Welcome to the Field Guide to Plant Pathogens. This guide is currently in active development by Jake Dalzell. I have had a passing interest in these organisms since I was a child looking at flower smuts on some of my favourite plants, but since early 2024 I have become really very deeply interested in them. I want to share my interest and experience through this project and provide all the information you need to get started recording plant pathogens in Europe. My focus here is on recording with iNaturalist, as this is an international platform and the one where your records are most likely to get verified, passed on to databases, and engaged with by the community. To get started, check out guides to plant pathogens by habitat, to learn some common species you are likely to encounter:
If you already have a plant pathogen you want to identify, take a look at the guide to the major groups of plant pathogens.
Keep reading this page if you want some fun facts, to see why you should be interested in plant pathogens! These organisms are highly specialised to manipulate their host plants and spread to new hosts. Also check out the acknowledgements.
Many pathogens have evolved to be spread by pollinating insects. This means a given spore is more likely to meet another host plant than if it was spread by the wind. Many also produce resting spores in the petals or ovaries of the flower which go into dormancy in the soil.
The floricolous downy mildews (a group in the genus Peronospora) have evolved sticky spores (conidia) that are brushed onto pollinators by tall conidiophores.
A single species like Devil's-bit Scabious Succisa pratensis can have many different pathogens. Here I have shown three different species that I found on the flowers on one visit to Murlough National Nature Reserve, v.c. Down (H38). Each of these three represents a very distant branch of the tree of life from the other two: Oomycota, Fungi, and Insecta. The fungus and the oomycete show a degree of convergent evolution, having both evolved to be spread by pollinators.
I found all of these by noticing specific symptoms in the plants I walked past which made them stand out from the rest. I find Peronospora violacea particularly fascinating because it alters the form and even the colour of the flowers in such a distinctive way. The anthers and ovaries are sterilised (Horáková & Skalický, 1989) and the petals are enlarged and reddened. This appears to attract more pollinators to spread the conidia, which are produced at the end of conidiophores that grow on the petals.
Microbotryum succisae similarly sterilises the anthers, replacing the pollen with its own spores, but apart from that does not really seem to affect the development of the flower.
Contarinia dipsacearum was not a species I had seen or heard of before, but as I was walking through the reserve I noticed many plants had flowers that seemed late to open, and did not open in the normal order. Usually the top- and bottom-most flowers in the head open first, and a wave moves towards the middle band as more flowers open. In some plants however, I noticed that a few flowers were still closed even as those in the same band or further towards the middle were already open. Eventually I had seen quite a few plants with this and I knew it was not just a random abberation, so I stopped to investigate. Carefully opening up the flower revealed the orange midge larvae. Looking on the Plant Parasites of Europe page for the host, I found Contarinia was a match. Until this record it had been seen once in Britain and Ireland, in 1947 in London. The adult fly lays its eggs in the developing flower buds, and the larvae gall the flowers, preventing the buds from opening. They stay inside the bud as they develop, and eventually leave to pupate in the soil.
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 host. 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.
New plant species are introduced to Britain and Ireland every year. With lax biosecurity, endophytic fungi and oomycetes can be brought in on plants imported for gardens. They may not even show symptoms until after they have passed through a port. Coinvasion refers to multiple species with a close symbiosis invading a new area at the same time.
New species of plants and fungi can alter the interactions of native species. Here the native hyperparasite of ergots, Fusarium heterosporum, has formed a new relationship with the ergot Claviceps spartinae and its grass host Sporobolus (Spartina) anglicus, both of which have been introduced to Ireland in the last 120 years.
Many thanks to my mentor Chris Preston for teaching me a huge amount and encouraging my interest in plant pathogens, and for suggesting some of the additions to this site. Thanks to the iNaturalist community for helping with identification of many of the species here.
Horáková, J., & Skalický, V. (1989). Contribution to the ecology of Peronospora violacea Berk. Česká Mykologie, 43(1), 13–29.
Thines, M., & Choi, Y.-J. (2016). Evolution, Diversity, and Taxonomy of the Peronosporaceae, with Focus on the Genus Peronospora. Phytopathology, 106(1), 6–18. https://doi.org/10.1094/PHYTO-05-15-0127-RVW