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Authors
Introduction
Symptoms/Signs
Disease Management

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Figure 1. Dark reddish-brown specks associated with TSWV infection in young flue-cured tobacco plants.

Figure 2. Older plants appear wilted because of the twisting and distortion the virus causes. Symptoms are usually most severe on one side of the plant.

Figure 3. Necrotic local lesions on oriental tobacco infected by TSWV.

Figure 4. Vein clearing on oriental tobacco infected by TSWV.

Figure 5. Chlorotic and necrotic ringpots in line patterns on systemically infected leaves of oriental tobacco infected by TSWV.

Figure 6. Leaf necrosis on systemically infected leaves of oriental tobacco infected by TSWV.

Figure 7. Necrotic local lesion on old leaves of oriental tobacco infected by TSWV.

Figure 8. Stem necrosis and top curve of oriental tobacco infected by TSWV.

Figure 9. dwarfing or oriental tobacco plant infected by TSWV.

Figure 10. Recovery of top leaves of an oriental tobacco plant infected by TSWV.

Authors

A.L. Mila, Extension Plant Pathology
E.K. Chatzivassiliou, Plant Pathology, Democritus University of Thrace, Greece

Introduction

United States

Tomato spotted wilt (TSW) is a potentially devastating disease of tobacco caused by the tospovirus, tomato spotted wilt virus (TSWV). The host range is large, including many weeds, vegetables and ornamentals. TSWV was first reported in Australia in 1918 and now has been reported in the United States, Canada, Central America, South America, Japan, and Europe. In US the incidence of TSWV in tobacco was light in Georgia and 0 in North Carolina from 1989 through 1994. Since 1995 the incidence increased dramatically in Georgia; a similar increase occurred in North Carolina after 2002.

Greece

Tomato spotted wilt virus (TSWV) (Bunyaviridae: Tospovirus) is one of the most destructive viruses of tobacco crops in Greece. The first TSWV infected plants were reported in oriental tobacco crops in 1956. The virus was officially described in 1972 and its presence was associated with the thrips species Thrips tabaci. Afterwards, TSWV became endemic in crops of all tobacco varieties throughout Northern Greece (Thrace, south-east and central Macedonia) causing severe epidemics when environmental conditions favored the growth of T. tabaci populations. From 2000 to 2003, severe epidemics were recorded for the first time in new tobacco areas (West Macedonia and Pieria) although TSWV presence had remained previously unnoticed. Nowadays, TSWV is widespread in the central and northern part of the country, infecting, except for tobacco, several cultivated or wild plant species.

Symptoms/Signs

Flue-Cured & Burley Tobacco

Symptoms of TSWV vary with plant age, virus strain, and environmental conditions. Newly transplanted seedlings die rapidly, then swiftly decay. Therefore, seedling infections are often misdiagnosed as other seedling diseases or transplanting problems. Wrinkling of the stem surface, with wrinkles often in elongated concentric circles, maybe observed during careful examination of seedlings just beginning to die. Ankle-high and taller plants will show some characteristic foliar symptoms. On small plants, dark reddish-brown specks and leaf distortion are common on the youngest leaves (Figure 1). Slightly older plants will show classic reddish-brown necrotic spots or ringspots, often with star-like projections into the green leaf tissue. Necrosis of tissue running adjacent to leaf veins is common and characteristic. Despite the term wilt in the name, older plants only appear wilted because of the twisting and distortion the virus causes. Symptoms are usually most severe on one side of the plant and in the bud (Figure 2). Infected plants near flowering may have black streaks running down one side of the stem, often resembling burn from contact suckercides. Streaks also occur within the pith. Plants that get infected near, during, or after flowering suffer little loss. Symptoms on these plants are generally local, being restricted to the leaf or leaves that were initially infected.

Thrips and the Movement of Tomato Spotted Wilt Virus

TSWV moves by thrips. Thrips usually spend the winter as adults or as pupae in the soil. Adults may hibernate in sheltered areas, but in mild winters-or at least during mild periods-they may be active on host plants such as winter weeds. In the spring, thrips begin to move more actively and can spread to other hosts, including tobacco. Most of this movement is over distances that may reach several hundred yards, but thrips can sometimes be carried hundreds of miles by the wind. Generations are short, about two weeks when it's warm, and there may be several generations during the growing season.

