Interactions between the rust fungus, Puccinia paludosa, and the alternate host Pedicularis palustris : A field study in the archipelago of Skeppsvik, northern Sweden

Plant-pathogen interactions are of interest due to their impact on our economy, social welfare and not to mention, our environment. One important group of pathogens are the rust fungi, many which require two host plants to survive. In this thesis I studied patterns and effects of a rust disease during an outbreak by the rust fungi Puccinia paludosa on its alternate host Pedicularis palustris. The main targets of the study was to examine if the severity of rust damage varies along the water gradient of the littoral zone where P. palustris grow, moreover, if it depends on the distance to the primary host Carex nigra. I also examined the rust’s influence on the host fitness by quantifying various life history characters on P. palustris individuals (i.e. mortality, growth, fecundity). The outcome showed that P. palustris plants growing in the upper levels of the littoral zone had higher proportions of rust damage, indicating that soil moisture can be a determining factor for how vulnerable the plants are to rust infection. Also, a short distance between the host species favours a higher proportion of rust damage, and, in the littoral level where C. nigra dominates the vegetation, the proportion of rust damage on P. palustris individuals were higher. This suggests that the dispersal range of the rust spores are limited. Finally, P. paludosa negatively influences the fitness of the alternate host P. palustris as mortality is dramatically increased while growth and fecundity are reduced