Bacteriophages are obligate parasites of bacteria characterized by the breadth of hosts that they can infect. This “host range” depends on the genotypes and morphologies of the phage and the bacterial host, but also on the environment in which they are interacting. Understanding phage host range is critical to predicting the impacts of these parasites in their natural host communities and their utility as therapeutic agents, but is also key to predicting how phages evolve and in doing so drive evolutionary change in their host populations, including through movement of genes among unrelated bacterial genomes. Here, we explore the drivers of phage infection and host range from the molecular underpinnings of the phage–host interaction to the ecological context in which they occur. We further evaluate the importance of intrinsic, transient, and environmental drivers shaping phage infection and replication, and discuss how each influences host range over evolutionary time. The host range of phages has great consequences in phage-based application strategies, as well as natural community dynamics, and we therefore highlight both recent developments and key open questions in the field as phage-based therapeutics come back into focus.

Summary: Bacterial canker is a major disease of Prunus species, such as cherry ( Prunus avium ). It is caused by Pseudomonas syringae pathovars, including P. syringae pv. syringae ( Pss ) and P. syringae pv. morsprunorum race 1 ( Psm1 ) and race 2 ( Psm2 ). Concerns over the environmental impact of, and the development of bacterial resistance to, traditional copper controls calls for new approaches to disease management. Bacteriophage‐based biocontrol could provide a sustainable and natural alternative approach to combat bacterial pathogens. Therefore, seventy phages were isolated from soil, leaf and bark of cherry trees in six locations in the south east of England. Subsequently, their host range was assessed against strains of Pss , Psm1 and Psm2 . While these phages lysed different Pss , Psm and some other P. syringae pathovar isolates, they did not infect beneficial bacteria such as Pseudomonas fluorescens . A subset of thirteen phages were further characterized by genome sequencing, revealing five distinct clades in which the phages could be clustered. No known toxins or lysogeny‐associated genes could be identified. Using bioassays, selected phages could effectively reduce disease progression in vivo , both individually and in cocktails, reinforcing their potential as biocontrol agents in agriculture.

Significance: Agrobacteria are plant-pathogenic bacteria that can deliver DNA to plant cells as part of their infection strategy. This property has been used for decades to generate transgenic plants and, more recently, to deliver gene-editing reagents to plant cells. Notwithstanding their importance for research and industry, laboratory strains have not been improved much over the years and several aspects of Agrobacterium biology and pathogenesis remain poorly understood. Here, we developed a CRISPR-mediated base-editing approach to efficiently modify the genome of Agrobacterium . We show that single-nucleotide changes can be introduced at targeted positions in both the Agrobacterium tumefaciens and Agrobacterium rhizogenes genomes. Whole-genome analysis of edited strains revealed only a limited number of unintentional mutations.

Summary: Xanthomonas campestris pv. campestris (Xcc) is a vascular pathogen that invades the xylem of Brassica crops. Current chemical and antibiotics‐based control measures for this bacterium are unsustainable and inefficient. After establishing a representative collection of Xcc strains, we isolated and characterized bacteriophages from two clades of phages to assess their potential in phage‐based biocontrol. The most promising phages, FoX2 and FoX6, specifically recognize (lipo) polysaccharides, associated with the wxc gene cluster, on the surface of the bacterial cell wall. Next, we determined and optimized the applicability of FoX2 and FoX6 in an array of complementary bioassays, ranging from seed decontamination to irrigation‐ and spray‐based applications. Here, an irrigation‐based application showed promising results. In a final proof‐of‐concept, a CaCl ₂ ‐formulated phage cocktail was shown to control the outbreak of Xcc in the open field. This comprehensive approach illustrates the potential of phage biocontrol of black rot disease in Brassica and serves as a reference for the broader implementation of phage biocontrol in integrated pest management strategies.

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 Φ16, Φ17A and Φ31, 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 Φ31 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 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.