Introducing the Field Guide to Plant Pathogens

Since I started seriously recording plant pathogens, I have been really encouraged by the reception and interest other biological recorders have shown. I think there is a huge potential for recorders on platforms like iNaturalist to generate a lot more data on the distribution of these overlooked taxa. When people have told me my interest is “niche” however, I have had to pause. What does “niche” mean? Is it that, despite being more diverse than their plant hosts, plant pathogens are somehow less important? Is it that because of their small size, people will always dismiss them in favour of iconic plants and vertebrates? Maybe some people will never catch the bug, but plenty have shown an interest — the biggest problem is that plant pathogens are inaccessible. When there are few resources for a group, anyone with a passing interest will struggle to develop a passion for it. When fewer than ten people in Ireland are recording a group, of course it will be dismissed as “niche”.

This is a problem with an obvious solution: write a field guide! So, that is what I’ve done. I am very pleased to introduce the Field Guide to Plant Pathogens, which aims to explain the best practice for recording and identification. There are pages showing common species you are likely to encounter in different habitats, like grassland and urban areas, including illustrations I have made of different pathogens you might see on certain hosts.

There is a guide to the major groups that most pathogen species fall into, including a rudimentary key, and descriptions of each group. There is a guide to identification techniques including microscopy, and I have included a particular focus on how to effectively use existing resources like the Plant Parasites of Europe Database which I feel aren’t known widely enough. I have also included a book of essays which explain why pathogens are an interesting subject in the first place:

A single plant species like Devil’s-bit Scabious Succisa pratensis can have many different pathogens. On a single visit to Murlough National Nature Reserve, I found three different species infecting its flowers, representing three distant branches of the tree of life: Fungi, Oömycota, and Insecta. The first, Microbotryum succisae, is a smut fungus. It alters the development of the anthers, replacing the pollen they would normally produce with its own bright white spores. It is easy to spot while wandering through a field of Scabious, the pale halo of an infected flowerhead standing ghostly against the purple background. This is just one of many smut fungi that infect the flowers, fruits, and leaves of a host of different flowering plants. Like most flower smuts, it is spread by pollinators: more on that later.

— Introduction, In Defence of Disease

This project is still in active development, so don’t be surprised if a few of the pages are a bit sparse. There may be mistakes — please email me to point them out!

I would like to acknowledge the huge generosity of the Irish Naturalists’ Journal in providing a grant that allowed me to buy a new microscope. This will contribute enormously to my work on Irish plant pathogens and the description of biodiversity more broadly. The photomicrographs I take will be useful for illustrating the field guide.