P-020 - Virulence and aggressiveness of diverse Pseudomonas syringae pv. aptata strains on resistant and susceptible table beet and Swiss chard cultivars

Pseudomonas syringae pv. aptata is a genetically diverse group of bacterial pathogens that vary in virulence and aggressiveness on chenopods and cucurbit hosts. The pathogen causes bacterial leaf spot disease on table beet and Swiss chard globally, significantly reducing crop quality and yield. Previous studies demonstrated resistance in table beet and Swiss chard to a single pathogen genotype. We assessed disease severity on 14 germplasm accessions of table beet and Swiss chard inoculated with 10 genotypes of P. syringae pv. aptata in replicated greenhouse trials. One-month-old seedlings were inoculated on the abaxial leaf surface with a bacterial suspension of approximately 1 × 10⁸ CFU/mL. Disease severity was assessed weekly for 4 weeks using a 0–7 rating scale. There was substantial variation in P. syringae pv. aptata strains for aggressiveness and virulence on table beet and Swiss chard. Two genotypes of P. syringae pv. aptata were the most aggressive on all cultivars. A third genotype of P. syringae pv. aptata was pathogenic on table beet but non-pathogenic on Swiss chard. When the two most aggressive genotypes were inoculated on Touchstone Gold, W452C, Rainbow, and Fordhook Giant there was significantly lower disease severity, whereas Pablo, Red Ace, Bresko, Detroit Dark Red, Manolo, Ruby Queen, W451C, and Silverado exhibited higher disease severity. These findings highlight the pathogenic variability of P. syringae pv. aptata strains across different plant genotypes and provide essential information for advancing the study of molecular host-pathogen interactions.