilt-susceptible crops in five areas across North Carolina. Tomato, pepper, and tobacco plants in commercial fields were sampled and 30 species of thrips were collected over a 3-year period. The most common species overall was Frankliniella tritici (Fitch). The most common thrips species that are known to vector Tomato Spotted Wilt Virus (TSWV) were F. fusca (Hinds), and F. occidentalis (Pergande). Relatively low numbers of Thrips tabaci Lindeman, another reported vector, were collected. The spatial and temporal occurrence of vectors varied with sampling method, crop species, region of North Carolina, and localized areas within each region. In a laboratory experiment, no difference was detected between the ability of F. fusca and F. occidentalis to acquire and transmit a local isolate of TSWV. Based on vector efficiency and occurrence, F. fusca is considered the most important vector of TSWV in tobacco, whereas both F. fusca and F. occidentalis are important vectors of TSWV in tomato and pepper.     

Patterns of spread of Tomato spotted wilt virus in field crops of lettuce and pepper: spatial dynamics and validation of control measures

Patterns of spread of Tomato spotted wilt virus (TSWV) were examined in lettuce and pepper plantings into which thrips vectors spread the virus from external virus sources. These plantings were: 1) seven separate field trials into which TSWV ‘infector’ plants of tomato were introduced alongside or near to plantings of lettuce or pepper, and 2) three commercial lettuce plantings into which spread from nearby external infection sources was occurring naturally. The vector thrips species were Frankliniella occidentalis, F. schnitzel and Thrips tabaci, at least two of which were always present. Spatial data for plants with TSWV infection collected at different stages in the growing period were assessed by plotting gradients of infection, and using Spatial Analysis by Distance IndicEs (SADIE) and maps of spatial pattern. Despite the persistent nature of TSWV transmission by thrips vectors, in both lettuce and pepper plantings there was a steep decline in TSWV incidence with distance from external infection sources that were alongside them. The extent of clustering increased over time and was greatest closest to the source. The relationship between percentage infection and assessment date suggested that spread was predominantly monocyclic with only limited polycyclic spread. Development of isolated clusters of infected plants distant from TSWV sources within both crops was consistent with only limited polycyclic spread. Spread to lettuce was greater downwind than upwind of virus source, with magnitude and proximity of source determining the amount of spread. When 15 m wide fallow or non-host (cabbage) barriers separated TSWV sources from lettuce plantings, spread was slower and there was much less clustering with the latter. In commercial lettuce plantings, spread was favoured by TSWV movement within successive side-by-side plantings. The spatial data from the diverse scenarios examined enabled recommendations to be made over ‘safe’ planting distances between external infection sources of different magnitudes and susceptible crops that were short-lived (e.g. lettuce) or long-lived (e.g. pepper). They also helped validate the inclusion of isolation and ‘safe’ planting distances, planting upwind, prompt removal of virus sources, avoidance of side-by-side plantings, and deploying intervening non-host barrier crops as control measures within an integrated disease management strategy for TSWV in field vegetable crops.     

Selection of tomato plants resistant to a local Polish isolate of tomato spotted wilt virus (TSWV)

We found that the Sw-5 gene confers resistance to one of the Polish isolates of tomato spotted wilt virus (TSWV). A series of tomato breeding accessions was analysed along with standards of resistance and susceptibility to TSWV. The presence of the Sw-5 gene was determined using the available PCR marker. Subsequently plants from these accessions were grown in the presence of the TSWV isolate from Poland. Some of them developed severe symptoms of the TSWV disease. Expression of the virus proteins was also assayed in tissues of the investigated plants. We found general agreement between either lack or presence of the disease symptoms, virus proteins and resistance gene. Some observed discrepancies of these data are also discussed. Our results indicate that marker-assisted selection can be used for breeding of the TSWV-resistant tomato in Poland.     

