Fungicides & their modes of action

Plant pathogens are just as likely to evolve resistance to fungicides as insect pests are to insecticides, and for the same reasons, which are outlined in my recent blog on insecticide resistance.

Botrytis and downy mildews, for example, are regarded as being at high risk of this, thanks to their rapid rates of reproduction and a genetic makeup that’s particularly quick to respond to selective pressure. Such pressure can come from repeated use of the same fungicide or from the mechanisms plant breeders harness to introduce disease resistance into crops ­– breeding for downy mildew resistance, for example, is something of an ‘arms race’.

On our Advice Hub pages you can find help with planning resistance management strategies against some of the pathogens most prone to developing fungicide resistance, including Botrytis, downy mildews and powdery mildews. We give examples of fungicides with different modes of action – and which therefore target different aspects of the pathogen’s biochemistry – that are active against them.

Oh, fungi or oomycetes?

Understanding fungicide modes of action is not only essential to resistance management, it’s important for choosing products effective against the type of pathogen you’re looking to manage. Although we tend to refer to all disease-control products as fungicides, not all plant diseases are caused by true fungi.

Most are – Botrytis, powdery mildews and rusts, for example.

But there’s another distinct group with rather different biology, even though they behave in much the same way as fungi. These are the oomycetes, which include downy mildews and the tricky-to-control root diseases Pythium and Phytophthora.

Oomycetes have some of the superficial characteristics of fungi but much of their genetics and biochemistry is completely different. For example, their cell walls are made of cellulose (like plants), whereas those of true fungi are chitin (the same material insect exoskeletons are formed from); and they have some unique genetic mechanisms too.

Research into their genetics has suggested they’re no more closely related to fungi, in evolutionary terms, than you or I and their fungus-like characteristics have evolved quite independently from fungi themselves.

Some fungicides are designed to target cell walls; others to interfere with the way genetic information is managed during growth and reproduction. These modes of action won’t necessarily be effective against both fungi and oomycetes.

Our fungicide Subdue, for example, remains highly effective against Pythium and Phytophthora. Its active ingredient, metalaxyl-M, targets the mechanism by which oomycetes copy their genetic information, the starting point for many key aspects of their metabolism.

I’ll say more about how fungicides are classified according to how they work in my next blog.