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.
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.
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 Mode of Action (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.
Fungicide Resistance Action Committee (FRAC) – Download their MoA app!
Grains Research and Development Corporation (GRDC)
GRDC Groundcover Supplement: Resistance in Weeds, Pests and Diseases. Issue 139: March – April 2019.
An overview of chemical resistance in weeds, pests and diseases. Largely in laymen’s terms, without compromising on depth or quality.
Umina et al. Your guide to agricultural chemical resistance in a nutshell.
Young. Your guide to ag chemical resistance mechanisms in laymen’s terms. Available at:
Young. What drives the pace of resistance development in agricultural chemicals? Available at:
McDonald. Changing up chemical groups essential to preserve longevity of actives. Available at:
Poole et al. Label rates for effective control of weeds, pests and diseases. Available at:
Hoffman & Lopez-Ruiz. Common tactics for managing agricultural chemical resistance. Available at:
Poole et al. Strategies must differ for weeds, insects and fungal pathogens. Available at:
Oliver. Overseas fungicide resistance experience guides Australia. Available at:
Van de Wouw. Fungicide resistance needn’t be a blot on the landscape. Available at:
Evaluating the risk of fungicide resistance
Principles of fungicide resistance
Vincelli, Uni of Kentucky
Fungicide resistance and management strategies
National Institute of Agricultural Botany (NIAB) UK
Fungicide resistance management
Fungicide Resistance Action Committee (FRAC)