Identification of a new type of resistance breaking strain of tomato spotted wilt virus on tomato bearing the Sw-5b resistance gene

Sw-5b is a widely used resistance gene in tomato breeding to control tomato spotted wilt virus (TSWV). The NSm protein encoded by TSWV is identified as the avirulence (AVR) determinant in Sw-5b -mediated resistance. In the last decades, Sw-5b resistance breaking (RB) isolates were found and identified in many locations around the world. The resistance-breaking phenotype in all the previously verified TSWV Sw-5b RB strains is associated with the NSm C118Y or NSm T120N mutations. In the summer of 2022, a Sw-5b RB TSWV strain was recognized in a greenhouse in Hungary. In inoculation experiments this strain was able to infect tomato plants with the Sw-5b resistance gene. Molecular analysis of the NSm avirulence determinant revealed a single alteration in the NSm protein, D122G mutation was identified. To our knowledge, this is the first report to identify this amino acid alteration associated with resistance-breaking phenotype in Sw-5b resistant tomato plants.