Diseases remain a yield-limiting factor for crops despite the availability of control measures for many pathogens. Indigenous soil microorganisms can suppress some plant pathogens, yet there is little systematic information on the effects of cropping systems on disease-suppressive populations in soil. Streptomyces have been associated with suppression of plant diseases in several naturally occurring disease-suppressive soils. Pathogen-suppressive activity of Streptomyces communities is correlated with higher bacterial densities and with inhibitory phenotypes, driven by competition among indigenous soil bacteria. We sought to characterize relationships between cropping practices and pathogen suppression among soil Streptomyces. We evaluated bacterial and Streptomyces densities and inhibitory activities in soils from a long-term crop rotation experiment. Signaling interactions that altered inhibitory phenotypes among sympatric populations were also evaluated for a subset of samples. Soils from longer rotations, which had a higher number of plant species over time, had larger bacterial and Streptomyces densities, and more inhibitors than soils from shorter rotations. In addition, signaling occurred more frequently among isolates from higher-density communities. Our work shows that bacterial density, pathogen suppression and signaling are interrelated and are affected by crop rotation, suggesting the potential for management to optimize suppressive populations.