Groundsel (Senecio vulgaris L.), healthy or infected with the rust fungus Puccinia lagenophorae Cooke, was grown at a range of nutrient concentrations in sand culture. There were statistically significant interactions between the effects of infection and nutrient supply upon the dry weights of stems, leaves, roots and reproductive tissues, leaf area and cumulative capitulum production. This interaction occurred since infection caused significant inhibitions of growth only at moderate or high nutrient concentrations. At low concentrations rusted plants were similar to or slightly larger than controls. Both in controls and rusted plants root: shoot ratios increased as nutrient supply declined. The ratio of root: shoot dry weight was consistently reduced by infection whilst root length: leaf area ratio was relatively unchanged.
More detailed investigations confirmed that infection had little effect on plant growth under nutrient deficient conditions despite suppression of the host's ability to increase root: shoot ratios in response to nutrient stress. This reflected the inhibition of relative growth rates in rusted plants at high but not low nutrient concentrations, which in turn reflected reduced net assimilation rates (NAR). Increases in leaf-area ratio (LAR) often ameliorated the decline in NAR in rusted plants.