Prepared by James W. MoyerProfessor, Dept of Plant Pathology, North Carolina State University, Raleigh, NCThomas GermanProfessor, Dept of Plant Pathology, University of Wisconsin, Madison, WIJohn L SherwoodProfessor and Head, Dept of Plant Pathology, University of Georgia, Athens, GADiane UllmanProfessor, Dept of Entomology, UC-Davis, Davis, CA
Moyer, J.W., German, T., Sherwood, J.L. and Ullman, D. 1999. An Update on Tomato Spotted Wilt Virus and Related Tosposviruses. APSnet Features. Online. doi: 10.1094/APSnetFeatures-1999-0499
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Tospoviruses are "emerging " viruses not only in the sense of their increasing economic importance as pathogens on a worldwide basis, but also in that our understanding of the complexity of this new viral taxon only began to emerge during the last decade. Although diseases attributed to tomato spotted wilt virus (TSWV), the type species for the Tospoviruses, were first reported in Australia about 1915, it was not until after 1990 that we came to realize that TSWV was not unique. Prior to 1990, TSWV was considered a monotypic group of plant viruses. There are now at least twelve distinct viruses (species) in the Tospovirus genus. The genus has been classified in the Bunyaviridae family of viruses. Tospoviruses are the only viruses in that family that infect plants.
Since then several new viruses have been discovered such as peanut bud necrosis virus that causes significant disease losses to peanut production in India and other similar viruses have caused serious diseases of cucurbits in Japan and Taiwan. Additional viruses have also been identified from vegetables and ornamentals in the United States, Israel and Brazil (eg., Figure 4).
When larvae feed on infected plants, ingested virus crosses the midgut barrier and enters the salivary glands. Transmission then occurs when the virus moves into the plant with the saliva during feeding. A midgut barrier in adults prevents virus ingested during this stage from moving to the salivary gland. While the insects remain infective for life, there is no evidence of transovarial passage from one generation of thrips to the next. Seed transmission of Tospoviruses is not known to occur, but some of these viruses are commonly spread in infected propagation material when crops are vegetatively propagated.
Control of Tospoviruses remains problematic. Cultural practices and varietal selection have proven effective in minimizing losses due to TSWV in some field crops. A series of risk factors including prior history, planting date, cultivar selection and plant and row spacing have been identified as critical factors in peanuts. In other high-risk areas, such as Hawaii, highly susceptible crops cannot be grown profitably. In greenhouse grown crops, such as floral crops, extreme measures including screening of production areas with fine-meshed cloth, preventative thrips management strategies and use of propagation material shown to be free of TSWV and INSV are necessary for control of these viruses. While forms of resistance have been introduced into various crops, they have nearly always been overcome by the rapid occurrence of resistance-breaking strains of the virus. TSWV is thought to exist in nature as a complex heterogeneous mixture of distinct isolates that can exchange genetic information through reassortment of genome segments. This provides a readily available reservoir of genetic information to facilitate adaptation.
One or a small number of RNA-dependent, RNA polymerase (RdRp) molecules are associated with each segment of the genome. As the RdRp is required in the initial stages of infection, its activity must be preserved during the transmission process and therefore these viruses have well defined tolerances for the transmission process. This characteristic results in a relatively unstable virus. The ambisense genome organization consists of one open reading frame in the viral sense at the 5’ end of the molecule and a second open reading frame near the 3’ end in the viral complementary sense. A large A-U rich intergenic region separates the two open reading frames. Interestingly, nonstructural proteins on the small (NSs) and middle NSm) genome segments are located near the 5’ end and the open reading frames nearer the 3’ end code for structural proteins (N and G1/G2). The N protein encapsidates the RNA genome segments and may have other functions involved in replication. G1/G2 are found in the envelope and may be involved in recognition of receptors in the vector. NSm has been associated with cell-to-cell movement. NSs accumulates to very high concentrations in infected cells, but its function remains a mystery.
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