Interactions between conidia of Botrytis cinerea and aeciospores of Puccinia lagenophorae (rust of groundsel, Senecio vulgaris) were studied because rusted groundsel exhibits exceptionally high mortality when additionally infected by B. cinerea. The latter acts as a secondary invader via rust pustules on leaves and stems. Conidia of B. cinerea, collected dry from parent cultures, failed to germinate on agarose in Petri dishes but did so in the presence of aeciospores, germination generally increasing as aeciospore concentration increased. Germination on agarose was also stimulated by diffusates from groundsel leaves and by the removal of agarose discs to the surface of groundsel leaves. Germination on such discs was higher over sporulating lesions on rusted leaves than on healthy leaves and decreased with radial distance from the centre of individual lesions. When conidia were sown directly onto the leaf, germination over epidermis at the edge of a lesion was greater than that within a lesion, but germination in both these positions was greater than that at a distance from the lesion. The promotive effects of rust on the germination of B. cinerea conidia in vivo were quantitative rather than qualitative but they explain in part the synergy between P. lagenophorae and B. cinerea.