Tomato spotted wilt virus (Orthotospovirus tomatomaculae, but referred to as TSWV) was first described in 1915 in Australia on tomato plants, where it caused characteristic spotting and wilting symptoms. The virus was later identified as the causal agent of the disease in the 1930s. The global spread of TSWV accelerated in the 1980s and 1990s, primarily due to the expansion of its most efficient vector, the western flower thrips (Frankliniella occidentalis). During the mid to late 1980s, TSWV was observed in multiple crops across the southeastern United States. The virus was first confirmed in North Carolina tomato and tobacco fields in 1988. By 1997, TSWV had been reported in nearly every county in the state, with average incidence ranging from 10 to 15 percent in the Coastal Plains region and crop losses reaching up to 50% in heavily affected fields. TSWV continues to expand its host and geographic range, with at least 26 new “first reports” since 2020, more than 70% of which have been documented in Asian countries.

Tomato spotted wilt virus (TSWV) belongs to the Orthotospovirus genus, a group of plant-infecting viruses vectored by arthropods. However, the only known vectors of TSWV are thrips. In North Carolina and surrounding states, the primary vectors of TSWV are tobacco thrips (Frankliniella fusca) and western flower thrips (F. occidentalis). Among these, tobacco thrips are the most important vector in tobacco production. However, tobacco is not a host for reproducing populations of tobacco thrips, meaning that the immature stages do not develop in tobacco plants, and only adult thrips dispersing from neighboring areas are found feeding on tobacco. While large numbers of adult thrips may be observed feeding on individual tobacco leaves (10-20 thrips per leaf in some cases, Figure 1), the number of thrips per plant is not usually directly related to TSWV incidence, so it is possible to observe many thrips on plants and still have low levels of TSWV. TSWV incidence is influenced not only by the number of dispersing thrips, but also by the age of healthy plants that adult thrips feed on, the abundance of infected source plants (Table 1), and environmental conditions such as temperature and rainfall events in winter and early spring.

A wide range of broadleaf winter and summer weeds serve as good reproductive hosts for reproducing populations of tobacco thrips. These insects go through multiple generations per year and feed on other solanaceous crops such as peppers and tomatoes. TSWV itself is highly polyphagous and is now known to infect more than 1,000 plant species, including numerous weeds and economically important crops such as tomatoes, peppers, lettuce, peanuts, and tobacco.

Figure 1. Young tobacco leaf with >12 tobacco thrips

Lorena Lopez, NC State University

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Figure 1. Young tobacco leaf with >12 tobacco thrips

Lorena Lopez, NC State University

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Most TSWV-infected weeds do not show symptoms or become necrotic. In tobacco, not all TSWV-infected plants develop disease symptoms and symptoms of TSWV can range from mild to severe. Early infections can significantly reduce plant uniformity, yield, and cured leaf quality, while plants infected at or after flowering typically experience minimal yield loss. Symptoms often do not appear until 2 to 3 weeks after initial infection, making it difficult to detect and manage the disease in its early stages.

In tobacco, TSWV often causes zonate necrotic spots (specks) and concentric rings (ringspots) on the leaves (Figure 2). These spots initially appear yellow but quickly turn rusty brown, sometimes forming distinct concentric patterns. Rusty brown necrotic spots may also accumulate along the midrib and secondary veins, giving the appearance of “rusty veins,” which are visible on both the upper and lower surfaces of the leaf (Figure 3). Apex leaves can become distorted and may bend downward. Leaf distortion causes stunting and disproportionate damage on the sides of the leaves, resulting in asymmetrical wilting (Figure 4). Brown to black elongated lesions can also develop on the veins, petioles, and stems (Figure 5). As the disease progresses, leaves may yellow, wilt, and dry out (Figure 6). For rapid in-field detection, an Agdia ImmunoStrip® provides a quick and reliable test for TSWV, functioning similarly to a COVID-19 test strip and producing results within minutes.

Figure 2. Necrotic spots and leaf symptoms of TSWV in tobacco.

Lorena Lopez, NC State University

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Figure 2. Necrotic spots and leaf symptoms of TSWV in tobacco.

Lorena Lopez, NC State University

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Figure 3. Rusty brown necrotic spots along midrib and secondary veins

Daisy Ahumada, NC State University

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Figure 3. Rusty brown necrotic spots along midrib and secondary veins

Daisy Ahumada, NC State University

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Figure 4. Assymetrical leaf or bending downward symptoms of TSWV in tobacco.