Occurrence of Different Tospoviruses in Vegetable Crops in Argentina

Significant damage on vegetable crops by tospoviruses had occurred sporadically in Argentina in the past but since 1994, severe outbreaks have been recorded every year. The crops that have been most affected, tomato, lettuce, and pepper, were surveyed in the provinces of Mendoza and Buenos Aires in 1994–95 and 1995–96. A few weeds and miscellaneous crops were also collected. A total of 543 samples showing symptoms typical for tospoviruses were analysed by double-antibody sandwich-enzyme-linked immunosorbent assay with polyclonal antibodies to groundnut ringspot tospovirus (GRSV), impatiens necrotic spot tospovirus (INSV) and tomato spotted wilt tospovirus (TSWV). The 339 samples collected in 1995–96 were also assayed for tomato chlorotic spot tospovirus (TCSV). In addition, the incidence of tospoviruses in tomato crops was assessed in 41 farms representing 310 ha and 10 cultivars. GRSV was identified in 222 samples (40.8%), TSWV in 194 samples (32.7%), TCSV in 50 samples (14.7%), INSV was not detected and 77 samples did not react with the antisera used. TSWV was found to prevail in Buenos Aires and GRSV in Mendoza. Mixed infections were not found in this survey. In tomato crops the mean incidence of tospoviruses was 33%. These results show that the disease formerly assigned to TSWV, is caused by at least three tospoviruses.     

Integrated management tactics for Frankliniella thrips (Thysanoptera: Thripidae) in field-grown pepper

In a 2-yr study, the impacts of different plastic soil mulches, insecticides, and predator releases on Frankliniella thrips and their natural enemies were investigated in field-grown peppers. Ultraviolet light (UV)-reflective mulch significantly reduced early season abundance of adult thrips compared with standard black plastic mulch. This difference diminished as the growing seasons progressed. Late season abundance of thrips larvae was higher in UV reflective mulch compared with black mulch plots. The abundance of the predator Orius insidiosus (Say) was significantly lower in UV-reflective mulch compared with black mulch treatments. Infection of plants with tomato spotted wilt virus, a pathogen vectored by Frankliniella occidentalis (Pergande), was <6%. In the year with the higher disease incidence (2000), UV-reflective mulch plots had significantly less disease (1.9%) compared with black mulch plots (4.4%). Yield was significantly higher in UV-reflective mulch (24,529 kg/ha) compared with black mulch (15,315 kg/ha) during this year. Effects of insecticides varied with species of thrips. Spinosad reduced abundance of F. occidentalis, but not Frankliniella tritici. In contrast, esfenvalerate and acephate reduced numbers of F. tritici and Frankliniella bispinosa, but resulted in higher populations of F. occidentalis. Spinosad was the least disruptive insecticide to populations of O. insidiosus. Releases of O. insidiosus and Geocoris punctipes (Say) reduced populations of thrips immediately after releases; naturally occurring predators probably provided late season control of thrips. Our results suggest that UV-reflective mulch, combined with early season applications of spinosad, can effectively reduce abundance of thrips in field-grown pepper.     

A CAPS marker to assist selection of tomato spotted wilt virus (TSWV) resistance in pepper.

The hypersensitive resistance to tomato spotted wilt virus (TSWV) in pepper is determined by a single dominant gene (resistant allele: Tsw) in several Capsicum chinense genotypes. In order to facilitate the selection for this resistance, four RAPD (among 250 10-mer primers tested) were found linked to the Tsw locus using the bulked segregant analysis and 153 F2 individuals. A close RAPD marker was converted into a codominant cleaved amplified polymorphic sequence (CAPS) using specific PCR primers and restriction enzymes. This CAPS marker is tightly linked to Tsw (0.9 +/- 0.6 cM) and is helpful for marker-assisted selection in a wide range of genetic intercrosses.     

