Solanum pimpinellifolium exhibits complex genetic resistance to Pseudomonas syringae pv. tomato

<i>Pseudomonas syringae pv. tomato</i> (<i>Pst</i>) is the causal agent of bacterial speck disease in tomatoes. The <i>Pto/Prf</i> gene cluster from <i>Solanum pimpinellifolium</i> was introgressed into several modern tomato cultivars and provided protection against <i>Pst</i> race 0 strains for many decades. However, virulent <i>Pst</i> race 1 strains that evade <i>Pto</i>-mediated immunity now predominate in tomato-growing regions worldwide. Here we report the identification of resistance to a <i>Pst</i> race 1 strain (<i>Pst</i>19) in the wild tomato accession <i>S. pimpinellifolium</i> LA1589 (hereafter LA1589), using our rapid high-throughput seedling screen. LA1589 supports less bacterial growth than cultivars, and does not exhibit a hypersensitive response to <i>Pst</i>19. We tested an existing set of 87 Inbred Backcross Lines (IBLs) derived from a cross between susceptible <i>Solanum lycopersicum</i> E-6203 and <i>Solanum pimpinellifolium</i> LA1589 for resistance to <i>Pst</i>19. Using single-marker analysis, we identified three genomic regions associated with resistance. Bacterial growth assays on IBLs confirmed that these regions contribute to resistance <i>in planta</i>. We also mapped candidate genes associated with resistance in a cross between the <i>Solanum lycopersicum</i> var. <i>lycopersicum</i> cultivar Heinz BG-1706 and <i>S. pimpinellifolium</i> LA1589. By comparing candidates from the two mapping approaches, we were able to identify 3 QTL and 5 candidate genes in LA1589 for a role in resistance to <i>Pst</i>19. This work will assist in molecular marker-assisted breeding to protect tomato from bacterial speck disease.