Not every thrips you see on your tobacco is spreading TSW. Two species that do carry the virus and do feed on tobacco are the tobacco thrips (Frankliniella fusca) and the western flower thrips (Frankliniella occidentalis). The virus must be picked up by a very young thrips within a day or two of hatching from an egg. The same thrips cannot move the disease to another plant until it matures into an adult.

The Weed Connection

Many plant species can be infected by TSWV. However, some are much better hosts than others: in tobacco the most important sources for infection are several species of winter weeds (i.e. smallflower buttercup, mousear chickweed, common chickweed, and spiny sowthistle, dandelion, and Rugel's plantain). As the winter annuals begin to die in the spring, adult thrips are forced to move to alternative plants, including tobacco. If the plant on which they developed was infected, they carry the virus with them. The virus can also move back and forth between winter annuals and summer annuals and perennials.

Oriental Tobacco

TSWV can infect all tobacco varieties cultivated in Greece and all stages of tobacco growth; symptoms tend to be more severe during plant youth and highly depend on the tobacco variety, plant age, virus strain and environmental conditions.

Diseased plants show chlorotic and subsequently necrotic local lesions on the leaf sites that the virus is "injected" by the thrips (Fig. 1). Those lesions initially resemble those caused by blue mold (Peronospora tabacina). Two weeks later, diseased plants develop a vein clearing (Fig. 2) and subsequently chlorotic and necrotic ringspots and line patterns on systemically infected leaves (Fig. 3) that result in necrosis of large leaf parts (Fig. 4). Older leaves usually do not develop symptoms, or express only necrotic lesions (Fig. 5).  Characteristic symptoms are more severe on the one side of the plant that usually presents a side stem necrosis and top curve (Fig. 6). Early infections, especially in the seedlings, result in severe plant dwarfing (Fig. 7) and occasionally death of the plant. When the temperature is high plants recover, and leaves may develop mottling, mosaic, or vein clearing instead of necrosis (Fig. 8). However, necrotic symptoms re-emerge especially in top leaves, when temperature decreases and especially after rain.

Epidemiology

The thrips vector: The vector of TSWV in the tobacco grown in South East Europe differs from that in the USA. In this area, TSWV epidemics in tobacco are closely and exclusively associated with the thrips Thrips tabaci, widely known as "the onion thrips". Highly specialised populations of T. tabaci that contain both male and female individuals, are highly competent in transmitting the virus and consist the only vector colonizing tobacco. T. tabaci is called "the tobacco thrips" in this area, as Frankliniella fusca (Hinds) in the States.

Young wingless larvae of those T. tabaci populations acquire the virus from infected tobacco plants even in short feeding times (5 min). Subsequently TSWV spread is closely associated with the number of T. tabaci flying individuals. Those are the main vectors of the disease and transmission rates to tobacco can reach 67% depending on the temperature.

Virus sources: TSWV is not seed transmitted and its spread within tobacco crops mainly depends on infections coming from outside sources; spread between tobacco plants is rather limited. Infected weeds are probably the main reservoirs from which the virus spreads to seedbeds and crops. Other cultivated hosts such as vegetables or ornamentals, may serve as additional sources when planted close to seedbeds. Viruliferus thrips that overwinter either as pupae in the soil of infected fields, or as females on winter plants especially during mild winters, can also infect tobacco seedlings in the new cultivation period. Early tobacco plantings or abandoned seedbeds may also serve as additional virus and vector sources for the late plantings.

The role of weeds: In spring, T. tabaci population increase in the fields before tobacco is transplanted, so at least one thrips generation develop on weeds. Among the summer weeds, prickly lettuce (L. serriola), thorn apple (Datura stramonium) and black nightshade (Solanum nigrum) support high viruliferous T. tabaci populations. Those summer weeds are present throughout summer and autumn, so they may also serve as the bridge for the viruliferous thrips between crop harvest and the emergency of winter annual weed hosts as sow thistle (Sonchus oleraceus) and pigweed (Amaranthus retroflexus). Thrips grown on weeds move to tobacco plants when disturbed by wind, farming activities or weed maturity, but also due to increased preference they show for tobacco.

Favourable conditions for epidemic spread:  The spread of the virus depends on the availability of virus sources and the size of the T. tabaci population; the last one is determined mainly by temperature and rainfall. During winter, heavy rainfalls and frosts decrease the number of overwintering thrips individuals. During spring and summer, thrips populations increase and TSWV spread is faster in hot and dry conditions; the opposite happens when the weather is rainy. Many oriental varieties are cultivated in non-irrigated fields and as TSWV is endemic in many tobacco cultivation areas, when favorable environmental conditions occur, a geometrical increase of vector population results in an epidemic virus spread. This is the situation the oriental tobacco plantations in Greece experience almost every year, for the past years.

