Abstract Based on the observation that A cidovorax citrulli switches from saprobic to pathogenic growth for seed‐to‐seedling transmission of bacterial fruit blotch of cucurbits ( BFB ), we hypothesized that quorum sensing ( QS ) was involved in the regulation of this process. Using aacI ( luxI homologue) and aacR ( luxR homologue) mutants of AAC 00‐1, we investigated the role of QS in watermelon seed colonization and seed‐to‐seedling transmission of BFB . aacR and aacI mutants of AAC 00‐1 colonized germinating watermelon seed at wild‐type levels; however, BFB seed‐to‐seedling transmission was affected in a cell density‐dependent manner. There were no significant differences in BFB seedling transmission between watermelon seed infiltrated with approximately 1 × 10 6 CFU of AAC 00‐1, the aacR or aacI deletion mutants (95.2, 94.9 and 98.3% BFB incidence, respectively). In contrast, when seed inoculum was reduced to approximately 1 × 10 3 CFU /seed, BFB seed‐to‐seedling transmission declined to 34.3% for the aacI mutant, which was significantly less than the wild type (78.6%). Interestingly, BFB seed‐to‐seedling transmission for the aacR mutant was not significantly different to the wild‐type strain. These data suggest that QS plays a role in regulation of genes involved in seed‐to‐seedling transmission of BFB .