Tomato Spotted Wilt Virus is one of the worst plant pathogens and impacts more than 1,000 different species, some of which include major food crops like tomato, pepper, peanut, potato, and tobacco.
This disease spreads primarily by small thrips that act as the insect vector acquiring and furthering the disease. Yield loss by this disease globally has been of huge importance and a major threat.
Causal organism
The Tomato Spotted Wilt Virus belongs to the Orthotospovirus genus from the Tospoviridae family. With over 1,000 types of infected crops in different farming and gardening aspects, TSWV is highly prevalent and plays a very great role economically worldwide. Main infections are mostly done on tomatoes, peppers, potatoes, peanuts, lettuce, as well as the ornamental species, leading to severe losses during harvest time.
Classification and Taxonomy of Tomato Spotted Wilt Virus (TSWV)
TSWV falls in the kingdom Riboviria, phylum Kitrinoviricota, and the order Bunyavirales. It belongs to the family Tospoviridae and is classified under the genus Orthotospovirus, which other economically important viruses include Impatiens Necrotic Spot Virus (INSV). The genus name Orthotospovirus is derived from Tospovirus, which refers to the tomato spotted wilt disease first described in Australia during the early 1900s. Due to its wide range of hosts and adaptability to different environmental conditions, it is one of the most challenging plant pathogens to manage.
Morphology and Structure of Tomato Spotted Wilt Virus (TSWV)
TSWV is an enveloped virus that is quasi-spherical in shape and is one of the only few plant viruses having a lipid envelope, originating from the cell membrane of the host. The diameter is from 80 to 120 nm. Enveloped with these two glycoproteins Gn and Gc are in this lipid envelope. Inside the envelope, the viral genome is protected by nucleocapsid (N) proteins, which give it a complex structure known as the ribonucleoprotein complex (RNP). Because of its envelope, TSWV is structurally more complex than most plant viruses.
Genomic Organization of Tomato Spotted Wilt Virus (TSWV)
TSWV has a tripartite, single-stranded, negative-sense RNA genome, which is made up of three separate RNA segments known as Large, Medium, and Small. The Large (L) RNA segment encodes an RNA-dependent RNA polymerase (RdRp), which is necessary for virus replication. The Medium (M) RNA segment encodes glycoproteins (Gn and Gc) that enable thrips transmission. The Small (S) RNA segment encodes two proteins: the nucleocapsid (N) protein, which packages the viral RNA, and a non-structural protein (NS) that suppresses the host plant’s immune response.
Symptoms of Tomato Spotted Wilt Virus (TSWV)
Tomato Spotted Wilt Virus causes a wide variety of symptoms on infected plants affecting different parts like leaves, stems, flowers, and fruits. Symptoms vary from plant to plant based on host plant species, the growth stage at infection, environmental conditions, and virus strain. Infected plants may display mild to severe symptoms, leading to reduced yield, poor fruit quality, and, in extreme cases, death of the plant.
Symptoms on Leaves- The first visible symptoms of TSWV infection occur on the young leaves of infected plants. Initially, small dark brown to purple necrotic spots form on the leaf surface. Later, chlorosis may be present, followed by a characteristic bronzing or purpling effect, especially under direct sunlight. Vein necrosis may also occur in some leaves, where the veins turn dark brown or black. Infected leaves often curl down, giving the plant a wilting or drooping look. Gross necrosis with large areas of dead tissues can lead to defoliation or total loss of leaves from weakened plants, decreasing their ability to photosynthesize.
Symptoms on Stems and tips- The leaves of the infected plants have dark streaks and elongated brown-purple lesions, usually around the nodes and petioles. These streaks may eventually enlarge and result in stem splitting or cracking, thereby predisposing the plant to secondary infections by bacteria and fungi. The growing tips of the infected plants can exhibit symptoms like stunting, distortion, or complete dieback, which inhibits the formation of new leaves and branches. The growth of infected young seedlings also gets completely stunted, and they may even die before reaching the flowering stage.
Symptoms on Flowers- TSWV also affects the reproductive structures of plants, leading to visible symptoms on flowers. Infected flowers may have color breaking, with irregular streaks of white or necrotic (dead) tissue on the petals. Flowers may become deformed, twisted, or curled and fail to carry out their normal reproductive functions. In severe cases, flowers may wilt and drop prematurely, resulting in poor fruit sets and lower yields. This is highly destructive in horticultural tomatoes and peppers where high fruit yields are necessary.
Fruit Symptoms- One of the most notable symptoms of TSWV infection is found on the fruits of infected plants. Tomato fruits may display clearly defined circular ring spots, giving the fruits an appearance of concentric rings of yellow, brown, or green discoloration. Fruits may sometimes present mosaic-like mottling or uneven ripening, giving an unappealing patchy look. Necrotic patches may develop on the fruit surface, sometimes causing sunken, black, or leathery lesions. In more severe infections, fruit deformation, cracks, or uneven growth render the fruit unsuitable for commercial sale. The same symptoms are observed in peppers, peanuts, and other susceptible crops, lowering both yield and market quality.
