Edaphic quality and plant-pathogen interactions: effects of soil calcium on fungal infection of a serpentine flax

Spatial variation in the frequency and outcome of interspecific interactions is thought to play a central role in shaping geographic patterns of biodiversity. Previous empirical studies and modeling exercises suggest that negative species interactions should occur with greater frequency and intensity in high-quality/low-stress environments. I tested this hypothesis through a four-year epidemiological study of interactions between the plant Hesperolinon californicum and the pathogenic rust fungus Melampsora lini. By virtue of its association with serpentine soils, H. californicum is exposed to edaphic conditions that are stressful to most plants. Notable among these is the low availability of calcium, an element that plays critically important roles in the immune responses of plants to attacking pathogens. As a serpentine generalist, however, H. californicum grows in soils with a wide range of calcium concentrations, and this should lead to differences in the frequency and/or severity of rust infection among host populations. I investigated geographic variation in disease pressure by conducting annual surveys in 16 populations spanning the host's distribution and representing a range of soil calcium concentrations. Results indicated that plants growing in more stressful low-calcium soils experienced higher rates of rust infection, suggesting that soil calcium may modulate host susceptibility in a manner opposite to that predicted by the a priori hypothesis. Epidemiological surveys further revealed a latitudinal cline in disease prevalence, with high infection rates in northern host populations decreasing gradually toward the south. Studies of the fitness effects of disease demonstrated that rust infection caused significant reductions in host survival and fecundity, and there was evidence of a demographic feedback between infection prevalence and host density across survey years.