Tailored phage cocktail with resistance management for controlling Dickeya solani in potatoes

<i>Dickeya solani</i>, the causal agent of soft rot and blackleg diseases in potatoes, is responsible for substantial economic losses worldwide, with no effective environmentally acceptable chemical control measures available. Bacteriophage-based biocontrol represents an environmentally sustainable alternative to chemical treatments; however, the rapid emergence of resistant bacterial strains remains a major limitation to their efficacy. In this study, we developed a phage cocktail consisting of three bacteriophages (LMST, PDS1, and PDS2) through a rational, sequential isolation strategy to effectively target <i>D. solani</i> and its emerging phage-resistant strains. High-throughput sequencing and genomic analyses revealed that LMST, PDS1, and PDS2 possess double-stranded DNA genomes of 153,878, 254,449, and 133,665 bp, respectively, and lack genes associated with lysogeny, virulence, or antibiotic resistance. Transmission electron microscopy (TEM) analyses revealed a myovirus-like morphology and confirmed efficient replication of the phages on <i>D. solani</i> cells. Host range assays demonstrated that the phages were specific to <i>D. solani</i> strains, with LMST additionally infecting <i>D. fangzhongdai</i>, while thermal stability tests indicated that all three phages maintained infectivity across a broad temperature range (-20 °C to 60 °C), collectively supporting their potential as suitable biocontrol agents. Phylogenetic analyses classified LMST and PDS1 within the genera <i>Limestonevirus</i> and <i>Salmondvirus</i>, respectively, whereas PDS2 represented a highly divergent lineage, expanding the anti-<i>Dickeya</i> phage repertoire. Turbidity assays revealed that the phage cocktail more effectively suppresses <i>D. solani</i> growth than individual phages and substantially reduces the development of phage resistance. Furthermore, preventive and curative applications of the phage cocktail resulted in 68.75 and 59% reductions in <i>D. solani</i>-induced symptom severity in potato tubers, respectively. This study demonstrates that a rationally designed phage cocktail can significantly control <i>D. solani</i> infections while mitigating the emergence of phage-resistant strains.