Disease Management

Flue-Cured & Burley Tobacco

While no current management practices will completely control the effects of TSWV on tobacco crops, some tools that can help moderate the disease have emerged in the last few years. Proper application of these strategies can significantly reduce TSWV incidence in tobacco fields, but they may not provide adequate suppression under extremely high virus pressure.

Pesticides
Thrips are able to transmit TSWV very quickly, and most of these virus-carrying thrips come from outside the tobacco field. Over-the-top insecticides do not kill these thrips quickly enough to stop the spread of the virus. This type of spraying has not been successful in reducing disease incidence. However, some disease suppression has been noted on imidachloropicrin-treated plants in Georgia and North Carolina. Therefore, applying Admire in the greenhouse to control aphids and other insect pests may help suppress TSWV.

The application of Actigard, alone or in combination with an insecticide, as a foliar spray (drench) to seedlings in the greenhouse shows promise for being an effective and economical management tactic. Most economically important TSWV infections apparently occur within the first week or two after transplanting; many may occur during the first few days. Thus, protection should be in place before transplanting. Application of any chemicals after the virus has infected the plant will be of little, if any, benefit. The best treatment in our studies averaged about 45 percent control.

Cultural Practices
Field selection and the transplanting date do impact disease, but the effect of the transplanting date is not consistent enough from year to year to include in a management plan. TSW is most severe in early-planted fields in most years; but in some years late-planted tobacco is most affected. Generally, tobacco planted in the middle of the transplanting season is least affected. Removing infected plants does not prevent disease spread because infected thrips do not usually spread the virus from tobacco plant to tobacco plant.

Weed Management
Research is still under way to determine how we might use weed management to manage TSWV. However, a few management tools appear promising.

1. Weedy small grain fields and fallow fields destined for no-till soybeans or cotton are potentially important sources of virus-carrying thrips. Be careful not to disrupt these fields (for example, do not use a broad-spectrum herbicide) just before or at the time you are transplanting tobacco. Thrips will be forced from the dying weeds into a very susceptible tobacco crop. Weeds in these fields should be dead for at least three weeks before transplanting.

2. Movement of the virus from summer annuals back to winter annuals is an important step in the virus cycle. If summer annuals can be killed before the winter annuals emerge, the cycle might be disrupted. This is another argument for a vigorous and early stalk-and-root destruction program in tobacco (including cultivation) and for good general weed control in late summer and early fall. Pay particular attention to fields with substantial carpetweed populations because this plant generates large numbers of thrips and is a reservoir for the virus.

3.Whenever possible, manage your field borders to favor grassy vegetation over broad-leaved weeds. Grasses don't generate vector species of thrips and are poor hosts for the virus.

Oriental Tobacco

The ability of T. tabaci to efficiently spread TSWV, even during short feedings and thrips insecticide resistance make TSWV control in tobacco crops difficult.

Cultural measures: An effective control of TSWV demands several preventive measures. Seedbeds should be spatially separated from ornamentals and vegetables. Only healthy seedlings must be transplanted in the field, the diseased ones should be removed. Fields with a history of a high disease incidence, as well as the vicinity with early planted infected crops should be avoided. Rotation with cereals may be also applied. In the fields, the first generation of overwintering thrips develop mainly on weeds, therefore an intensive weed control is needed before the built up of thrips populations. However, when weeds are already infested by thrips, their management demands particular attention, as those thrips, when disturbed, will move toward the crop. In this case the application of an insecticide is absolutely necessary before weeds' removal.

Insecticide applications: Seedlings in oriental tobacco crops in Greece are usually protected by three applications of systemic soil insecticides (carbofuran, methomyl); the first when the plants have four leaves, followed by a second 2-3 weeks later and the third, one week before transplantation. The last application in the field can be alternatively done during transplantation. Subsequently, plants in the field are protected by foliage insecticides (methomyl, cypermethrine), starting one week after transplantation in weekly intervals, till flowering. None of the insecticides used fully protect tobacco plants. The most promising results in the control of the disease were recorded when data concerning thrips population fluctuation and vector presence, were used to direct insecticide applications performed by groups of farmers in large areas.

**
August 2007