Department of Ecological Botany, University of Umea, 90187 Umea, Sweden

Background and objectives
The potential role of pathogens in structuring plant communities has been increasingly acknowledged during recent years [1]. Several studies have addressed how parasitic fungi may affect their host plants [2]. However, the role of these interactions for host-population dynamics and plant community structure is still limited, although it is evident that the life cycle of the parasite and the host are important components. The aim of this study is to increase understanding of the role of the macrocyclic, obligate and host-alternating annual rust fungus Puccinia caricis var. caricis for the occurrence of its primary host Urtica dioica. The alternative hosts are clonal species of the Carex Distigmaticae group.

Materials and methods
The work has been undertaken in the agricultural landscape bordering the Tornešlven river in northernmost Sweden. The horse-nettle is patchily distributed in this landscape, and the alluvial shores are fringed by extensive sedge stands. I report data from a 10-year study including (i) spatial and temporal variations in disease incidence; (ii) results for reciprocal transplantations of horse-nettle; (iii) results from experiments where the alternative host has been removed.

Results and conclusions
Disease incidence and severity showed profound spatial and temporal variations. Horse-nettle populations growing close to the river usually showed higher disease frequency and severity. This pattern was often changed in the transplantation experiments. Further, removal of the alternative host often had a significant effect, resulting in increased reproductive output and biomass of the horse-nettle. Long-term exclusions of the alternative host and/or the pathogen showed a clear treatment effect favouring the horse-nettle. The data support the view that this annual, macrocyclic rust has an effect on its sexual host operating over comparatively small spatial scales. These data are discussed in relation to other annual pathogens.

1. Real LA, 1996. Bio Science 46, 88-97.
2. Jarosz AW, Davelas AL, 1995. New Phytologist 129, 371-387.