Lorena Lopez, NC State University

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Figure 4. Assymetrical leaf or bending downward symptoms of TSWV in tobacco.

Lorena Lopez, NC State University

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Figure 5. Black streaking along stem of TSWV-infected tobacco

Daisy Ahumada, NC State University

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Figure 5. Black streaking along stem of TSWV-infected tobacco

Daisy Ahumada, NC State University

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Figure 6. Later stages of TSWV infection leading to severe yellowing, wilting, and necrosis

Matthew Vann, NC State University

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Figure 6. Later stages of TSWV infection leading to severe yellowing, wilting, and necrosis

Matthew Vann, NC State University

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Tomato spotted wilt virus (TSWV) overwinters in alternative host plants (reservoirs) such as weeds, ornamentals, and volunteer crops, allowing both the virus and its thrips vectors to persist between growing seasons. Thrips acquire the transmissible virus during their first instar larval stage (the first stage after egg hatching), which lasts only 1–2 days, by feeding on TSWV-infected, non-necrotic plant tissue. This makes it likely that larvae hatch on infected plants. Once infected, the virus replicates in the thrip’s salivary glands, enabling transmission to healthy plants at random (but not every time) when feeding as adult thrips. Adult thrips need to feed on plants for several minutes before transmitting the virus to the plant host. Once inside the plant, TSWV moves systemically through the vascular system, replicating and causing symptoms.

Adult thrips and non-vector insects can acquire TSWV in their midgut when feeding on infected plants, but the virus cannot move to their salivary glands. This is because the parotid ligaments, critical for virus movement from the midgut to the salivary glands, are absent or non-functional in adult thrips and non-vectors. As a result, thrips that acquire the virus as adults and non-vector insects cannot transmit the virus. The cycle continues as more TSWV-infected thrips feed on tissue, lay eggs, acquire the virus as larvae, and transmit it as adults. Effective management depends on breaking this cycle by controlling thrips populations and removing virus reservoirs.

Once a plant is infected with Tomato spotted wilt virus (TSWV), there is no cure. The virus is transmitted exclusively by thrips, and control strategies focus on prevention. Total elimination of thrips is not practical, but reducing their populations can significantly limit TSWV spread and damage.

Removing Reservoirs

TSWV does not survive in crop debris or soil within the field and must be reintroduced each season from infected thrips on nearby reservoir plants. TSWV survives between growing seasons in alternative host plants, such as weeds, ornamentals, and volunteer crops that also support thrips populations. These virus reservoirs allow both TSWV and thrips to persist year-round. Volunteer peanuts do not attract tobacco thrips to tobacco more than other volunteer crops or weeds. Individual thrips (not swarms) gradually disperse from these reservoirs as plants dry down or become overcrowded. If weeds or infected plants are burned down or removed, large numbers of thrips may move at random to the nearest available host, including newly planted tobacco or nearby weeds. Remove weeds and volunteers at least 2 weeks before transplanting, with 2 to 3 weeks being ideal. This significantly limits thrips movement and early infection pressure.

Other Cultural Management Strategies

• Transplant timing: Transplanting after April 7 reduces infection risk.

• Host resistance: The only known highly susceptible variety is PVH2310. Avoiding this variety in high-risk areas can reduce losses.

• Sanitation: Transplants are not a significant source of TSWV. Removing diseased plants in the field has no significant effect on virus spread, and secondary transmission is minimal. Labor costs for plant removal and bagging should also be considered.

Chemical Control

There are no chemical products that directly target TSWV in tobacco. Management efforts focus on reducing thrips populations and help prime plant defenses.

The most commonly used management tools are:

• Acibenzolar-S-methyl (Actigard®): A plant activator that stimulates natural defense mechanisms, typically applied in the float house.

• Imidacloprid (Admire Pro® or Wrangler®): A systemic insecticide, typically applied in the float house.

• Thiamethoxam (Platinum®): A systemic insecticide, typically applied in the float house.

• Chlorantraniliprole+thiamethoxam (Durivo®): A systemic insecticide often used at transplant against thrips and early populations of budworms.

• Spinosad (Blackhawk®): A contact insecticide often used at transplant against thrips and early populations of budworms.

In areas with <10% historical TSWV incidence, additional treatments beyond standard insecticides may not be necessary due to minimal expected economic loss.

