Characterization of Tomato spotted wilt virus isolates that overcome the Sw-5 resistance gene in tomato and fitness assays

Abstract

Resistance-breaking (RB) isolates of Tomato spotted wilt virus (TSWV) that overcome the resistance conferred by the Sw-5 gene in tomato have had only a limited spread since they were first detected in north-eastern Spain in 2002. Symptom expression, homogeneity, stability and the transmission capacity of RB and non-resistance breaking (NRB) isolates were biologically compared. The fitness of both types of isolates infecting tomato plants was determined in competition assays. All TSWV isolates induced similar systemic symptoms in a wide range of plant species, except RB isolates in tomato carrying the Sw-5 resistance gene and pepper carrying the Tsw resistance gene. The mechanical transmission of RB isolates to tomato plants with the Sw-5 gene failed in some trials, although NRB isolates did not differ noticeably in transmission efficiency when tested with the thrips Frankliniella occidentalis. Biological clones from individual local lesions obtained by mechanically inoculating Nicotiana glutinosa in some TSWV field samples showed that they were biologically homogeneous. Mixed infections of RB and wilt-type isolates were not found. The RB isolates were relatively stable because no reversion to NRB isolates was seen after serial passages in susceptible tomato plants. In competition assays between RB and NRB isolates, after serial passages in susceptible tomato plants, the prevalence of a particular isolate was not related to its capacity to overcome Sw-5 gene resistance. The low spread of the RB isolates in Spain does not seem to be related to a loss of fitness in tomato plants or to differences in transmission capacity by thrips, but it could be related to the reduction of the selection pressure of RB isolates as consequence of the gradual replacement of susceptible tomato plants by resistant tomato plants by growers.