Disease Cycle of Tomato Spotted Wilt Virus (TSWV)
Primary Source of Inoculum (Virus Reservoirs) – The virus survives in weeds, ornamental plants, and volunteer crop plants, which are the reservoirs of infection. Common hosts of TSWV include weeds, such as chickweed, sowthistle, and pigweed, and various cultivated plants including tomatoes, peppers, peanuts, and lettuce. These infected plants are a reservoir for the continuous supply of the virus, enabling it to be present even out of the growing season.
Virus Acquired by Thrips – Thrips species mainly transmit TSWV, where Western flower thrips, Frankliniella occidentalis, and onion thrips, Thrips tabaci have been identified to be the best vectors. The virus enters the thrip only during the larval phase because it acquires it when feeding on the infected plant tissue. Once in the insect’s gut, it is passed up to the salivary glands, where it replicates and stays for its lifetime. However, the critical point is that adult thrips cannot acquire the virus; instead, they transmit it if infected as larvae. Therefore, management of young thrips populations has to be aimed at reducing disease spread.
Virus Transmission– Once infected, mature thrips transport TSWV to a healthy plant when they go there to feed. When an infected thrips causes a puncture wound in a plant cell for feeding purposes, it injects into the tissue saliva that contains viruses. The viruses then enter the cells of the plant and start multiplying and dispersing within the vascular tissues to new tissues and organs. A few days to weeks show symptoms including the development of necrotic spots on the plant surface, bronzing of the leaf, and irregularities of fruits. The plant does not show recovery but turns into an infected source that triggers new infection through additional thrips.
Systemic Movements– TSWV moves systemically within a plant once it penetrates inside, spreading from the primary feeding site to other tissues. It moves through the phloem and xylem to infect leaves, stems, flowers, and fruits. Systemic movement causes a range of symptoms that include ring spots, necrotic lesions, leaf curling, and fruit distortion. Infection at the seedling stage leads to stunted growth and death, whereas older plants may survive but yield low-quality fruits.
Secondary Spread– As infected thrips continue to feed and move between plants, they disperse the virus over the field, causing secondary infections. This results in localized outbreaks that can easily become large epidemics if not controlled. Rapid multiplication of thrips populations under warm, dry conditions makes greenhouses and open fields particularly vulnerable. The disease can be spread over long distances by the movement of infected plant material such as seedlings or cuttings.
Seasonal Carryover and Disease Persistence– At the end of the growing season, TSWV remains in infected weeds and alternate host plants, thus surviving through winter or dry periods. In mild climatic regions, the virus can survive throughout the year, causing non-stop infection. Moreover, thrips can survive overwintering host plants, thereby being able to transmit the virus with the appearance of new crops. This cyclic nature makes it almost impossible to eradicate TSWV from agricultural areas.
Environmental and Biological Factors influencing infection and spread
The spread of TSWV is influenced by several environmental and biological factors. Warm temperature (25–30°C) and dry conditions enhance the activity of thrips, while the existence of alternate host plants such as weeds, ornamentals, or volunteer crops increases the breeding grounds of thrips for the virus multiplication. Certain plant varieties with the resistance gene Sw-5 are partially resistant to TSWV. The new strains of the virus can develop ways to break this resistance. Moreover, the resistance of thrips to insecticides in their population has reduced the efficiency of chemical control and made integrated disease management inevitable.
Disease Management of Tomato Spotted Wilt Virus (TSWV)
Tomato Spotted Wilt Virus is a very challenging virus to manage because it infects a wide range of plant species and can only be transmitted by thrips vectors. As there is no cure for the virus once the plant is infected, the most effective management strategies for disease focus on prevention, vector control, resistant varieties, cultural practices, and integrated pest management. This approach minimizes yield losses and prevents the infection from spreading to other parts of the field.
Resistant varieties– The TSWV-resistant varieties are perhaps the most practical way of managing TSWV. Plant breeders have bred TSWV-resistant hybrids harboring the Sw-5 gene, which can confer strong protection against the virus. Resistant varieties reduce the severity of infection and can inhibit the spread of the virus in a field. However, resistance is not absolute as new virus strains may emerge that overcome genetic resistance. Some of the popular resistant tomato varieties include ‘Crista,’ ‘Amelia,’ and ‘Talladega’. Farmers should consult with local agricultural experts to select the best varieties for their region.
Thrips Management (Vector Control)- Since TSWV is transmitted only by thrips, the management of thrips populations is an important strategy in the control of the disease. Thrips management can be achieved through a combination of chemical, biological, and cultural methods.
Chemical control- Spinosad, abamectin, and imidacloprid are insecticides that may be used to control thrips populations. Thrips, however, develop resistance to pesticides rapidly, Overuse of pesticides also kills beneficial insects, and so chemical treatments should be applied cautiously and selectively.
