Date of Award
Doctor of Philosophy (PhD)
In light of large-scale urbanization and rising global temperatures, it is essential to understand how environmental and climatic changes impact the ecology and evolution of species interactions. This thesis investigates interactions between wild plants and their pathogens across gradients in land use and latitude, and specifically tests the effects of multiple scales of climate variation on such interactions. In this work, I focused on a system of common herbaceous plants in the genus Plantago and their fungal powdery mildew pathogens. I studied their interactions across an urbanization gradient transecting the metropolitan area of St. Louis, Missouri and along a latitudinal transect from southern Mississippi to northern Wisconsin. Both of these transects feature climate gradients, where temperature generally increases with the level of urbanization and with decreasing latitude. Moreover, microclimate conditions experienced by plants can vary dramatically within populations due to fine-scale heterogeneity in habitat features. As such, my thesis projects examine how plant–pathogen interactions change across three distinct spatial scales of climatic variation. However, climate conditions are not the only factors varying within and between these populations. Therefore, my projects also tease apart the roles of variation in host-pathogen compatibility, pathogen encounter rates, and other covariates (e.g., herbivory and mowing damage) in explaining patterns of disease. This research is important for understanding the factors that drive current variation in disease pressure in wild plant populations, and for informing predictions of how continued land use and climate change may impact future plant–disease dynamics. My thesis research focused on three main questions: 1) How does the phenology of plant–pathogen and plant–herbivore interactions change with increasing intensity of urbanization? 2) What is the effect of within-site microclimate variation on plant–pathogen interactions across an urbanization gradient? And finally: 3) Are pathogens locally adapted to host populations or temperatures along a latitudinal gradient?
In Chapter One, I studied how the prevalence of disease and herbivory on two co-occurring Plantago species changed across an urbanization gradient over the course of two growing seasons. I found that Plantago in more urban populations experienced more early-season herbivory damage as well as earlier and larger powdery mildew epidemics. Results of field and laboratory experiments suggested that the observed differences in disease prevalence were not driven by variation in genetically based host susceptibility across the urbanization gradient but may reflect variation in pathogen encounter rates. Chapter Two focuses on the effect of microclimate variability on infection prevalence across the same urbanization gradient. In observational field surveys, I found that plants located in shaded microhabitats were more likely to be infected. This effect of shade varied through time but was consistent across levels of urbanization. I then performed a field experiment manipulating shade while controlling for host genetic background and pathogen inoculation status. Results of this experiment suggested a causal role of shade in promoting pathogen growth and transmission. In Chapter Three, I performed a laboratory experiment to test whether powdery mildew strains are locally adapted to host genotypes and temperature regimes along a latitudinal gradient. I found evidence for powdery mildew local adaptation to temperature, but not to sympatric host genotypes. Finally, in Conclusions and Future Directions, I summarize and synthesize each chapter.
Chair and Committee
Rachel M. Penczykowski
Natalie Mueller, Jonathan Myers, Kenneth Olsen, Adam Smith,
Fox, Quinn N., "Wild Plant–Pathogen Interactions Across Gradients of Urbanization and Latitude" (2023). Arts & Sciences Electronic Theses and Dissertations. 2850.
Available for download on Sunday, April 07, 2024