Field Guide to Plant Pathogens

Welcome to the Field Guide to Plant Pathogens. This guide is currently in active development by Jake Dalzell — if you've been here before, clear your browser cache to ensure it displays properly!

The habitats section of this guide focuses on European species, particularly those found in Ireland. The other sections are relevant more broadly.

Click or tap on a photo to enlarge it, and do the same to shrink it again.

What are plant pathogens?

I use the term plant pathogen here to refer to any organism that has evolved to manipulate a plant biochemically in order to acquire resources. This is a somewhat arbitrary definition in order to include gall-forming invertebrates but exclude leaf-miners and e.g. ‘normal’ caterpillars that eat plant tissue without influencing the plant's development or biochemistry.

Pathogens include fungi, oömycetes, bacteria, viruses, phytoplasmas, and many groups of gall-forming animals.

If you already have a plant pathogen you want to identify:

Make an iNaturalist observation, including the location, date, and photos of the pathogen and its host. Check out my guide on what information to include.

Consider collecting a specimen, making sure to follow the Botanical Society of Britain and Ireland (BSBI) code of conduct (pdf) when picking plants. For more information about specimen collection look at the page on herbaria.

Identify the host plant. Ideally to species, though sometimes just genus is enough. Obviously this is beyond the scope of this site, so I will refer you to the BSBI's page on getting started in botany.

Try to identify the major group the pathogen belongs to. First take a look at the guide to the major groups of plant pathogens.

Try to identify the pathogen to species. Take a look at the Plant Parasites of Europe database page for your host, and filter by the group you think it is in. Look at my advice on using the database.

Other resources on this site

Check out guides to plant pathogens by habitat, to learn some common species you are likely to encounter:

If you want to find out more about plant pathogens and why they are so interesting, check out In Defence of Disease.

You will note that I have included English names in my species accounts. If I have not said otherwise, these are common names from iNaturalist. Some are names I have come up with myself, as I think common names can be a helpful way to engage people who are averse to scientific Latin. I have tried to make them both descriptive and memorable.

N.B. you may notice I use diaeresis in words like oömycete, oöspore, and coëxist. This shows the two adjacent vowels are in separate syllables. I think it's a fun spelling convention so I decided to include it.

A short film

My friend Ana McCrae and I made this short film about some of the plants and plant pathogens found at Killard National Nature Reserve, County Down. Ana is studying film and so we had access to some great equipment as well as her excellent camera, audio, and editing skills.

Acknowledgements

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.

Thanks to the Irish Naturalists' Journal for a grant that allowed me to buy a high-powered microscope.

Thanks to Astrid Biddle for providing images of Puccinia adoxae.

Thanks to those who have given feedback about the site, including Nicolas Schwab and Geoff Newell.