Plant Pathogen Temperature Ecology

Plant pathogenic fungi and oomycetes (hereafter ‘plant pathogens’) pose serious risks to crop production and natural landscapes, thereby threatening global food security and ecosystem health. Temperature directly affects plant pathogen physiology and is a key determinant of pathogen distributions and resultant crop risk, both in space and time. Global mean surface temperature is increasing and extreme weather events, such as heat waves, are becoming more common. To protect the warming biosphere we need to better understand how plant pathogens respond to temperature. In this thesis I interrogate previously published data on temperature responses for hundreds of plant pathogens, as well as utilise the fungal wheat pathogen, Zymoseptoria tritici, as a model pathogen system for studying plant pathogen temperature ecology. In Chapter 1, I summarize existing literature on plant pathogen temperature ecology and present the major aims of research detailed in this thesis. Materials and Methods that are used in more than one data Chapter are described in Chapter 2. In Chapter 3, I investigate the geometry and evolution of two ecological niche axes, temperature response and host range, for hundreds of potentially destructive plant pathogens. I find evidence for Hutchinson’s distinction between the fundamental and realized niche. I also find that abiotic and biotic niches can evolve independently in these microbial species. Chapter 4 explores how climate change will directly shift global infection risk, for a suite of plant pathogens. I find that infection risk will likely spread polewards during the 21st century. This Chapter also highlights the importance of considering plant pathogen infection risk, when quantifying changes in agricultural productivity under climate change. A mechanistic, weather-dependent model of Septoria tritici blotch (STB) disease risk is developed in Chapter 5. The model was unable to accurately predict annual STB disease risk across the UK. This Chapter nevertheless presents a new approach to mechanistically modelling STB and could provide a useful resource for future model development. In Chapter 6, I investigate Z. tritici responses to short-term elevated temperatures. I report the first preliminary evidence of within-species variation in survival at elevated temperature in Z. tritici, the first evidence for thermal priming in Z. tritici, as well as the first description of morphological changes of Z. tritici in response to fluctuating temperatures. In Chapter 7, I discuss the findings of this thesis and detail outstanding questions that require future investigation. Previously published (but poorly accessible) plant pathogen temperature response data collated and used in this thesis are deposited in an open-access repository, representing the most comprehensive dataset of its kind currently available for plant pathogens