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

In spring 2002, typical symptoms of Tomato spotted wilt virus (TSWV) occurred on c.30% of tomato plants cv. Bodar growing in a field in Barcelona province. When DNA extracted from these infected plants was amplified by polymerase chain reaction (PCR) and digested with the restriction enzyme MseI, it produced two bands characteristic of the Sw-5 gene, a dominant gene originating from tomato cv. Stevens that confers resistance to TSWV (Folkertsma et al., 1999). Enzyme-linked immunosorbent assay (ELISA) confirmed the presence of TSWV in the infected tomato plants. A virus isolate from a naturally-infected tomato plant, after three single lesion passages in Nicotiana glutinosa, induced symptoms typical of TSWV in 12 mechanically inoculated seedlings grown from the same seed lot as the field-grown tomato plants. The virus isolate, designated GRAU, was inoculated to nine plants of each of four commercial tomato hybrids (cvs Bodar, Bond, Lisboa and Maresme) that had previously shown resistance to different TSWV isolates (Aramburu & Rodriguez, 1999), and was later detected by ELISA in most inoculated plants of each hybrid. Systemic infection occurred a few days before local lesions appeared in inoculated leaves as a consequence of the typical hypersensitivity reaction caused by the Sw-5 gene.

The only previous report of a TSWV strain that infected tomato plants carrying the Sw-5 gene is that of the JF strain in South Africa in 1993, which however, did not become established and failed to spread beyond the original field (Thomson & van Zijl, 1996). The GRAU strain, by contrast, seems to be widespread in north-east Spain because five isolates from nearby pepper fields and tomato plants grown more than four kilometres away also broke the Sw-5 resistance.

Although further work is needed to characterise this GRAU strain, it is suitable for use in current screening programmes in the search for possible new sources of resistance to TSWV.