| Summary: | Bacterial leaf blight, which is caused by <i>Xanthomonas axonopodis</i> pv. <i>allii</i>, annually causes significant yield losses to Welsh onion in many producing countries, including Vietnam. In this study, we isolated and characterized lytic phages Φ16, Φ17A and Φ31, specific to <i>X. axonopodis</i> pv. <i>allii</i> and belonging to a new phage species and genus within the <i>Autographiviridae</i>, from four provinces 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 individually or as a three-phage cocktail at 10<sup>8</sup> PFU/mL in greenhouse conditions, our results show that treatment with Φ31 alone provides higher disease prevention than the two other phages or the phage cocktail. Furthermore, we compared phage concentrations from 10<sup>5</sup> to 10<sup>8</sup> and showed optimal disease control at 10<sup>7</sup> and 10<sup>8</sup> PFU/mL. Finally, under field conditions, both phage Φ31 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.
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