Erwinia tracheiphila is among the few plant pathogenic species of Erwinia retained in this genus after several other species have been re-assigned to new genera. E. tracheiphila strains fall into clades that exhibit differences in host-specific virulence based on their ability to rapidly wilt muskmelon but not wilt squash (Et-melo clade) or to rapidly wilt squash and less rapidly wilt muskmelon (Et-C1 and Et-C2 clades). We characterized whole genome sequences and physiological traits of 24 E. tracheiphila strains in two of these clades, leading us to propose that the Et-melo and Et-C1 clades be delineated as E. tracheiphila subsp. tracheiphila and E. tracheiphila subsp. lentilata, respectively. Here, we explored how the genetic basis of E. tracheiphila sub-speciation is associated with host-specific virulence. We identified the distribution of the type III secreted effector (T3SE) repertoire of two host-specific strains, SCR3 (Et-melo) and BHKY (Et-C1), and characterized the expression profile of selected effectors in planta using RT-qPCR. We explored the role of one highly expressed effector, Eop1, as a host-specificity candidate, and found that loss of eop1 did not alter the virulence of two Et-melo strains or an Et-C1 strain on their respective hosts. However, over-expression of eop1 from an Et-melo strain in an Et-C1 strain increased its virulence on muskmelon, but not on squash, indicating that Eop1 functions as a host-specific virulence factor. The expression profiling in planta also highlighted poor expression of a key T3SS pilus gene, hrpA, in Et-melo in squash. We demonstrated experimentally that over-expression of hrpA in an Et-melo strain enabled this strain to infect squash, demonstrating that host-specific hrpA expression is another factor contributing to host-specific virulence in E. tracheiphila. We also examined the role of the effector DspE in pathogenicity, and found that loss of dspE from Et-melo strains significantly reduced, but did not eliminate, virulence on muskmelon, whereas loss of dspE from Et-C1 strains resulted in a complete loss of pathogenicity on squash and muskmelon. Thus, DspE has distinct roles in the two E. tracheiphila clades. This work represents the first characterization of the major molecular drivers of host specificity and pathogenicity among E. tracheiphila strains. Finally, using pyramided mutant analysis, we demonstrated that, consistent with their expression profiles in planta, effectors DspE, OspG, Eop1 and AvrB4 contributed to the virulence of Et-melo on muskmelon, whereas DspE, OspG, Eop1, HopL1 and HopO1 contributed to the virulence of Et-C1 on squash and muskmelon, with highly overlapping functions among many of the effectors. This work thus illustrates the contribution of distinct sets of effectors to the virulence of E. tracheiphila strains on their hosts. This work sets the foundation for a better understanding of the molecular interaction between the E. tracheiphila pathosystem and its cucurbit hosts which may help guide mechanistic approaches that could lead to the development of durable resistant plant cultivars.