Impact of Phage Therapy on <i>Pseudomonas syringae</i> pv. <i>syringae</i> and Plant Microbiome Dynamics Through Coevolution and Field Experiments

Bacteriophages (phages) are viruses that infect and lyse bacteria and have the potential for controlling bacterial diseases. Isolation of phages targeting the cherry pathogen Pseudomonas syringae pv. syringae (Pss) led to five distinct phage genotypes. Building on previous in vitro coevolution experiments, the coevolution of the five phages (individually and as a cocktail) with Pss on cherry leaves was conducted in glasshouse and field experiments. Phages effectively reduced Pss numbers on detached leaves, with no evidence of phage resistance emerging in the bacterial population. Field application of phages in a cherry orchard in Southeast England evaluated phage survival, viability and impact on bacterial populations and the microbial community. The bacterial population and phages persisted in the leaf and shoot environment as long as the bacterial host was present. In contrast to in vitro studies, the plant environment constrained the emergence of phage resistant Pss populations. Application of phage cocktail in the orchard did not affect the cherry leaf microbiome. These observations provide essential knowledge for using phage treatments to control bacterial diseases while minimising the impact on the plant microbiome, highlighting phages' potential to safely control bacterial diseases in trees.