Use of Pseudomonas fluorescens-based formulations for management of tomato spotted wilt virus (TSWV) and enhanced yield in tomato

Strains of Pseudomonas fluorescens were investigated for biocontrol efficacy against tomato spotted wilt virus (TSWV) in tomato both alone and in mixtures. P. fluorescens strains applied to seed, soil and foliage or as a seedling dip significantly reduced TSWV, with a concomitant increase in growth promotion in both the glasshouse and field. Two native strains (CoP-1 and CoT-1) and one foreign strain (CHAO) reduced TSWV. In P. fluorescens-treated tomato plants, increased activity of polyphenol oxidase, β-1,3-glucanase and chitinase was observed, and induction of chitinase was confirmed by western blot analysis. Induction of new protein (18 kDa) detected by SDS-PAGE in P. fluorescens-treated tomato plants was not found in healthy and P. fluorescens-untreated virus inoculated control plants. Indirect ELISA clearly showed a reduction in viral antigen concentration in P. fluorescens-treated tomato plants corresponding to reduced disease ratings. All the P. fluorescens-treated tomato plants also showed enhanced growth and yield compared to control plants. Hence, plant growth promoting rhizobacteria (PGPR) could play a major role in reducing TSWV and increasing yield in tomato plants.     

Long-term resistance to tomato spotted wilt virus in transgenic tobacco cultivars expressing the viral nucleoprotein gene: greenhouse and field tests

We examined the resistance phenotype of a large number of transgenic tobacco plants originating from 12 commercial (Nicotiana tabacum) cultivars expressing the sense form of the nucleoprotein (N) gene of L3, a Bulgarian isolate of tomato spotted wilt virus (TSWV). The analysis revealed that transgenic plants are completely protected against the homologous L3 isolate of TSWV irrespective of whether or not they contain detectable levels of translational product. The effectiveness of protection against the virus was investigated upon mechanical inoculation under greenhouse conditions and in field trials. Non-segregating resistant lines were selected and the inheritance of the resistance to TSWV was analysed in successive generations (R₃–R₆). Extensive tests under controlled conditions and two-year field trials proved that the resistance to TSWV is stable in different environments and is a stably inherited trait.     

Population dynamics of Frankliniella spp. and tomato spotted wilt incidence as influenced by cultural management tactics in tomato

We investigated the effects of ultraviolet (UV)-reflective mulch and two rates of nitrogen fertilization on populations of Frankliniella spp. thrips and on the incidence of tomato spotted wilt in field-grown tomato in northern Florida. The higher of the two soil nitrogen fertilizer treatments significantly increased populations of Frankliniella occidentalis (Pergande), whereas mulch reflectance significantly decreased populations of F. occidentalis. Populations of Frankliniella tritici (Fitch) were decreased only by UV-reflective mulch. Decreased thrips populations in UV-reflective mulch plots were probably due to disruptions in host-finding behavior. Increased thrips populations in tomatoes treated with the higher nitrogen fertilization rate were probably due in part to increased nutrients available in flowers. Incidence of tomato spotted wilt was significantly decreased in tomatoes grown on UV-reflective mulch, whereas disease incidence was significantly greater in increased nitrogen-fertilized plots. This research reveals that cultural practices resulted in up to 45% reduction in the numbers of vector and nonvector species of flower thrips and up to 50% reduction in tomato spotted wilt.     

Seasonal dispersal patterns of Frankliniella fusca (Thysanoptera: Thripidae) and tomato spotted wilt virus occurrence in central and eastern North Carolina

The seasonal abundance and temporal pattern of Frankliniella fusca Hinds dispersal were monitored from 1996 to 2000 at 12 locations in central and eastern North Carolina. The predominant vector species of tomato spotted wilt virus (TSWV) captured across all locations was F. fusca (98%). The temporal patterns of F. fusca dispersal observed during spring seasons varied among locations in all years except 2000. Regression analysis estimated that times of first flight in the spring seasons varied among locations, whereas flight duration intervals were similar. Temporal patterns of F. fusca captured varied significantly between aerial traps placed 0.1 and 1.0 m above the soil surface. Fewer total thrips were captured at 0.1 m, although thrips dispersal occurred earlier and over a greater time interval compared with 1.0-m traps. Temporal patterns of TSWV occurrence differed among locations in the spring seasons of 1999 and 2000, whereas patterns of virus occurrence were similar during the fall seasons. Patterns of F. filsca dispersal and subsequent TSWV occurrence were synchronous at locations in 1999 and 2000 where the greatest number of TSWV lesions was recorded. Knowledge of the temporal patterns of F. fiasca dispersal and TSWV occurrence may be a useful indicator for describing the time when susceptible crops are at highest risk of TSWV infection.     

