Regional variation in the incidence of the systemic floral-smut fungus Sparisorium amphilophis (Syd.) Langdon & Fullerton an the perennial grass Bothriochloa macra (Steud.) S. T. Blake was investigated through three surveys over a 12 yr period (1981–93). In all three surveys a marked north-south trend in percentage of infection was detected with a greater proportion of plants in northerly populations being infected than in populations located to the south. The incidence of disease in populations was negatively correlated with the frequency of days with temperatures <0°C in winter. Detailed exploration of local variation in a subset of five populations showed that the incidence of S. amphilophis was density-dependent and was greater in the edge areas of host populations than in the less disturbed core areas. Smut prevented seed production and negatively affected different aspects of the morphology of the plants, such as the height and basal diameter. Infection also significantly increased the number of inflorescences per plant. These field results were complemented by glasshouse-based competition experiments which indicated that, despite causing reductions in size, S. amphilophis only affected the competitive ability of infected plants grown under sub-optimal conditions. The results of this study indicate that an interplay between physical and biotic aspects of the environment determines regional and local levels of disease.