Wait! There is no such thing as a ‘specialist’ or ‘generalist’ plant pathogen

The niche is a fundamental concept in ecology, defining the habitat of a species by a set of environmental (e.g. temperature) and biotic (e.g. prey, hosts) requirements. There are ‘generalist’ species which can survive in a wide variety of conditions with different resources and ‘specialist’ species which have narrower requirements and tolerances. The niche theory is much less understood in plant diseases. 

Thomas Chaloner and colleagues at the University of Exeter investigated the temperature and host range for hundreds of plant pathogenic fungi and oomycetes. The researchers revealed that temperature responses for in vitro growth in culture differ to those which involve interactions with host species, such as infection and disease development. They also found that having more plant hosts does not lead to a wider temperature range for a plant disease. Thomas Chaloner is a PhD student who researches plant pathogen temperature ecology across various scales of biology, supervised by Prof. Dan Bebber (lead author) and Prof. Sarah Gurr (co-author) at the University of Exeter.

Chaloner and colleagues compiled a large dataset (it will be available through Dryad) of 631 fungi and 64 oomycetes and 15,982 hosts of 309 pathogens. The researchers collocated the minimum, optimum and maximum temperatures, which are termed the ‘cardinal temperatures’, of five life cycle processes (disease development, fructification, infection, spore germination and sporulation) and in vitro growth of plant diseases using Togashi’s book, and previous research findings from 2007 and 2012. Thermal performance curves were calculated for all the microbes and the influence of geographical distribution and co-phylogeny with host plants was investigated for Phytophthora species.

“This paper began before I started my PhD in 2015 when Prof. Dan Bebber sourced Togashi, K. Biological characters of plant pathogens: temperature relations, from an antiquarian bookseller in Paris”. said Chaloner in an interview for Botany One. “This book is very rare and contains plant pathogen temperature response data hitherto poorly accessible to the scientific community. The paper then developed naturally as we considered the various pre-existing datasets available to us to investigate niche geometry and evolution”.

Chaloner said that the biggest challenge of this study was “probably the sheer scale of the analysis. For example, the study uses various pre-existing datasets where species names may differ due to changing binomial nomenclature, so ensuring the various datasets matched each other was a complex process.”

The scientists used the Index Fungorum and associated Species Fungorum databases, as well as the Mycobank database to double-check the species names and they accessed the Plantwise database (CABI) for pathogen-host interaction records. Firstly, they identified 1,016 hosts of 302 pathogens but when the host plants were not identified down to species level (e.g. order level), all plant species were added as potential hosts. After these assumptions, they identified 15,982 hosts of 309 pathogens.

The results showed a lot of overlap between fungi and oomycete cardinal temperatures for lifecycle processes. The scientists found that the biological processes, especially growth in culture and spore germination, were reduced more at higher temperatures compared to lower temperatures. The study provides empirical evidence for Hutchinson’s distinction between the fundamental niche and the realised niche, where the latter is limited by interactions with other species. Also, pathogens with more diverse host plants did not have a wider temperature range. In comparison, the temperature range of 101 Phytophthora varied in the same locations which suggest high general adaptability. The further analysis of 35 Phytopthotra species showed co-phylogeny (similar divergence) with their hosts.

“Our analyses demonstrate that plant pathogens show remarkable adaptability to new climates and new plant hosts. Generalists are sometimes called ‘Jack of all trades, master of none’. Our analyses show that many plant pathogens are ‘Jack of some trades, master of others’. I think this departure from traditional thinking is particularly exciting”. He added, “I also think plant pathogens are a very interesting and tractable group of species to investigate hypotheses fundamental to ecology and evolution,” Chaloner said.

This research indeed answered many questions about the role of temperature and plant hosts in plant disease distribution and evolution. What next? 

“I am coming to the end of my PhD and starting to think about what I would like to do next. I’m considering applying for an Independent Research Fellowship to continue working on plant pathogen temperature ecology, but trying to keep my options as open as possible,” Chaloner said.

“We live in an era of growing global population size, climate change, and emerging threats to crop production and food security. We hope our paper stimulates conversation about the importance of understanding the processes that enable plant pathogens to spread and cause disease in space and time.”