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Significance

Although worldwide in distribution, TSWV was of limited and sporadic significance until the mid-1980s. In peanut, for example, TSWV had not caused significant losses in the United States until 1982, but now the disease has become epidemic in peanuts and other crops. The emergence of the western flower thrips, Frankliniella occidentalis, as a worldwide resident, and the interstate and intercontinental shipping of plant materials likely contributed to TSWV becoming a global problem.

The emergence of TSWV as a significant problem at the time that tools of molecular biology were becoming readily available was fortuitous. The almost instantaneous drop in infectivity in plant extracts and the novel transmission of these viruses by thrips had long made them hard to study. Difficulties associated with purification of TSWV from infected host plant tissues created problems in the production of good quality antiserum needed for detection. At one time, it was proposed that the genome of TSWV, like most plant viruses, had a positive sense RNA genome; we have come to know this is not the case. The uncertainty and frustration in working with TSWV was addressed as the techniques of cloning and sequencing allowed the genome organization of TSWV to be unveiled, and monoclonal antibody production permitted the production of useful diagnostic reagents. Yet the biology of the virus is still poorly characterized. For instance, understanding of the molecular mechanisms associated with the replication and transcription of the TSWV genome has been obstructed by the absence of a system to study infectious transcripts synthesized from cloned viral cDNAs. One of the reasons for the lack of such a system is the complexity of the genome organization and replication of TSWV and the tospoviruses in general. Tospoviruses have a unique strategy of ambisense translation in two of the three genomic RNAs. This means the genome segments have a "positive"(or messenger) polarity for one gene and a "negative" (or anti-messenger) polarity for the other gene on that RNA segment. In the future, methods to use reverse genetics, an approach used for viral genomes that are entirely positive or negative sense, may be developed for TSWV.

Another significant event in the study of TSWV was the identification of a number of different tospoviruses. When TSWV was initially described, it was assumed that it was the only virus that had its unique characteristic features. However, Impatiens necrotic spot virus was described in 1989, and, subsequently, approximately a dozen other members of the genus Tospovirus have been identified.

There is now a modest understanding of genome organization and replication strategies of the tospoviruses, and reasonable quality antisera are now available for diagnostics. However, the diseases caused by TSWV and other tospoviruses are still recalcitrant to effective management. Studying TSWV and other tospoviruses will add to our understanding of how viruses evolve to overcome plant host resistance. There also is much to learn about the interaction of TSWV with thrips. Characterization of a receptor for TSWV in the thrips midgut would be a significant breakthrough. Hence, as in the study of all plant pathogens, there are always additional questions to be answered.