Phage biocontrol of bacterial leaf blight disease on welsh onion caused by xanthomonas axonopodis pv. Allii

Bacterial leaf blight, which is caused by Xanthomonas axonopodis pv. allii, annually causes significant yield losses to Welsh onion in many producing countries, including Vietnam. In this study, we isolated and characterized lytic phages ?16, ?17A and ?31, specific to X. axonopodis pv. allii and belonging to a new phage species and genus within the Autographiviridae, from four prov- inces in the Mekong Delta of Vietnam. Moreover, we evaluated their efficacy for the biocontrol of leaf blight in greenhouse and field conditions. When applying the three highly related phages indi- vidually or as a three-phage cocktail at 108 PFU/mL in greenhouse conditions, our results show that treatment with ?31 alone provides higher disease prevention than the two other phages or the phage cocktail. Furthermore, we compared phage concentrations from 105 to 108 and showed optimal dis- ease control at 107 and 108 PFU/mL. Finally, under field conditions, both phage ?31 alone and the phage cocktail treatments suppressed disease symptoms, which was comparable to the chemical bactericide oxolinic acid (Starner). Phage treatment also significantly improved yield, showing the potential of phage as a biocontrol strategy for managing leaf blight in Welsh onion.