| Summary: | Tomato yellow leaf curl virus (TYLCV) is a disease that is damaging to tomato production worldwide. Resistance to TYLCV has been intensively investigated, and single resistance genes such as <i>Ty-1</i> have been widely deployed in breeding programs. However, resistance-breaking incidences are frequently reported, and achieving durable resistance against TYLCV in the field is important. In this study, gene-specific markers for <i>Ty-2</i> and <i>ty-5</i>, and closely-linked markers for <i>Ty-4</i> were developed and applied to distinguish TYLCV resistance in various tomato genotypes. Quantitative infectivity assays using both natural infection in the field and artificial inoculation utilizing infectious TYLCV clones in a growth chamber were optimized and performed to investigate the individual and cumulative levels of resistance. We confirmed that <i>Ty-2</i> could also be an effective source of resistance for TYLCV control, together with <i>Ty-1</i>. Improvement of resistance as a result of gene-pyramiding was speculated, and breeding lines including both <i>Ty-1</i> and <i>Ty-2</i> showed the strongest resistance in both field and artificial infections.
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