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

Bacterial spot of the pepper (BSP) and the tomato (BST) caused by multiple Xanthomonas spp. remains a major constraint to production of both crops worldwide. The widespread breakdown of dominant resistance genes, such as Bs2, due to the emergence of virulent races, like Xanthomonas euvesicatoria P6, has underscored the need for more durable, non-race-specific resistance. The recessive genes, bs5; bs6; and bs8, 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 Xanthomonas species, X. euvesicatoria (Xe), X. hortorum pv. gardneri (Xhg), and X. perforans (Xp). 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 bs5, particularly when combined with either bs6 or bs8, significantly reduces bacterial growth and disease severity across all three Xanthomonas species. The triple-stacked line (ECW568 (i.e., bs5, bs6, and bs8)) consistently displayed the strongest suppression of pathogen proliferation and symptom development. By contrast, bs6 and bs8, alone or in combination, were largely ineffective. In some cases, combining bs6 with bs8 was less effective than bs8 alone. These findings reinforce the central role of bs5 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, X. perforans, to grow in planta, 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.