Field studies at multiple sites in North Carolina and Georgia have confirmed that the greatest reduction in TSWV occurs when insecticides are applied as float tray treatments, either alone or in combination with Actigard. Actigard may be applied in the float house at the same time as the insecticide application, after the insecticide but prior to transplant, or at transplant. Insecticides may also be applied as transplant, in-furrow, or foliar applications, following all label restrictions. Post-transplant foliar applications of Actigard come with a risk of stunting and are not recommended by the label, but if foliar applications are conducted, the most effective timing for Actigard varies by year and location depending on the timing of the spring flight of tobacco thrips. It is important to use a decision-support tool, as Actigard requires several days to activate and typically provides protection for approximately two weeks, depending on environmental conditions.

Decision-Support Tools

From mid-February to mid-October, over 95% of tobacco thrips have full wings and are capable of flight. Since tobacco thrips (Frankiella fusca) are the most important vector of TSWV in tobacco, chemical control decisions can be guided by the NC State University Tobacco Thrips Flight and TSWV Intensity Predictor, a region-specific forecasting tool developed using TSWV data in tobacco. It provides updates on tobacco thrips activity and TSWV pressure to help growers make informed insecticide application decisions.While the tool is most accurate for tobacco, the biology of TSWV in other southeastern crops is similar. It can be used to reasonably estimate relative risk in other crops, though numerical precision is highest for tobacco.

For more information on using this tool, see the Tobacco Thrips Flight and TSWV Intensity Predictor guide.

Tobacco Streak Virus (TSV, Figure 7)

Tobacco streak virus (TSV) typically causes fine chlorotic or necrotic streaks along the leaf veins, often accompanied by mottling, leaf distortion, and sometimes mild stunting. It can also be transmitted by thrips vectors. Affected leaves may appear twisted or curled, and the streaking is most visible on young, expanding leaves. Field-level detection for TSV may also be conducted with an Agdia ImmunoStrip®

Tobacco Mosaic Virus (TMV, Figure 8)

Tobacco mosaic virus (TMV) causes mosaic or mottled light and dark green patterns on the leaves, often accompanied by leaf distortion, wrinkling, and a characteristic blistered appearance. Infected plants may also exhibit stunted growth and, in severe cases, narrow, leathery leaves. Symptoms are usually most visible on young leaves and can become more pronounced under stress. Field-level detection for TMV may also be conducted with an Agdia ImmunoStrip®

Tobacco Etch Virus (TEV, Figure 9)

Tobacco etch virus (TEV) causes noticeable vein clearing and numerous small chlorotic spots that gradually become tan/brown and necrotic. In severe cases, veins on lower leaves may turn brown and die. Early infection often leads to plant stunting. Field-level detection for TEV may also be conducted with an Agdia ImmunoStrip®

Figure 7. Streaking symptoms on TSV-infected tobacco

Daisy Ahumada, NC State University

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Figure 7. Streaking symptoms on TSV-infected tobacco

Daisy Ahumada, NC State University

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Figure 8. Light and dark green mottling on leaves of TMV-infected tobacco

Brittany Pendleton, N.C. Cooperative Extension Agent, Nash County

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Figure 8. Light and dark green mottling on leaves of TMV-infected tobacco

Brittany Pendleton, N.C. Cooperative Extension Agent, Nash County

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Figure 9. Etching symptoms on TEV-infected tobacco

Daisy Ahumada, NC State University

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Figure 9. Etching symptoms on TEV-infected tobacco

Daisy Ahumada, NC State University

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The NC State University Plant Disease and Insect Clinic provides diagnostics and control recommendations. The NC State Extension Plant Pathology Portal provides information on crop disease management.

The North Carolina Agricultural Chemicals Manual provides pesticide information for common diseases of North Carolina. The manual recommendations do not replace those described on the pesticide label, and the label must be followed. Updates related to tobacco season and on-season TSWV risk reports are available at the NC State University Tobacco Growers Information Portal.

For assistance with a specific problem, contact your local N.C. Cooperative Extension agent.

Kennedy, G. G. (n.d.). Tomato spotted wilt virus: USDA RAMP Project. University of Georgia, College of Agricultural and Environmental Sciences.

Khatun, M. F., Hwang, H.-S., Kang, J.-H., Lee, K.-Y., & Kil, E.-J. (2024). Genetic diversity and DNA barcoding of thrips in Bangladesh. Insects, 15(2), 107.

Tiberini, A., et al. (2025). Tomato spotted wilt virus (Orthotospovirus tomatomaculae), a cyclically occurring threat to crop production worldwide.

2025 Flue-Cured Tobacco Production Guide. (2025). NC State Extension.