Ecology and Epidemiology

- Survival and virus reservoirs

As mentioned earlier, tomato spotted wilt virus (TSWV) infects a large number of cultivated and non-cultivated hosts. More than three hundred hosts have been identified. TSWV can survive easily in the vicinity of tobacco crops, including various weeds such as Capsella bursa-pastoris, Chenopodium amaranticolor, Convolvulus arvensis, Solanum nigrum, Sonchus oleraceus, Stellaria media etc.. Many cultivated plants serve as hosts for TSWV, for example vegetables (lettuce, eggplant, tomato, cucumber, pepper, potato, peas) (figure 1) or ornamentals (chrysanthemum, dahlia, zinnia, begonia, cyclamen, gladiolus, lily etc.). It is also found on some edible crops in hot areas: vines, peanuts, chayote, pineapple etc. These various plants serve as virus reservoirs and that makes the virus control extremely difficult.

- Transmission and dissemination (figure 2)

TSWV is transmitted by several species of thrips in a persistant manner. In France, only Thrips tabaci and Frankliniella occidentalis are present. The latter seems a much more efficient vector than Thrips tabaci, for  which biotypes unable to transmit the virus have been  detected. Frankliniella fusca and Frankliniella shultzei also ensure the transmission of TSWV in other countries where it occurs. Recently this virus has been transmitted experimentally to peanuts by two other thrips species: Thrips palmi and Scirtothrips dorsalis. Only the larvae are capable of acquiring the virus after feeding, in about 15 minutes minimum time.
Inoculations occur mainly during feeding of the adult. The latter puncture the leaf surface cells, inject saliva causing lysis of the cell contents and aspire it. This procedure takes only 5 to 15 minutes. The latent period lasts at least four days. Thrips, which have a reduced size, move over short distances. Sometimes thrips can be carried by the wind over several hundred meters or more away. Adults live from 30 to 45 days, and are viruliferous until they die.
TSWV is also transmitted with seeds and vegetative reproductive organs in many plants, but not in tobacco. It may sometimes be transmitted during  topping.

U.S. situation

Research in the southeast U.S. indicates that the most important sources for infection of tobacco are several species of winter weeds. Some of these include the annual small flower buttercup (Ranunculus abortivus), mouse-ear chickweed (Cerastium vulgatum), common chickweed (Stellaria media), and spiny sowthistle (Sonchus asper), as well as the perennials dandelion (Taraxacum officinale) and Rugel’s plantain (Plantago rugelii). In most years, the tobacco thrips is apparently the most important vector of TSWV in the early season. 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.

The movement of TSWV into tobacco is complex. Several things must happen for the epidemic to occur. First, there must be infected plant hosts in the area that harbor the virus. Second, these plants must also be hosts of one of the thrips species that can carry the disease. Third, these thrips must be one of the species that attack tobacco. Fourth, there must be some reason for the adult thrips to move from the host to tobacco. Finally, this movement must take place when the tobacco is in the field and in a susceptible stage.

(Mina Mila - North Carolina State University)