AbstractClassical biocontrol of Senecio jacobaea (ragwort) has generally utilised herbivorous insects, although the nisi Puccinia expansa has also been considered. Although this rust is specific and damaging in the glasshouse. It has not been used in the field.Research into the ecophysiology of Senecio vulgaris (groundsel) infected by the rust Puccinia lagenophorae has revealed the extent to which the effects of infection are dependent on environmental factors. The damage caused by rust is enhanced under mild drought conditions, during periods of frost in winter and by competition between groundsel and neighbouring plants, bui is reduced by nutrient deficiency. Rust injury is also greatly increased by secondary infection of pustules by necrotrophic fungi. Such secondary infection can be achieved artificially with a range of opportunistic necrolrophs and can selectively kill groundsel: the effective inoculum dose of both fungi is significantly reduced. Attempts to apply our understanding of rust‐necrotroph injury to ragwort have been partially successful but we have not succeeded in causing significant mortality of this host.This paper discusses these, and other weed—pathogen—environment interactions, and their possible application to biocontrol.