Pyramiding Recessive Resistance Genes Enhances Bacterial Leaf Spot Resistance in Peppers by Suppressing In Planta Bacterial Growth

Bacterial spot of the pepper (BSP) and the tomato (BST) caused by multiple <i>Xanthomonas</i> spp. remains a major constraint to production of both crops worldwide. The widespread breakdown of dominant resistance genes, such as <i>Bs2</i>, due to the emergence of virulent races, like <i>Xanthomonas euvesicatoria</i> P6, has underscored the need for more durable, non-race-specific resistance. The recessive genes, <i>bs5</i>; <i>bs6</i>; and <i>bs8</i>, have emerged as promising alternatives, conferring broad-spectrum resistance without triggering a hypersensitive response. In this study, we systematically evaluated the individual and combinatorial effects of these three recessive resistance genes against three <i>Xanthomonas</i> species, <i>X. euvesicatoria</i> (<i>Xe</i>), <i>X. hortorum</i> pv. <i>gardneri</i> (<i>Xhg</i>), and <i>X. perforans</i> (<i>Xp</i>). Using near-isogenic lines (NILs) developed in the susceptible Early Calwonder (ECW) background, we assessed the in planta bacterial population growth and symptom development across a panel of eight genotypes carrying different gene combinations. Our results demonstrate that <i>bs5</i>, particularly when combined with either <i>bs6</i> or <i>bs8</i>, significantly reduces bacterial growth and disease severity across all three <i>Xanthomonas</i> species. The triple-stacked line (ECW568 (i.e., <i>bs5</i>, <i>bs6</i>, and <i>bs8</i>)) consistently displayed the strongest suppression of pathogen proliferation and symptom development. By contrast, <i>bs6</i> and <i>bs8</i>, alone or in combination, were largely ineffective. In some cases, combining <i>bs6</i> with <i>bs8</i> was less effective than <i>bs8</i> alone. These findings reinforce the central role of <i>bs5</i> in conferring quantitative resistance and highlight the additive benefit of pyramiding recessive resistance genes. Furthermore, we have demonstrated that these recessive resistance genes are effective in limiting the ability of the emerging pathogen, <i>X. perforans</i>, to grow <i>in planta</i>, and thus are predicted to offer a high level of resistance in the field. Our work provides key insights for breeding durable, broad-spectrum resistance into commercial pepper cultivars and offers a framework for integrated disease management strategies in the face of rapidly evolving bacterial pathogens.