Characterisation and In Planta Activity of Bacteriophages That Infect the Key Bacterial Species Associated With Acute Oak Decline

Acute oak decline (AOD) is a complex disease of oak trees (Quercus spp.) which results in bleeding stem cankers and potential tree mortality. A polymicrobial complex is implicated in causing AOD, of which the most commonly isolated species are Brenneria goodwinii and Gibbsiella quercinecans. Bacteriophages (phages), natural predators of bacteria, can act as biocontrol agents that reduce bacterial infections in trees. In this study, the first phages that lyse B. goodwinii and G. quercinecans, BREN6 and GIB1 phages respectively, were isolated from AOD-impacted trees. Genomic analyses and transmission electron microscopy showed that BREN6 is a novel phage type with siphovirus morphology, whereas GIB1 possesses characteristics typical of N4-like phages. While in vitro assays showed that each phage could effectively reduce their hosts' population growth for at least 24 h, application of the phages to either single or dual species communities of B. goodwinii and G. quercinecans in the stems of oak saplings showed that the phages had a minimal impact on their host populations, and in some trees actually resulted in higher bacterial populations. Our data show that the application of these phages does not alter the in planta population dynamics between B. goodwinii and G. quercinecans, and emphasises the importance of assessing phage effectiveness within natural settings.