Abstract: Bacterial leaf blight, which is caused by Xanthomonas axonopodis pv. allii, annually causes
significant yield losses toWelsh onion in many producing countries, including Vietnam. In this study,
we isolated and characterized lytic phages F16, F17A and F31, specific to X. axonopodis pv. allii and
belonging to a new phage species and genus within the Autographiviridae, 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 108 PFU/mL in greenhouse conditions, our results show that treatment
with F31 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 disease control
at 107 and 108 PFU/mL. Finally, under field conditions, both phage F31 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.

Keywords: bacterial leaf blight; bacteriophages; Welsh onion; Xanthomonas axonopodis pv. allii;
biocontrol