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POSTERS: Late Breaking I

P-639 - Diversity of seedborne Pseudomonas syringae pv. aptata isolates associated with bacterial leaf spot of table beet and Swiss chard

Monday, August 4, 2025
4:30 PM - 5:15 PM HST
Location: Exhibit Hall 3
Abstract Text:

Pseudomonas syringae pv. aptata is a seedborne pathogen causing bacterial leaf spot on a variety of crops. To investigate the diversity of seedborne P. syringae pv. aptata associated with beet and Swiss chard (Beta vulgaris), P. syringae strains were isolated onto semi-selective KBBC agar medium from samples of 66 table beet and 43 Swiss chard seed lots grown in the US (66), New Zealand (38), or France (5). Pathogenicity of 1,442 seed isolates was determined on 2-week-old plants of ‘Red Ace’ table beet and ‘Silverado' Swiss chard, and fluorescence assessed on KMB agar medium. The genomes of 210 pathogenic and 36 non-pathogenic isolates were sequenced, and a 19-housekeeping-gene-phylogeny generated to compare with pathotype strains of P. syringae pathovars in phylogroup 2 and formerly characterized bacterial leaf spot pathogens. Of the 246 isolates, 117 (48%) were highly virulent and in phylogroup 2b of P. syringae sensu stricto. Of these, 47% were non-fluorescent and in genotype MLST1, and 19% were fluorescent isolates in genotype MLST3. The remaining 34% of isolates were less virulent and belonged to a diversity of genotypes in phylogroup 2b. To confirm Koch’s postulates, we inoculated 4-week-old ‘Red Ace’ and ‘Silverado’ plants with a subset of isolates. Identity of reisolates was confirmed by rep-PCR assay. All MLST1 and 3 isolates were highly virulent on ‘Red Ace’ but had moderate to low virulence on ‘Silverado’, regardless of host of origin. The results demonstrated two highly virulent genotypes of P. syringae pv. aptata isolated from table beet and Swiss chard seed.