TEMPORAL AND SPATIAL CHANGES IN A METAPOPULATION OF THE RUST PATHOGENTRIPHRAGMIUM-ULMARIAE AND ITS HOST, FILIPENDULA-ULMARIA

1 The numerical dynamics of a rust disease caused by the pathogen Trip hragmium ulmariae was studied in a metapopulation of 129 discrete popu lations of the host plant Filipendula ulmaria. 2 Over the four years 1 990-1993, 37% of the populations were consistently infected by the pat hogen while 43% were always healthy. In the remaining 20% of populatio ns, the presence of disease fluctuated from year-to-year. 3 In each of the four years the incidence of disease was strongly positively corre lated with the logarithm of host population size (P < 0.001). Disease incidence was also weakly affected by the type of shore on which host populations grew but not by their degree of exposure. 4 The severity o f disease occurring in infected populations was weakly positively corr elated with the logarithm of population size (P < 0.05). However, the relationship between the density of individuals within populations and disease prevalence showed no density-dependence. 5 The distribution o f disease among populations df the metapopulation had a significant sp atial component in two of the years, with infected populations being c loser together than would be expected by chance.