Aims: To positively select Pectobacterium atrosepticum (Pa) mutants with cell surface defects and to assess the impact of these mutations on phytopathogenesis. Methods and Results: Several phages were isolated from treated sewage effluent and were found to require bacterial lipopolysaccharide (LPS) for infection. Two strains with distinct mutations in LPS were obtained by transposon mutagenesis. Along with a third LPS mutant, these strains were characterized with respect to various virulence-associated phenotypes, including growth rate, motility and exoenzyme production, demonstrating that LPS mutations are pleiotropic. Two of the strains were deficient in the synthesis of the O-antigen portion of LPS, and both were less virulent than the wild type. A waaJ mutant, which has severe defects in LPS biosynthesis, was dramatically impaired in potato tuber rot assays. The infectivity of these novel phages on 32 additional strains of Pa was tested, showing that most Pa isolates were sensitive to the LPS-dependent phages. Conclusions: Native LPS is crucial for optimal growth, survival and virulence of Pa in vivo, but simultaneously renders such strains susceptible to phage infection. Significance and Impact of the Study: This work demonstrates the power of phages to select and identify the virulence determinants on the bacterial surface, and as potential biocontrol agents for Pa infections.