One of the consequences of spatial and temporal patchiness is that pathogens do not exert a homogeneous selective force on their hosts. We have been studying the spatial and temporal dynamics of Silene acaulis, a long-lived perennial, and Microbotryum violaceum (= Ustilago violacea), a pollinator-transmitted fungal pathogen that sterilizes its host, to better understand the impact the pathogen has on host population dynamics. Complete spatial maps of S. acaulis were made for six 60 m2 plots; host density ranged from 2 to 27 plants per m2 and disease frequency ranged from 1% to 26%. We have been recording changes in flowering status, disease status, appearance of new recruits, and death for six years. The spatial pattern of disease, host gender, and juveniles was analyzed at the level of nearest-neighbors and over the scale of several meters. Diseased plants were significantly clustered at a scale of less than one meter, but plants were not segregating by gender at the scale of meters. Clustering of diseased plants was weaker in plots with high host densities. Juvenile plants were clustered and tended to not have either healthy or disease adults as nearest neighbors. The temporal dynamics of disease spread and host population growth show that the number of new juveniles was variable across years, but was always higher than either the rate of disease spread (<1%) or host mortality (<5%). Although the rate of disease spread per year was low, plants rarely lost infection. The combination of stable infections, host longevity, slow rate of disease spread, and higher rate of host population growth suggests that the interactions between S. acaulis and M. violaceum are unlikely to result in rapid extinction of either host or pathogen populations.

Key words: Microbotryum violaceum, pollinator-transmitted pathogen, Silene acaulis, spatial structure, temporal dynamics, Ustilago violacea