Biological control of bacterial spot of tomato using hrp mutants of Xanthomonas campestris pv. vesicatoria

Xanthomonas campestris pv. vesicatoria strain 75-3 hrpG, hrpX, hrpF and hrpE1 mutants were evaluated for control of bacterial spot of tomato under both greenhouse and field conditions. In greenhouse studies, the 75-3 hrp mutants were spray-inoculated onto tomato foliage 48 h prior to inoculation of the wild-type pathogen, X. campestris pv. vesicatoria 75-3, and bacterial spot severity was assessed 10 days after pathogen inoculation. Statistically significant reductions in disease severity relative to the buffer treatment were provided by all four 75-3 hrp mutants. Unexpectedly, however, disease reductions differed significantly among the mutants with the 75-3S hrpG mutant consistently providing the greatest disease suppression (mean reduction in disease severity compared to the buffer treatment of not, vert, similar58%), followed by the hrpX mutant (mean not, vert, similar40%), the hrpF mutant (mean not, vert, similar30%), and the hrpE1 mutant, which provided the least control (mean not, vert, similar21%). Disease control efficacy of these mutants was evaluated under field conditions in Alabama (AL) and Florida (FL). Only the 75-3S hrpG mutant and the 75-3 hrpF mutant provided statistically significant reductions in disease severity compared to the pathogen-only control in all three field trials. In the two AL trials, the 75-3S hrpG mutant was significantly more effective in reducing bacterial spot severity than the previously most effective biological control agents Pseudomonas syringae Cit7 and Pseudomonas putida B56. In the two AL trials, the 75-3S hrpG was as effective as the plant activator acibenzolar-S-methyl in one experiment but less effective in the other experiment. Averaged across all three field trials the 75-3S hrpG mutant provided a mean reduction in foliar disease severity of not, vert, similar76%, compared to the mean of not, vert, similar29% for P. syringae Cit7 averaged across all previous field trials. This study is significant not only for the selection of a biological control agent superior to previously selected strains and comparable in efficacy to the plant activator acibenzolar-S-methyl, but also for the finding that not all hrp mutants of a given bacterial strain are equally effective as biological control agents of that pathogen.