Pseudomonas syringae pv. porri, the causative agent of bacterial blight in leek (Allium porrum), is increasingly frequent causing problems in leek cultivation. Because of the current lack of control measures, novel bacteriophages were isolated to control this pathogen using phage therapy. Five novel phages were isolated from infected fields in Flanders (vB_PsyM_KIL1, vB_PsyM_KIL2, vB_PsyM_KIL3, vB_PsyM_KIL4, and vB_PsyM_KIL5), and were complemented with one selected host range mutant phage (vB_PsyM_KIL3b). Genome analysis of the phages revealed genome sizes between 90 and 94 kb and an average GC-content of 44.8%. Phylogenomic networking classified them into a novel clade, named the “KIL-like viruses,” related to the Felixounalikevirus genus, together with phage phiPsa374 from P. syringae pv. actinidiae. In vitro characterization demonstrated the stability and lytic potential of these phages. Host range analysis confirmed heterogeneity within P. syringae pv. porri, leading to the development of a phage cocktail with a range that covers the entire set of 41 strains tested. Specific bio-assays demonstrated the in planta efficacy of phages vB_PsyM_KIL1, vB_PsyM_KIL2, vB_PsyM_KIL3, and vB_PsyM_KIL3b. In addition, two parallel field trial experiments on three locations using a phage cocktail of the six phages showed variable results. In one trial, symptom development was attenuated. These data suggest some potential for phage therapy in controlling bacterial blight of leek, pending optimization of formulation and application methods.
Pseudomonas syringae pv. porri causes bacterial leaf spot and blight of leek (Allium porrum) and is in wet crop seasons responsible for substantial losses. The local diversity within this pathogen in Flanders, Belgium, was investigated to obtain insights into its epidemiology. Therefore, symptomatic leek leaves were collected from 112 fields and bacteria were isolated. An oxidase negative, HR positive, fluorescent Pseudomonas was consistently recovered from the diseased tissues. Isolates were identified as P. syringae pv. porri by rpoD gene sequencing and by confirmation of pathogenicity in leek. Genomic profiles generated with BOX-PCR subdivided them into two groups, with one group containing 5 of the 37 analyzed strains. Those five isolates were all obtained in 2012 and the plant origins indicated seed transmitted infection. Draft genome sequences were produced for a P. syringae pv. porri strain from each BOX group and sequences of seven housekeeping genes were extracted for multi locus sequence analysis (MLSA). This resulted in the clustering of both P. syringae pv. porri strains with the P. syringae pv. oryzae strain 1_6 as did the whole genome sequence comparisons by ANI analysis. The P. syringae pv. porri isolates, designated LMG 28495 and LMG 28496, differed in a type III effector gene, HrpW, and in the number of mobile elements in the genome. Overall, the data demonstrate that two P. syringae pv. porri variants are present in symptomatic leek in Flanders which can be discriminated and possibly traced using a genomic profiling method such as BOX-PCR. Furthermore, the draft genome sequences of both strains will facilitate the development of sensitive and specific methods for early detection.
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