Lytic bacteriophages (phages) are viruses that infect and kill bacteria, thus holding promise for treating bacterial diseases.

The isolation of phages that infect the cherry pathogenic bacterium, Pseudomonas syringae pv. syringae (Pss), has identified five genomically distinct phage genotypes. Each phage, along with a cocktail of all five phages, can effectively kill Pss, but phage resistant Pss rapidly emerge in vitro, rendering the phages ineffective.

Two experiments examined phage resistant mutants: a 66 h co-inoculation of Pss and the phages, and a 10-passage co-evolution experiment of phages and bacteria. Genome sequence analysis of phage-resistant mutants identified mutations in lipopolysaccharide (LPS) biosynthesis genes. The use of a five-phage cocktail also led to the emergence of LPS mutants. Curiously, the use of single phages led to a wider spectrum of mutations manifesting in the Pss population compared to the use of the five-phage cocktail.

Replicating these treatments in planta, in a controlled glasshouse and in a field orchard, enabled an assessment of whether phage-resistance occurs in a more natural setting. Coevolution of phage, individually and as a five-phage cocktail, with Pss in the cherry leaf environment revealed no evidence of phage resistance emergence in the bacterial population. The Pss population could be reduced in planta by the application of phages. The field trial in a cherry orchard also revealed that the phages could persist in the leaf environment as long as the bacterial host was present.

These observations suggest that, in contrast to in vitro studies, the plant environment plays an important role in constraining the emergence of phage-resistant LPS mutants. Together, these observations provide the key knowledge required to complete the rational design of phage treatments of bacterial diseases in plants.

It raises the possibility that phage treatments can be used safely for controlling bacterial diseases in trees.