Bacteriophages infecting Acidovorax citrulli, the causal agent of bacterial fruit blotch, have been proven to be effective for the prevention and control of this disease. However, the occurrence of bacteriophage-resistant bacteria is one of hurdles in phage biocontrol and the understanding of phage resistance in this bacterium is an essential step. In this study, we aim to investigate possible phage resistance of A. citrulli and relation- ship between phage resistance and pathogenicity, and to isolate and characterize the genes involved in these phenomena. A phage-resistant and less-virulent mutant named as AC-17-G1 was isolated among 3,264 A. citrul- li Tn5 mutants through serial spot assays and plaque assays followed by pathogenicity test using seed coat- ing method. The mutant has the integrated Tn5 in the middle of a cupin protein gene. This mutant recovered its pathogenicity and phage sensitivity by complemen- tation with corresponding wild-type gene. Site-directed mutation of this gene from wild-type by CRISPR/Cas9 system resulted in the loss of pathogenicity and acquisi- tion of phage resistance. The growth of AC-17-G1 in King’s B medium was much less than the wild-type, but the growth turned into normal in the medium supple- mented with D-mannose 6-phosphate or D-fructose 6-phosphate indicating the cupin protein functions as a phosphomannos isomerase. Sodium dodecyl sulfa analysis of lipopolysaccharide (LPS) extracted from the mutant was smaller than that from wild-type. All these data suggest that the cupin protein is a phosphoman- nos isomerase involved in LPS synthesis, and LPS is an important determinant of pathogenicity and phage susceptibility of A. citrulli.