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. And the more taxonomists study a group, the larger and larger the estimates of its true diversity grow, to the point where we realise we have discovered just a tiny fraction of the many millions of species on our planet.
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.
Even Entyloma is relatively well studied compared to many groups of plant pathogens. The diversity of leaf spot fungi is a rabbithole that seems to go on forever. Many hosts have five, six, seven different species of leaf spots that only infect them, and it is likely that the majority are still undescribed.
One reason for this is that leaf spots are small, unattractive, and difficult to study. For example, the leaf spot Septoria anthyllidicola looks rather like nothing. It looks like a brown dead patch on a leaf of Kidney Vetch. When you look more closely, you see that in that dead patch are embedded small black dots – pycnidia. When you look even more closely, with a high-powered microscope, you see that these pycnidia contain sausage-shaped conidia, the asexual spores of the fungus. It is only by measuring these conidia that we can actually work out which species we have. Kidney Vetch is host to two different Septoria species that differ only in the size of their conidia.
Small wonder, then, that Septoria anthyllidicola does not have many records. In fact, the only records I am aware of are from its original description from Croatia, in 19142, and my own records from my garden in Belfast.
The other reason that leaf spots are poorly described is that they are extremely abundant and diverse. Septoria anthyllidicola may not even be a particularly rare species, but with so few people making the effort to record leaf spots, most go under the radar. When only one species is known from a host, people often record all leaf spots on that host as that species, without checking microscopically that they haven't found something new. It seems unlikely this will be solved any time soon!
This does leave us with the rather exciting prospect that many of the fungi in Ireland are likely still undescribed. In 2025, I have found two strong contendors for new species of leaf spot fungi: an Ascochyta on Navelwort, and a Septoria on Bluebell. Whether they are truly new remains to be seen, but it is exciting nonetheless. It shows the importance of paying attention to ‘boring’ taxa.
Good collections are essential so that taxonomists can investigate evolutionary relationships and 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 started an extremely non-exhaustive 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.