Acidovorax citrulli (Ac) causes bacterial fruit blotch (BFB) of cucurbits, a disease that represents a serious threat to melon and watermelon production worldwide. Despite the economic importance of BFB, there is a significant gap in the understanding of basic aspects of Ac-host interactions. Based on biochemical traits, two main groups of strains exist within Ac. The groups also differ in their association with cucurbits: while group I strains have been mainly isolated from melon and other non-watermelon cucurbits, most group II strains were isolated from watermelon. Ac requires a functional type III secretion system (T3SS) for pathogenicity. We hypothesize that differences in the repertoire of type III-secreted effectors (T3Es) between group I and II strains contribute to their distinguished host preferential association. Based on similarity to T3E genes of other phytopathogenic bacteria, we identified eleven putative T3E genes in the sequenced strain AAC00-1. The ORFs of these genes were sequenced from a set of 21 additional strains isolated from different geographic locations. Comparative analyses revealed that T3E genes cluster according to the group I/II classification. We are using marker exchange mutagenesis, virulence assays and heterologous expression to assess the role of individual effectors in virulence and host range determination in Ac. Bioinformatics approaches are also being employed to identify novel putative effectors.