Why are high doses bad news for fungicide resistance?

The best dose to counter pesticide resistance depends on whether the target is a weed, a pest or a disease. Ellie Dearlove, Knowledge Transfer Manager (Cereals & Oilseeds), explains.

The maximum individual dose of any pesticide is clear. It’s written on plant protection product labels.

Knowing when and how much to reduce rates isn’t as straightforward, but your choice will impact control in current crops (because of pest pressures) and future crops (because of resistance).

Many factors affect the appropriate dose, such as:

• Conditions at or around spraying
• Growth/developmental stage
• The pesticide used

When these factors were discussed at our Resistance Roadshow (winter 2025/26) it revealed that many people still get confused about dose.

With a limited number of pesticides and mode of action (MoA) groups, chemistry must be used in the right way to protect efficacy.

In particular, the spray target (weed, pest or disease) makes a big difference to the best approach.

In this blog, I look at each target in turn, so you know when it's best to cut back or opt for a rate closer to or at the full label rate.

Although reduced rates may kill many weeds in a population, the ‘tougher’ ones can survive.

Initially, survivors may not be fully resistant. However, repeated exposure to the same herbicide MoA encourages resistance to build up in the weed population. 

To manage herbicide resistance (target site and non-target site resistance), using a diversity in control approaches is key.

Combining cultural controls (such as rotation, delayed drilling and cultivation) and chemical diversity (through rotating MoA, as much as possible) will make it much harder for weeds to break all defences.

Close field monitoring to spot and remove survivors quickly is also essential to stop resistance spreading.

Critically, it’s important to follow the manufacturers' label recommendations to set the optimum dose for a complete kill (and to determine the appropriate spray timing/application method).

In UK arable crops, pyrethroids are the only synthetic chemical option approved to control many insect pests (and some of these are already resistant to them).

As alternating MoA is not usually an option, insecticide resistance management means: 

• Using insecticides less often
• Using them at the full label rate

Once again, if you underdose, resistant individuals can survive and multiply very quickly.

The lack of insecticides is intensifying research into alternative solutions, including synergists.

Scientists have found that some synergists inhibit the activity of key metabolic enzymes associated with pyrethroid detoxification in resistant individuals (including cabbage stem flea beetle).

This helps to restore insecticide efficacy. However, it may be a while before such solutions are on the market.

Unlike herbicides and insecticides, using higher/highest fungicide doses can increase resistance pressure, especially with modern single-site products.

High doses wipe out sensitive disease strains, allow resistant ones to build up and can eventually impact control.

By reducing the dose, it keeps a mix of sensitive and resistant strains in the population. This slows down resistance and maintains product efficacy. 

It’s a tricky balance, because too-low-doses will lead to unacceptable disease control.

Additionally, if a fungicide dose is too low to control disease sufficiently, other fungicides used in a mixture or in later sprays may be at a higher risk of selecting for resistance.

A higher number of applications of any MoA is also a key driver of resistance. In DMI fungicides (azoles), this may be more important than dose.

However, all fungicides should be applied at the minimum effective dose (no higher than needed).  

Integrated pest management (IPM) will reduce disease pressures and the dose required.

Resistance management also requires good use of available MoA (by mixing and alternating), which should include the use of multi-site fungicides (which have a relatively low resistance risk).

Genetics is a key reason for the difference between targets, especially the gene copy number (as well as reproduction).

Most plants and insects are diploid with two copies of each gene.

In a population of insects or weeds, there may be individuals that are: 

• Fully susceptible (e.g. both genes susceptible)
• Partially resistant (e.g. one gene susceptible, one gene resistant)
• Fully resistant (e.g. both genes resistant)

Note: The above points are a simplification. Organisms may have several distinct resistance genes.

For insecticides and herbicides, higher doses will kill susceptible and partially resistant individuals (use recommended label rates to set the dose).

Decreasing the number of partially resistant individuals also reduces the frequency with which fully resistant offspring arise through sexual recombination.

In contrast, most plant pathogens are either haploid (one copy of each gene) and/or reproduce clonally.

This means that they don’t have a partially resistant stage (in the same way as insects/weeds) or they reproduce in a way that copies whole strains exactly.

A pathogen population often has a mix of sensitive and insensitive strains (to various degrees).

The higher the dose, the quicker the selection for the most resistant strains.

We are most worried about the strains that become so resistant that even the maximum label dose doesn't kill them.

For fungicides, minimum effective doses promote a good mix of sensitive and resistant strains in the population (use efficacy data to judge rates).

IPM will reduce the need to spray (and help you reduce fungicide doses).

For example: 

• Use cultural controls to reduce pest pressures and support beneficial insects  
• Monitor crops and stick to treatment thresholds  
• Use decision support tools to determine the need to spray and to get timing right 
• Use a variety of MoA (as many as possible)