Epidemiology, resistance structure, and the effects of soil calcium on a serpentine plant-pathogen interaction

While the impacts of diseases in agricultural settings have long been appreciated, little is known about the role of plant pathogens in natural ecosystems. Studies that quantify the ecological impacts of diseases on wild plant populations, and link these impacts to resulting evolutionary patterns, inform our understanding of the dynamics of natural systems and allow theories about variation in the frequency, intensity, and outcome of antagonistic coevolution to be assessed. Plant communities associated with serpentine soils offer an intriguing setting in which to conduct such studies. While essentially nothing is known about the diseases of serpentine flora, evidence from agricultural systems suggests that low soil calcium levels typical of serpentine soils should make associated plants vulnerable to attack by pathogens. In contrast, an ecological theory attempting to explain patterns of edaphic endemism characteristic of serpentine plant communities posits that these low calcium soils may provide a refuge from disease. Studying the interaction between California dwarf flax, Hesperolinon californicum, and the rust fungus Melampsora lini , I tested these conflicting predictions and examined patterns of epidemiology and host resistance structure across the latitudinal range of the host species. Using a greenhouse experiment in which I manipulated the calcium concentration of serpentine soils I found that infection rates were lower for plants growing in higher calcium soils. This finding was supported by results of three years of epidemiological surveys conducted in sixteen wild H. califomicum populations. Infection prevalence was positively correlated to host population latitude, altitude, and density, and negatively related to soil calcium concentration. Infection was associated with increased seedling mortality rates and reduced fecundity in adults. I documented a cline in infection, with high disease prevalence in northern plant populations decreasing steadily towards the south. A nearly identical pattern in host resistance structure was elucidated by greenhouse inoculations in which plants from each survey population were challenged with ten rust isolates. Genetic resistance was lowest in northern flax populations and increased gradually and continuously towards the south. This is one of a handful of studies that has investigated both epidemiological patterns and host resistance structure in a wild plant/pathogen interaction.