Plants of groundsel (Senecio vulgaris L.), either inoculated with rust Puccinia lagenophorae Cooke or uninoculated, were grown to capitulum production at a range of densities up to 2240 plants m−2, both in monoculture and together in mixture. Rust infection reduced vegetative growth and capitulum production in monocultures and mixtures. However, in mixture, unequal interference between inoculated and uninoculated plants amplified the effect of rust infection on growth. The relative crowding coefficient of uninoculated over rusted groundsel consistently exceeded unity but showed no clear trend in response to planting density. Rust infection resulted in bimodal frequency distributions of plant height, particularly in mixture, i.e. many rusted plants were stunted and were confined to the lowest layers of the shoot canopy. Thus, the reduced yield of the inoculated component of mixtures resulted from the very severe inhibition of growth of a proportion of rusted plants. However, it appeared that suppressed, rusted plants remained able to respond to limitation of available resources since both dry weight partitioning to the roots and specific leaf area increased. Perhaps because of the ability of groundsel to adapt to unfavourable environmental conditions, rust infection resulted in little mortality. Thus, even heavily rusted suppressed plants were able to produce some capitula and, potentially, seed.