Bacteriophage-Mediated Reduction of Bacterial Speck on Tomato Seedlings

One crucial first step in bacteriophage therapy is choosing a phage to apply, which involves screening for effectiveness in a meaningful way. Increasingly, research suggests that in vitro tests of phage-mediated bacterial lysis poorly translate to in planta effectiveness. We tested a seedling-based method for rapidly screening phage effectiveness in vivo. In three trials, phages were prophylactically applied to tomato seedlings in sterile conical tubes before flooding with the bacterial pathogen Pseudomonas syringae pv. tomato DC3000. We recorded seedling disease progression and quantified endpoint bacteria and phage densities. Phages replicated in all trials, but reduction of disease symptoms and endpoint P. syringae density varied across trials with different application densities. This resource-efficient method rapidly identified an effective phage and application density to mitigate disease on seedlings. We propose that this method could be used to screen candidate phages before testing in agricultural conditions.