How does fungicide resistance develop?

Fungicide resistance is all about selection pressure. Every time a fungicide is applied, we apply selection pressure to the fungal population(s) present in the crop.

It’s important to know – it’s a numbers game.

More fungicide applications equals more opportunities for resistant fungi to be selected.

AND

The larger the fungal population, the greater the chance for there to be resistant fungi within that population. So, when we do apply those fungicides, there are more resistant individuals which could survive and go on to reproduce, leading to reduced sensitivity or resistance at the whole fungal population/field scale.

This is why reducing disease pressure and minimising fungicide applications are both important for reducing the risk of fungicide resistance developing in your fields.

Fungicide resistance evolution. (Modified from CropLife Australia Fungicide Resistance Management Fact Sheet.)

The main driver for fungicide resistance development is the repeated use of fungicides with the same biochemical mode of action (mode of action or MoA groups).

You may have heard of MoAs in relation to herbicides and herbicide resistance. It’s the same concept – that the chemistry of the fungicide or herbicide works in a biochemically similar way, targeting the same target site in the fungal pathogen (in the case of fungicides) or weed (in the case of herbicides).

This has implications for resistance, because if the fungal pathogen or weed develops resistance to one chemistry in that MoA group, then it’s likely that other chemistries in that MoA group are also compromised, because they bind similarly.

In any fungal population there are likely to be individuals that have natural resistance and are less susceptible to some fungicides, even before chemicals are applied. This resistance arises through mutations (random changes in the genetic structure of the fungal pathogen). If resistant individuals are repeatedly exposed to the same fungicide, they may be selected for, which increases their frequency in the fungal population.

Continued use of the same fungicide or fungicides from the same (MoA) group can result in a significant build-up of resistant individuals in the fungal population – to the point where that product, or other products from the same MoA group, have reduced efficacy or are no longer effective.

The risk of developing fungicide resistance varies between different MoA groups, different fungal pathogens and different environmental factors. Consequently, specific strategies are recommended for those situations considered to carry the highest risk. You can check out some of these risks and recommendations for managing fungicide resistance on the Fungicide Resistance and Resources pages of this website.