Many freshwater algae are parasitized by aquatic fungi belonging to the Chytridiales Order. The top-down effects of parasitism result in individual losses within algal populations. We present data on the parasitism of the diatom Asterionella formosa Hassall by chytrids, occurring in response to hydrodynamic disturbances and the architecture of the algal host on the landscape scale. Among a series of nine reservoirs, the observed pattern of parasitism depends on the levels of hydrodynamic disturbance: the rate of parasitism is higher at intermediate disturbance levels. A. formosa cells are not randomly parasitized. Whatever the disturbance levels, fungi were found to mostly parasitize cell clusters forming non-stellate colonies.