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This thesis focuses on mechanisms and processes underlying spatial patterns of resistance and virulence and on local adaptations in plant–parasite interactions. The model system used comprises the plant host Salix triandra, the pathogenic rust fungus Melampsora amygdalinae, the leaf beetle Gonioctena linnaeana, and the galler Pontania triandrae. In this work, I (1) emphasize the most important factors determining the outcome of a plant–pathogen interaction, and the types of systems in which local adaptations can be expected, (2) examine the resistance structures of different populations of S. triandra, and whether the leaf beetle G. linnaeana responds to the local conditions of the populations of S. triandra in Sweden, and (3) address whether the distribution of parasites on S. triandra can be explained by the plant content of secondary metabolites. A review of several studies of the subject leads to the conclusion that adaptation of pathogens to their local hosts is more likely to be found in systems in which the pathogen is host-specific, non-systemic, and has a larger dispersal range and evolutionary potential than its host does. Furthermore, the scale of the study must be adjusted to that of the pathogen’s local population distribution. In addition, the temporary nature of host–pathogen interactions influences the importance of sample size, and too-small sample sizes can lower the chance of finding local adaptations, even though they may have evolved in a given system. The results of an inoculation experiment using material from physically isolated natural populations of S. triandra and M. amygdalinae confirm the importance of previous conclusions. Spatial variation in the resistance structure of S. triandra also has effects on the insect herbivore G. linnaeana, which has responded by adapting to the local hosts. However, local differences in secondary chemistry affect different parasites in different ways, and while P. triandrae is attracted by high levels of phenolic compounds, including tannins, M. amygdalinae and G. linnaeana are more rarely found on plant individuals with high concentrations of tannins. In addition, brood deposition by adult females of G. linnaeana and the performance of larvae are positively affected by luteolin-7-glucoside and an additional unidentified flavonoid, whereas they are negatively affected by the presence of (+)-catechin and high levels of tannins. Our results also show that plants traits that provide resistance to one type of parasite do not necessarily provide resistance to others. This indicates that different natural enemies potentially assert divergent selection pressure on S. triandra phenotypes which can be important for maintaining phenotypic variation in plant species.