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June 4, 2004
From: British Soc. Plant Pathol., New Disease Reports [edited]

Resistance breaking strain of tomato spotted wilt virus (tospovirus; Bunyaviridae) on resistant pepper cultivars in Almeria, Spain
P. Margaria, M. Ciuffo and M. Turina <m.turina@ivv.cnr.it>, Istituto di Virologia Vegetale, CNR- Strada delle Cacce 73, 10135 Torino Italy.
Accepted for publication 28/5/04

Tomato spotted wilt tospovirus (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 TSWV 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 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_ [Cc] resistant accession #152225, and no necrotic spots were observed on the inoculated leaves, whereas systemic symptoms were observed 4 days
post-inoculation on newly emerged un-inoculated leaves. ELISA (Roggero et al., 2002) confirmed the systemic infection of Cc with TSWV. Type strains Br01 and p105 (Roggero et al., 2002) were also inoculated to Cc and produced typical necrotic spots on the inoculated leaf, while 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 Cc (Roggero et al., 2002). After 3 passages by mechanical inoculation through single local lesions on _Nicotiana tabacum_, the virus was still able to systemically infect Cc. 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 No. AY598831). Comparison of the nucleocapsid amino acid sequence with homologue sequences of TSWV isolates in GenBank showed highest identity (>99 percent) to an isolate from Almeria described in 1995 (Accession No. X94550).

TSWV resistance-breaking strains have previously been reported from Italy in _Capsicum_ spp. 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 1st report of TSWV strains breaking the resistance provided by the Tsw gene introgressed in _Capsicum_ spp. under field conditions in Spain.

References

Aramburu J, Marti M, 2003. The occurrence in north-east Spain of a variant of tomato spotted wilt virus (TSWV) that breaks resistance in tomato (_Lycopersicon esculentum_) containing the Sw-5 gene. Plant Pathology 52, 407.

Roggero P, Masenga V, Tavella L, 2002. Field isolates of Tomato spotted wilt virus overcoming resistance in pepper and their spread to other hosts in Italy. Plant Disease 86, 950-954.

Jahn M, Paran I, Hoffmann K, Radwanski ER, Livingstone KD, Grube RC, Aftergoot E, Lapidot M, Moyer J, 2000. Genetic mapping of the Tsw locus for resistance to the Tospovirus tomato spotted wilt virus in Capsicum spp. and its relationship to the Sw-5 gene for resistance to the same pathogen in tomato. Molecular Plant-Microbe Interactions 13, 673-682.

Qiu WP, Geske SM, Hickey CM, Moyer JW, 1998. Tomato spotted wilt Tospovirus genome reassortment and genome segment-specific adaptation. Virology 244, 186-194.

[Tospoviruses, mainly Tomato spotted wilt virus (TSWV), constitute a severe threat to _Capsicum_ cultivation worldwide. TSWV is regarded as one of the 10 most economically destructive plant pathogens. Resistance to TSWV but not to other tospoviruses, based on a hypersensitive reaction, has been found only in accessions of Cc 'PI152225' and 'PI159236'. The resistance, carried by the dominant gene Tsw, is broken at high temperatures and depends on plant age, with young plants being more susceptible. The Tsw gene has been introduced into several commercial sweet and hot pepper cultivars with good agronomic performance. Resistance-breaking strains of TSWV systemically infecting resistant plants have been found under experimental conditions and in the field.

Reference (Roggero et al.,) online:
<http://www.ento.csiro.au/thysanoptera/Symposium/Section4/15-Roggero-et-al.pdf> - Mod.DH]