UC Riverside

Pseudomonas species are Gram-negative bacterial pathogens that affect a wide range of economically important crops, including almond. Increasing resistance of Pseudomonas syringae to conventional management strategies highlights the need for alternative disease control options. In this study, we isolated and characterized three lytic bacteriophages, including vB_PsyP_Mobley, vB_PsyP_Plaza, and vB_PsyP_Mission, targeting almond-infecting Pseudomonas strains. Host-range analysis across 36 isolates revealed partially overlapping infectivity profiles, with strongest activity against phylogroup 2 (PG2) almond-associated P. syringae pv. syringae and reduced infectivity against phylogenetically distinct isolates, including PG7 P. viridiflava and other crop-associated pathovars. Efficiency-of-plating assays quantitatively supported these host-range patterns. Plaque morphology and transmission electron microscopy demonstrated icosahedral capsids with short, non-contractile tails consistent with tailed dsDNA bacteriophages of the class Caudoviricetes, with vB_PsyP_Mission producing halo-associated plaques consistent with predicted extracellular polysaccharide-modifying proteins. Genomic analyses revealed compact (~ 40 kb) genomes lacking integrases, tRNAs, lysogeny-associated genes, and known virulence or antimicrobial resistance determinants. Comparative genomics showed that vB_PsyP_Plaza and vB_PsyP_Mission likely represent novel isolates within previously described species-level groups, whereas vB_PsyP_Mobley is more genomically divergent. Collectively, these results define the phenotypic and genomic characteristics of three related but distinguishable Pseudomonas phages and provide a basis for future in planta evaluation of their potential utility against almond bacterial blast.