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“Occurance of Tomatto spoted wilt virus
in
Eastern Uttar Pradesh ”
U.P. Gupta and Lokesh kumar Shukla,
Departement of Botany
Harischandra. P.G. College,
Varanasi 221001, India
The Tospovirus are a genus (Tospovirus) of negative RNA Virus found with in the family Bunyaviridae. They are the sole group of plant infecting viruses in the this family, as all other described members of the Bunyaviridae infect animals (Check Ophiovirus and Tenuivirus). The genus takes its name from the discovery of Tomato spotted Wilt Virus (TSWV) in Australia in 1915. It remained the only member of the family until the early 1990’s when genetic characterization of viruses discovered in plants became more common . There are now at least twenty viral “Species” in the family with more being recorded and described on a relatively regular basis. Together , these viruses have been documented infecting over eight hundred different plant species from 82 different families.
This virus has single stranded RNA genome with negative polarity, therefore it is classified as a Class V virus ((-)ss RNA viruses).The structure of the genome resembles that of the genus Phlebovirus. The genome is linear and is 17.2 kb in size. It is segmented into three segments termed S (2.9 kb), M(5.4kb) and L 8.9kb) . The M and S RNA segments encode for proteins in an ambisense orientation.
Tospoviruses are arboviruses usually vectored by a thrips. At least ten species of thrips belonging to family thripidae have been confirmed as vectors for the transmission of thirteen or more tospoviruses. The thrips vectors are not closely related, implying an independent origin of infection for each thrips, possibly transmitted horizontally through shared hosts. There may be other species of thrips competent to transmit similar viruses , but they have not been documented on crops of economics significance.
Recent research concludes that thrips can only be infected by tospovirus during the larval phases of development, as pupation and metamorphosis separate the connection between the salivary glands and the infected muscle tissue of the mid gut. Adults transmit the virus from infected salivary glands, and uninfected adults will not transmit the virus. Obviously, controlling the infection by limiting transmission from infected plants to larval thrips or by preventing adult dispersal from infected plants are key strategies in preventing an epidemic of the disease.
Infection with the virus results in spotting and wilting of the plant, reduced vegetative output, and eventually death. No antiviral cures have been developed for plants infected with a Tospovirus, and infected plants should be removed from a field and destroyed in order to prevent the spread of the disease.
There are a large number of plant families that are known to be affected by viruses of the Tospovirus genus. These include both food crops ( such as peanuts, watermelons, capsicums, tomatoes, zucchinis, et al.) as well as ornamental species which are important to flower farms ( calla lily, impatiens, chrysanthemums, iris, et al.) For a more complete list of hosts examine the Tospovirus host list at Kansas state University.
Early symptoms of infection are difficult to diagnose. In young infected plants the characteristic symptoms consist of inward cupping of leaves and leaves that develop a bronze cast followed by dark spots. As the infection progresses additional symptoms develop which include dark streeks on the main stem and wilting of the top portion of the plant. Fruit may be deformed, show uneven ripening and often have raised bumps on the surface. Once a plant becomes infected the disease cannot be controlled.
TSMV is prevalent in warm climates in regions with a high population of thrips. This virus is an agricultural pest in Asia, America, Europe and Africa. Over the past 15 years outbreaks of Tomato spotted wilt disease have become more prevalent in these regions. Therefore TSWV is desribed as an emerging viral disease of plants. The increased prevalence is largely beause of the successful survival of the thrips vector Frankliniella occidentalis. Another thrips, Scirtothrips dorsalis, has also been implicated in the transmission of at least three tospoviruses, but there remains some controversy over its efficiency as a vector. Immunological testing and vector-competence studies suggest that S. dorsalis may represents a non-transmitting carrier for some strains of virus.
The success of this virus has also been attributed t ;the acquisition of a gene in the M segment of the genome which encodes a movement protein. This protein allows the virus to infect a wide range of hosts. The gene encoding this protein was likely acquired by recombination from either a plant host or from another plant virus.
Control of this disease is difficult. One of the reasons for this is that the wide host range allows the virus to successfully over season from one crop to the next. To prevent spread of the virus infected plants should be immediately removed away fro neighboring plants. Control of insects, especially thrips, is important to reduce spread of the virus by vectors.
References :
1. Jones, D.R. 2005. Plant viruses transmitted by thrips. European Journal of Plant Pathology 113: 119-157.
2. Mound, L.A. 2002. So many thrips- so few tospovirus?,pp. 15-18. In L.A. Mound and R. Marullo [eds.] Thrips and Tospoviruses: Proceedings of the 7th International Symposium on Thysanoptera. CSIRO Entomology, Reggio Calabria, Italy.
3. Mortiz, G., S.Kumm, and L.A. Mound. 2004. Tospovirus transmission depends on thrips ontogeny. Virus research 100: 143-149.
4. Groves, R.L.,J.F. Walgenbach, J.W. Moyar, and G.G. kennedy. 2003. Seasonal Dispersal Patterns of Frankliniella Fusca (Thysanoptera: Thripidae) and Tomato Spotted Wilt Virus Occurance in Central and Eastern North Carolina. Journal of Economic Entomology 96: 1-11.
5. Nault, B.A., J.Speese III, D.Jolly, and R.L.Groves. 2003. Seasonal patterns of adult thrips dispersal and implications for management in Eastern Virginia tomato fields. Crop Protection 22: 505-512.
6. Whitfield AE, Ullman DE, German TL (2005). Tospovirus- Thrips interactions. Annual Review of Phytopathology. 43: 459-89.
