Senecio vulgaris L., either healthy or inoculated with rust (Puccinia lagenophorae Cooke), was grown in monoculture or in mixture with Capsella bursa-pastoris (L.) Medic, under high or low nutrient conditions. At harvests made 7, 9, and 11 weeks after sowing, uninfected S. vulgaris progressively outcompeted C. bursa-pastoris under nutrient-rich conditions. Rust infection eliminated the competitive advantage of S. vulgaris over C. bursa-pastoris under high nutrient conditions. In contrast, under low nutrient conditions C. bursa-pastoris outcompeted uninfected S. vulgaris and infection with P. lagenophorae resulted in no additional competitive disadvantage to its host. The reduced competitiveness of rusted S. vulgaris under high nutrient conditions was associated with reductions in root growth in mixtures that were greater than those caused by rust in monocultures.

The effects of nutrient supply, rust infection and competition on tissue concentrations of nitrogen, phosphorus and potassium were complex and often influenced by interactions between these factors. Increasing the nutrient supply increased nitrogen concentrations ([N]) in S. vulgaris, as did rust infection, but the effects of infection were confined to monocultures. In general [N] in C. bursa-pastoris showed rather less variation. Phosphorus concentrations ([P]) in C. bursa-pastoris increased with nutrient supply but under nutrient-rich conditions were reduced by competition. In S. vulgaris [P] increased with increasing nutrient supply and with infection but in both cases responses were confined to monoculture. Potassium concentration ([K]) in S. vulgaris was increased by rust, especially in mixtures. In C. bursa-pastoris [K] under high nutrient levels was higher in mixtures than in monoculture.

The mechanisms by which the effects of rust on host competitiveness differed under high and low nutrient conditions are considered and the implications for our understanding of the role of rusts in natural vegetation are discussed.