Populations of groundsel (Senecio vulgaris L.) were grown in the field in bottomless containers which allowed roots to penetrate freely into the natural soil profile. The 36 plants within each population (equivalent to 1250 m−2) were either all inoculated with the rust Puccinia lagenophorae Cooke, all rust free, or were a 1:1 mixture of the two. Rain was excluded from all populations by small shelters. Half the populations were subjected to mild drought, whilst remaining populations were well-watered. Rust significantly reduced shoot dry weights, especially in droughted populations. The frequency distributions of shoot dry weight showed significant inequality (were strongly skewed) in all treatments and populations. Inequality of the frequency distribution of shoot dry weight was increased in rusted populations because infection increased the proportion of suppressed individuals. Again the effect of rust was most pronounced under conditions of drought and particularly in mixture. Rust significantly lowered mean shoot water potential (ψ) in dry monocultures and in both wet and dry mixtures. Values of ψ at harvest in suppressed, rusted individuals in mixtures were often highly negative. Compared with dry monocultures, water stress in mixtures was ameliorated in the healthy, but exaggerated in the rusted, components. The more negative values of ψ in rusted populations were the result of changes in frequency distributions as well as in mean values. Under dry conditions status in the dry weight hierarchy was closely correlated with ψ These changes are interpreted as reflecting the reduced ability of the root systems of rusted plants to compete for limited water.
The competitive ability of control over rusted groundsel, as measured by relative crowding coefficient (Ker), was 1–09 under well-watered conditions but increased to 1˙51 under drought. In both well-watered and droughted populations, the greater impact of rust on the population hierarchy of mixtures was reflected in the inverse relationship between the status of sub-classes, based on shoot weight and class Ker. It is concluded that water stress probably plays a crucial role in determining the impact of rust on mixed populations in the field.