Selective sieves in the epidemiology of Melampsora lini

The effects of various potential selective sieves operating at different stages in the epidemiological cycle of pathogen populations were examined in the context of a natural interaction between Melampsora lini and Linum marginale. The establishment of self-sustaining pathogen populations in previously healthy host stands was significantly lowered only when the size of host populations was extremely low (1-3 plants). During the endemic phase of growth when interpustule competition was non-existent, differences in the latent period or size of individual pustules of 10 different pathogen isolates were minor compared to differences due to temperature. A competition experiment between two pathotypes of M. lini detected a marked shift in the relative frequency of the two pathotypes during the course of an epidemic lasting approximately five generations. Finally, the survival of two different pathotypes of the pathogen during off-season reductions in population size was significantly affected by site, year and pathotypic identity, Interactions between these variables were either marginal or non-existent. The net effect of the interplay of these genetic and ecological factors is to increase stochasticity and the potential for sustained differences between pathogen demes-a feature expected when host-pathogen co-evolution occurs at a metapopulation level.