Resistance breaking strain of Tomato spotted wilt virus (Tospovirus; Bunyaviridae) on resistant pepper cultivars in Almeria (Spain)

Tomato spotted wilt virus (TSWV) is endemic in the Almeria region of Spain, where the local agro-climatic conditions allow Frankliniella occidentalis, the main TSWV vector species, continuous generations throughout the year. So far the only efficient method to control for this virus in pepper crops has relied on the Tsw resistance gene. During autumn 2003, some of the pepper varieties previously shown to be resistant to TSWV displayed the symptoms commonly associated with TSWV infection. A serological test carried out in the field with lateral flow devices prepared in our institute was positive for TSWV. Sap from infected plants was mechanically inoculated to Capsicum chinensis resistant accession #152225 and no necrotic spots were observed on the inoculated leaves, whereas systemic symptoms were observed four days post-inoculation on the newly emerged un-inoculated leaves. ELISA (Roggero et al., 2002) confirmed the systemic infection of C. chinensis with TSWV. Type strains Br01 and p105 (Roggero et al., 2002) were also inoculated to C. chinensis and produced the typical necrotic spots on the inoculated leaf, while the newly emerging leaves were symptomless and tested negative for TSWV. ELISA tests were negative for Impatiens necrotic spot virus and Tomato chlorotic spot virus, the other tospoviruses able to systemically infect C. chinensis (Roggero et al., 2002). After three passages by mechanical inoculation through single local lesions on Nicotiana tabacum, the virus was still able to systemically infect C. chinensis. RT-PCR was carried out on upper un-inoculated leaves in order to amplify regions of the S segment of the TSWV genome (Qiu et al., 1998). An 800 bp PCR product corresponding to the nucleocapsid gene was amplified, cloned and sequenced (GenBank accession N° AY598831). Comparison of the nucleocapsid amino acid sequence with homologue sequences of TSWV isolates in GenBank showed a highest identity (>99%) to an isolate from Almeria described in 1995 (Accession N° X94550).

TSWV resistant-breaking strains have previously been reported from Italy in Capsicum species carrying the Tsw gene (Roggero et al., 2002) and from Spain in tomato species carrying the Sw5 gene (Aramburu & Marti, 2003). To our knowledge this is the first report of TSWV strains breaking the resistance provided by the Tsw gene introgressed in Capsicum spp under field conditions in Spain.