March 29, 2005
Source: British Society for Plant Pathology, New Disease Reports, Vol. 10 [edited]
<http://www.bspp.org.uk/ndr/jan2005/2005-08.asp>

First report in Italy of a resistance breaking strain of Tomato spotted wilt virus infecting tomato cultivars carrying the Sw5 resistance gene
M. Ciuffo, Istitituto di Virologia Vegetale, CNR- Strada delle Cacce 73,10135 Torino, Italy; M.M. Finetti-Sialer, Dipartimento di Protezione delle Piante e Microbiologia applicata, Universita degli Studi, and Sezione di Bari dell'Istituto di Virologia Vegetale, CNR, Via G. Amendola 165/1, I-70126 Bari, Italy; D. Gallitelli (as for Finetti-Sialer); and M. Turina (as for Ciuffo). Accepted for publication 25 Jan 2005.

In 2004, leaf samples of a processing tomato variety carrying the Sw5 resistance gene to Tomato spotted wilt virus (TSWV) were collected from field grown plants in Mesagne (BR), Apulia (Southern Italy).

Leaf extracts were tested by lateral flow and/or ELISA (Roggero et al. 2002) for TSWV, Impatiens necrotic spot virus (INSV), Cucumber mosaic virus (CMV), Tobacco mosaic virus (TMV) and Potato virus Y (PVY). Leaf dips were
also observed with a transmission electron microscope and 2 of these samples were inoculated mechanically on to a set of test plants. Only TSWV was detected in all the field samples tested.

One of the TSWV field isolates, T992, was investigated for the ability to overcome the resistance gene Sw5. T992 was mechanically inoculated onto 20 plants of each F1 hybrid tomato cultivar carrying the Sw5 gene (Cvs Donald,
York, Rovente, Valiente, Hermes, UGX 9233, Diaz, ISI 19343, Es 5302, Scipio and Herdon); cv Marmande was used as a susceptible control. Another set ofF1 hybrids was mechanically inoculated with strain p105; a wild-type
strain of TSWV (Roggero et al., 2002). Tomato plantlets were inoculated at the 4-5 true leaf stage and systemic infection was tested 20 days post-inoculation using ELISA.

All hybrids carrying the Sw5 gene were uninfected systemically by strain p105, with the exception of 4 plants of F1 UGX 9233. In contrast, T992 systemically infected all F1 hybrids tested. Marmande was infected systemically by both p105 and T992. These results showed that strain T992 can overcome Sw5 gene resistance.

Portions of the S and M genome of T992 were cloned and sequenced, and the data deposited in GenBank (accession numbers AY848922 and AY848921, respectively). Using a 560-bp fragment corresponding to part of the non-structural protein of the middle segment (NSm), the closest identity was to the SAN-1 isolate (AY124966), previously described from Apulia (Finetti-Sialer et al., 2002). Using a 780 bp N fragment the closest identity was to the LE98-527 strain from Bulgaria (99.6 percent identity at the nucleotide level) (Heinze et al., 2001). T992 was classified as an A-type isolate according to the MaeI restriction pattern used previously (Finetti-Sialer et al., 2002).

To our knowledge this is the 1st confirmed report of resistance-breaking (RB) strains of TSWV in tomato in Italy. Previously, RB strains of TSWV on tomato in Europe were identified only in Spain (Aramburu & Marti, 2003).

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[This is the 2nd instance of a resistance-breaking strain of TSWV reported in Europe in the past 3 years. 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). Resistance-breaking strains can spread, resulting in increased losses in fruit production and quality. Similarly,
in Australia, 3 of 1386 TSWV-infected _Capsicum chinense_ accessions also expressed systemic symptoms when manually inoculated with specific TSWV strains. Resistance-breaking strains are a dangerous factor in crop
production systems.

Links:
<http://www.ingentaconnect.com/content/aab/annals/2003/00000142/00000002/art00011>
<http://www.seedquest.com/News/releases/2004/june/8898.htm>  
<http://www.apsnet.org/online/feature/tospovirus>
- Mod.DH]