At low densities, Plantago lanceolata was less likely to have the inflorescence disease caused by the fungus Fusarium moniliforme var. subglutinans. Where the disease was found, as host density increased, the proportion diseased remained constant or decreased. The pathogen's inability to exploit high host densities may be due to both the long disease cycle and the reduced efficiency of disease spread in heterogeneous habitats. Diseased plants were randomly spaced early in the summer but were later found in clusters, probably as a result of localized spore dispersal. Spatial patterns of disease levels remained constant from year to year.