The infection cycle of the autoecious rust fungus Puccinia punctiformis on the clonal plant Cirsium arvense was described in terms of intensity (amount of pathogen per unit area and used for systemic infection) and extent (area occupied by the pathogen and used for local lesion infection) at four grassland sites in The Netherlands. The incidence of systemic infection was relatively low and systemically infected shoots were typically concentrated at a few locations within a site. Incidence of local lesion infection was relatively high and widespread in ail sites. Spatial and temporal distribution of teliospores, the inoculum for systemic infection, were quantified in order to explain the relatively low and spatially unequal incidence of systemic infection. Because of the large impact of systemic infection on C. arvense shoots, it is concluded that a spatially homogeneous distribution of teliospores may be a prerequisite for the biological control of C. arvense by P. punctiformis.