Quantifying the effect of bacterial antagonists on the relationship between phyllosphere population sizes of Xanthomonas translucens pv. translucens and subsequent bacterial leaf streak severity on wheat seedlings

The influence of bacterial antagonists on the relationship between leaf-associated population sizes of Xanthomonas translucens pv. translucens (Xtt) and subsequent bacterial leaf streak (BLS) severity on wheat seedlings was investigated when antagonists were challenged with the pathogen. Bacterial antagonists (10⁸ CFU/ml) were inoculated individually onto 10-day-old wheat seedlings maintained in a growth chamber. Antagonists were challenged 48 h later with a single inoculum density (10⁷ CFU/ml) or with a range of Xtt inoculum densities (0-10⁸ CFU/ml). Some antagonists significantly decreased both Xtt population sizes and BLS severity compared to a pathogen-only control over a range of pathogen inoculum densities, although statistically significant reduction in pathogen population size was not always coupled with significant disease reduction. There was no consistent effect of pathogen inoculum density on antagonist effectiveness. Specifically, the influence of antagonists on Xtt population size and BLS severity was variable among experiments at the same pathogen inoculum density and among pathogen inoculum densities within the same experiment. In all cases, the relationship between leaf-associated pathogen population size and subsequent BLS severity was not significantly altered by antagonists. Thus, bacterial antagonists studied here influence disease severity primarily by influencing the size of the pathogen population prior to disease onset.