Modified expression of alternative oxidase in transgenic tomato and petunia affects the level of <Emphasis Type="Italic">tomato spotted wilt virus</Emphasis> resistance

Background

Tomato spotted wilt virus (TSWV) has a very wide host range, and is transmitted in a persistent manner by several species of thrips. These characteristics make this virus difficult to control. We show here that the over-expression of the mitochondrial alternative oxidase (AOX) in tomato and petunia is related to TSWV resistance.

Results

The open reading frame and full-length sequence of the tomato AOX gene LeAox1au were cloned and introduced into tomato 'Healani' and petunia 'Sheer Madness' using Agrobacterium-mediated transformation. Highly expressed AOX transgenic tomato and petunia plants were selfed and transgenic R1 seedlings from 10 tomato lines and 12 petunia lines were used for bioassay. For each assayed line, 22 to 32 tomato R1 progeny in three replications and 39 to 128 petunia progeny in 13 replications were challenged with TSWV. Enzyme-Linked Immunosorbent Assays showed that the TSWV levels in transgenic tomato line FKT4-1 was significantly lower than that of wild-type controls after challenge with TSWV. In addition, transgenic petunia line FKP10 showed significantly less lesion number and smaller lesion size than non-transgenic controls after inoculation by TSWV.

Conclusion

In all assayed transgenic tomato lines, a higher percentage of transgenic progeny had lower TSWV levels than non-transgenic plants after challenge with TSWV, and the significantly increased resistant levels of tomato and petunia lines identified in this study indicate that altered expression levels of AOX in tomato and petunia can affect the levels of TSWV resistance.

     

Midgut infection by tomato spotted wilt virus and vector incompetence of Frankliniella tritici

Abstract:  The mechanism leading to vector competence of thrips species to transmit tomato spotted wilt virus (TSWV) is not well characterized. We investigated the interaction of TSWV and the non-vector species Frankliniella tritici . A monoclonal antibody to the non-structural protein (NSs) of the TSWV was used to detect TSWV replication within the thrips by immunofluorescence microscopy and enzyme-linked immonosorbent assay (ELISA). TSWV was acquired by F. tritici , replicated and moved within the alimentary canal of F. tritici similar to a known vector of TSWV, Frankliniella occidentalis . However, virus was not found in the salivary glands of F. tritici , which is a prerequisite to virus transmission. Thus, movement to the salivary glands may determine vector incompetence of F. tritici .     

Impact of early-season thrips management on reducing the risks of spotted wilt virus and suppressing aphid populations in Flue-cured tobacco

The influence of tray drench (TD) treatments, with and without foliar applications of the plant activator acibenzolar-S-methyl (Actigard), was examined in replicated field plots in 2000--2002. TD treatments of Actigard, imidacloprid (Admire), and these two products combined had little effect on seasonal mean thrips populations; however, thrips densities were lower in the Admire-treated plots at 4 and 5 wk after transplanting. Actigard and Admire TD treatments significantly reduced the seasonal incidence of tomato spotted wilt virus (TSWV) symptomatic plants in 2 yr in the study. The combination of both products was better in reducing TSWV than Actigard alone. Three early-season foliar sprays of Actigard had no effect on thrips population densities, but they did reduce TSWV incidence. The tobacco thrips, Frankliniella fusca (Hinds), comprised 92-95% of the thrips complex each year. Other thrips collected on tobacco foliage at very low densities included Haplothrips spp., Chirothrips spp., Limothrips cerealium (Haliday), other Frankliniella spp. and other unidentified species. Using nonstructural TSWV protein enzyme-linked immunosorbent assay, 1.5-2.3% of the F. fusca tested positive for nonstructural TSWV protein. Cured yields were higher in the TD treatments and the Actigard foliar treatments in the years with high TSWV in the untreated plots. The TD treatments and foliar Actigard had little impact on plant height or grade index; however, TD treatments with Admire had low tobacco aphid, Myzus nicotianae Blackman, populations through 10 wk after transplanting. The early-season Actigard and Admire treatment options are management decisions that can effectively reduce the risks of TSWV incidence in flue-cured tobacco.