The ecological genetics ofPseudomonas syringaefrom kiwifruit leaves

SUMMARY Interactions between commensal microbes and invading pathogens are understudied, despite their likely effects on pathogen population structure and infection processes. We describe the population structure and genetic diversity of a broad range of co-occurring Pseudomonas syringae isolated from infected and uninfected kiwifruit during an outbreak of bleeding canker disease caused by P. syringae pv. actinidiae ( Psa ) in New Zealand. Overall population structure was clonal and affected by ecological factors including infection status and cultivar. Most isolates are members of a new clade in phylogroup 3 (PG3a), also present on kiwifruit leaves in China and Japan. Stability of the polymorphism between pathogenic Psa and commensal P. syringae PG3a isolated from the same leaf was tested using reciprocal invasion from rare assays in vitro and in planta. P. syringae G33C (PG3a) inhibited Psa NZ54, while the presence of Psa NZ54 enhanced the growth of P. syringae G33C. This effect could not be attributed to virulence activity encoded by the Type 3 secretion system of Psa . Together our data contribute toward the development of an ecological perspective on the genetic structure of pathogen populations. ORIGINALITY-SIGNIFICANT STATEMENT Bacterial pathogen populations are often studied with little consideration of co-occurring microbes and yet interactions between pathogens and commensals can affect both population structure and disease progression. A fine-scale sampling of commensals present on kiwifruit leaves during an outbreak of bleeding canker disease caused by P. syringae pv. actinidiae reveals a clonal population structure. A new clade of non-pathogenic P. syringae (PG3a) appears to be associated with kiwifruit on a global scale. The presence of PG3a on kiwifruit has significant effects on the outcome of infection by P. syringae pv. actinidiae . This emphasises the value of studying the effect of co-occurring bacteria on pathogen-plant interactions.