Biological control– Biological control includes the introduction of natural predators of thrips, such as minute pirate bugs, predator mites, and lacewings. These beneficial insects help keep the thrips population under tight control naturally and reduce its dependence on chemical insecticides. Moreover, experiments on the biopesticidal use of Beauveria bassiana, a fungal pathogen infecting thrips, have been promising in controlling their population.
Reflective mulches, which include silver plastic mulch, can be utilized in fields for thrips repellents. Thrips cannot easily trace host plants where reflective mulch is present, as the plants reflect sunlight.
Research studies prove that using reflective mulch can drastically cut down thrip populations and reduce the frequency of TSWV occurrence within crops.
Cultural Practices for Tomato Spotted Wilt Virus (TSWV)
Good cultural practices are also an important management step for TSWV. Weed control is essential because many wild plants, including chickweed, sowthistle, and pigweed, act as reservoirs for the virus and its insect vectors. Removing weeds from field borders and nearby areas reduces the chances of thrips acquiring the virus and transmitting it to healthy crops.
Crop rotation– Crop rotation with crops not susceptible to the virus, such as corn or legumes, can break the disease cycle. Through this practice, the continuity of infected plants in the same field does not persist, hence reducing pressure time after time. Planting early will allow crops to grow before the peak thrips season, reducing the possibility of infection.
Row covers or insect-proof nets can also prevent thrips from reaching young seedlings. This is very useful in nurseries, where young plants must be protected from thrips to prevent disease. However, once the plants start flowering, row covers should be removed to allow for pollination.
Sanitation and removal of infected plants– Since there is no cure for the plants infected by TSWV, removing and destroying the symptomatic plants will help to reduce the further spread of the virus. Any plant showing symptoms like necrotic rings, bronzing, or deformed fruits should be removed as soon as possible. Such plants should not be composted but rather be burned or buried so that neither the virus nor thrips survive.
Farmers should also clean the tools and equipment coming into contact with infected plants. Although TSWV is not transmitted mechanically by sap, reducing cross-contamination in the field is good hygiene practice.
Monitoring and Early Detection- Monitoring populations of thrips is critical and should be conducted regularly for early intervention. Sticky traps in yellow or blue are used to monitor the movement of thrips to detect population increase. Early field scouting of the first signs and symptoms of TSWV such as bronzing of leaves and ring spots helps identify infected plants before the disease spreads widely.
The confirmation of TSWV in suspected plants can be done by laboratory tests such as ELISA (Enzyme-Linked Immunosorbent Assay) and PCR (Polymerase Chain Reaction). These tests offer accurate diagnoses to farmers for better management decisions.
Since no single method is entirely effective against TSWV, an Integrated Disease Management approach is required. This strategy involves the use of multiple control measures to reduce disease incidence and economic losses.
References
- Tomato spotted Wilt virus on tomato and pepper | NC State Extension Publications. (n.d.). https://content.ces.ncsu.edu/tomato-spotted-wilt-virus-on-tomato-and-pepper
- Tomato spotted wilt virus. (n.d.). SDSU Extension. https://extension.sdstate.edu/tomato-spotted-wilt-virus
- TOMATO SPOTTED WILT VIRUS. (2021, April 8). [Slide show]. SlideShare. https://www.slideshare.net/slideshow/tomato-spotted-wilt-virus/245918491
- Awan, M. J. A., Akram, A., Amin, I., & Mansoor, S. (2023). Viral vectors as carriers of genome-editing reagents. Trends in Plant Science, 28(9), 981–983. https://doi.org/10.1016/j.tplants.2023.05.010
- Ruark-Seward, C. L., Bonville, B., Kennedy, G., & Rasmussen, D. A. (2020). Evolutionary dynamics of Tomato spotted wilt virus within and between alternate plant hosts and thrips. Scientific Reports, 10(1). https://doi.org/10.1038/s41598-020-72691-3
- Spotted wilt virus in tomatoes. (n.d.). University of Maryland Extension. https://extension.umd.edu/resource/spotted-wilt-virus-tomatoes/extension.umd.edu/extension.umd.edu/resource/spotted-wilt-virus-tomatoes
- Tomato spotted wilt virus | Public | Clemson University, South Carolina. (2024, March 26). https://www.clemson.edu/public/regulatory/plant-problem/fact-sheets/tswv.html
- Marchiori, C. H. (2024). Tospovirus (family Bunyaviridae). Definitions. https://doi.org/10.32388/eka51n
- Gupta, R., Kwon, S., & Kim, S. T. (2018). An insight into the tomato spotted wilt virus (TSWV), tomato and thrips interaction. Plant Biotechnology Reports, 12(3), 157–163. https://doi.org/10.1007/s11816-018-0483-x.
- Tomato spotted wilt orthotospovirus (tomato spotted wilt) | CABI Compendium. (n.d.). CABI Compendium. https://www.cabidigitallibrary.org/doi/full/10.1079/cabicompendium.54086
