From so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.
—Darwin, Charles. 1861. “Conclusion.” In On the Origin of Species, 425. New York: D. Appleton and Company.
Natural history is a field with no end. I used to wonder if learning more about the natural world would make it lose its magic. The opposite is true: in learning about a new group of organisms, they suddenly become at once obvious everywhere you go and many times more mysterious. Before I was shown how to find pathogens, rusts and smuts were a minorly interesting, occasional find on a few host plants. Now, I see that I am surrounded by “endless forms most beautiful and wonderful”, many of which don't even have names.
Many plant pathogen species remain to be described. Generalist species in genera like Peronospora and Entyloma are often found to be made up of many distinct, host-specific species. Entyloma in particular is fantastically diverse, with many Ranunculus species having multiple Entyloma species that are exclusive to their host1. However, only fourteen species have been described from Ranunculus (and two from Ficaria), leaving the tantalising possibility that there are many hundreds more buttercup leaf smuts to be discovered and documented.
This possibility became real for me while walking somewhere I had been a thousand times before. The machair at An Caol in Acaill, Mayo, is a hotspot for plant diversity. It has very short, sheep-grazed turf where tens of tiny species can coexist in one ten-by-ten centimetre square. I passed golfers and hares on my walk, reaching the river known only as An tSruthail (“the Stream”). This river is somewhere I once saw Marsh Lousewort (Pedicularis palustris) and I wanted to see if I could find it again. I failed, but this was made up for by the wonderful display of flowering Water-crowfoots (Ranunculus sect. Batrachium).
Something I quickly noticed was that many of the floating leaves of Brackish Water-crowfoot were infected with a white leaf smut (Entyloma). This was exciting — I hadn't managed to find any aquatic plant pathogens before. What I quickly realised was that not only are the Entyloma on Water-crowfoots undescribed, the genus had never been recorded from this host species.
Good collections are essential so that we can describe new species without the struggle of refinding potentially rare or extinct taxa. If you find something that is on an unusual or new host, or in a new area, it is important to submit the specimen to a herbarium so that it can be studied. I have made a list of target taxa that would particularly benefit from more specimens here.
It is not just that the diversity of pathogens in an ecosystem arises from the diversity of hosts. Plant and pathogen diversity are reciprocally dependent on each other. In the next chapter we will explore just how intimate this relationship is, which underpins a huge amount of the variety of living things on our planet.