First report in Italy of a resistance breaking strain of Tomato spotted wilt virus infecting tomato cultivars carrying the Sw5 resistance gene

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 two 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 of F1 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 found 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% 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 first confirmed report of 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).