Pseudomonas azotoformans and Pseudomonas putida: Novel kiwifruit-native biological control agents against Pseudomonas syringae pv. actinidiae

Pseudomonas syringae pv. actinidiae (Psa), the etiological agent of the bacterial canker in Actinidia plants, remains the main threat to kiwifruit orchards worldwide. Though environment-friendly disease control methods based on biological control agents (BCAs) represent a promising alternative to xenobiotic pesticides, their efficacy in field conditions has often resulted erratic. The selection of beneficial microorganisms directly from the phyllosphere of the host plant is a promising approach to overcome this limitation since it ensures the adaptation of the isolates to the environment in which they are going to be applied. This work reports the screening of the kiwifruit epiphytic bacterial community from three Psa infected orchards in Portugal to identify potential bacterial BCAs capable of inhibiting Psa growth or interfering with its virulence. Strains of Pseudomonas putida and Pseudomonas azotoformans efficiently antagonized Psa on flowers and leaves and colonized all susceptible organs with high surviving rates in glasshouse conditions. In vitro metabolic analysis together with genome sequencing and annotation revealed siderophore production, in particular pyoverdine, which may limit iron availability to the pathogen. Moreover, several biosynthetic gene clusters of secondary metabolites, were predicted in the genome of both strains, including non-ribosomal peptides, and the bacteriocin pyocin was predicted in the genome of BG1. Overall, these results open new perspectives to develop commercial products for Psa management based on kiwifruit-native bacteria, well-adapted to common orchard management practices, with a high efficiency of host plant colonization, at Psa-conducive temperatures, and point out possible mechanisms of action for these two BCA candidates, supporting further steps to assess their effectiveness in orchard conditions.