Molecular characterization of resistance-breaking strains of Tomato spotted wilt virus (Tospovirus, Bunyaviridae) infecting resistant pepper carrying the Tsw geneMargaria P, Turina M Istituto di Virologia Vegetale, CNR, Torino 10135, Italy. Correspondence: m.turina@ivv.cnr.it Tomato spotted wilt virus (TSWV) (Tospovirus, Bunyaviridae) causes one of the most devastating diseases of pepper (Capsicum sp.) worldwide. The most effective and environmentally sound control strategy for this virus relies on the availability of resistant pepper cultivars. All known resistant cultivars possess a single dominant resistance gene, Tsw. Following TSWV infection, Tsw induces a hypersensitive-like reaction (HR), characterized by local necrotic lesions, that prevents the spread of the virus from the point of entry. Recently, naturally occurring resistance-breaking TSWV strains have been identified, causing major concern. We are utilising a collection of RB-TSWV strains to identify the specific genetic determinant that allows the virus to overcome the Tsw gene in Capsicum. As a reverse genetic approach is not feasible for minus-strand genome plant viruses such as TSWV, we set up a series of reassortment experiments between the wild-type strain Br01 and RB- TSWV strains. Our results confirm that the S RNA, which encodes both the nucleocapsid (N) and a nonstructural protein (NSs), carries the genetic determinant responsible for the breakdown of Tsw resistance. Sequence comparison between full-length S RNA segments of wild type and RB strains allows us to speculate about the specific genetic determinant of the ＾avirulence￣ factor.
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