Abstract:
Pseudomonas syringae pv. aptata and related strains are seed-borne pathogens that cause bacterial leaf spot of beet and chard. Management strategies include planting clean seed, cultural practices, and few chemical management options. Biological control agents could be a beneficial addition to the repertoire of management strategies for both conventional and organic production. Using a series of enrichment methods and semi-selective growth media, myxobacteria, gram-negative soil-dwelling bacterial predators, were isolated from soils with symptomatic beets and soils with non-host crops. 180 isolates were screened against three strains of Pseudomonas syringae through predation assays to determine the level of host specificity of the myxobacteria. It was hypothesized that myxobacteria isolated with enrichment steps would be better predators than those isolated with fewer or no enrichment steps. Predation of the three Pseudomonas syringae strains was inconsistent with enrichment at the soil level. Isolates were significantly better predators from enriched fields in one field pair, while better predators were isolated from non-enriched fields in a second field pair, and the third field pair did not have significantly better predators from either field.  Predation assay results indicate that soil selection may not influence of myxobacteria predation. Enrichment at all three steps of isolation (field, liquid culture, and solid medium) produced statistically better predators in two field pairs than myxobacteria isolated with no enrichment. Additional enrichment steps adding Pseudomonas syringae to a soil slurry and on an agar bait plate may encourage the isolation of myxobacteria that are better overall predators.