Fungicide resistance management in cereals

This FRAG publication outlines the fungicide-resistance status of the key cereal disease pathogens. It also describes the resistance risks associated with the main fungicide groups and provides guidance on how to protect long-term efficacy of chemistry.

Most modern fungicides have single-site modes of action, acting on specific biochemical pathways in the target fungal pathogen.

Repeated use of fungicides with the same mode of action can select for isolates of the fungal population that have a reduced sensitivity to the fungicide – leading to a loss of efficacy.

Tests, conducted on fungal isolates, can determine a population's EC50 value for specific fungicides.

An EC (effective concentration) of 50 is the fungicide dose that results in a 50% inhibition of the isolate, compared to a control (no fungicide applied).

If isolates with increased EC50 values remain at low levels, they may have no impact on the field performance of the fungicide when used at commercial doses.

Resistance can arise rapidly and completely, so that disease control is lost in a single step – such as the G143A mutation that affects the performance of strobilurin fungicides.

More commonly, resistance develops gradually so that the pathogen population becomes progressively less sensitive – such as the development of resistance in Zymoseptoria tritici to the azole fungicide group.

To have the greatest impact, anti-resistance strategies need to be implemented as soon as fungicides are introduced to the market, before any shifts in sensitivity are detected.

Multisite chemistry

The use of low resistance risk multisite chemistry to reduce selection pressure on higher resistance risk fungicides is an important component of anti-resistance management strategies.

The withdrawal of chlorothalonil in 2020 removed this active ingredient from the options available.

Chlorothalonil was an important component of anti-resistance management strategies, particularly when used as part of control of septoria leaf blotch.

However, the principle of using the lowest resistance risk options and limiting the use of higher-risk options within fungicide programmes remains core to resistance management advice. 

Although other multisite fungicides are not as effective as chlorothalonil against septoria leaf blotch and carry some additional cost, the inclusion of lower resistance risk chemistry – such as the multisite fungicides folpet and mancozeb – is still useful in reducing reliance on other higher-risk chemistry.

Limit the level of exposure of the target pathogen to the fungicide.

• Make use of varieties that exhibit a high degree of resistance to diseases known to be prevalent in your area, in addition to the main agronomic factors you desire. However, avoid growing large areas of any one variety
• Use integrated pest management to reduce the use of fungicides – e.g. disease-free seed lots, good hygiene and control of volunteers
• Only use fungicides in situations where the risk or presence of disease warrants treatment
• Use a dose appropriate for the variety and disease pressure that will give effective control
• Make full use of effective fungicides with different modes of action – in mixtures or as alternate sprays
• Ensure that mixing partners are used at doses that give similar efficacy and persistence
• Monitor crops regularly for disease and treat before the infection becomes well established
• Avoid repeated applications of the same product or mode of action and never exceed the maximum recommended dose or number of applications

• Septoria tritici blotch
• Powdery mildew
• Brown rust
• Yellow rust
• Other cereal rusts
• Tan spot
• Net blotch
• Rhynchosporium
• Eyespot
• Ramularia leaf spot
• Microdochium spp.

The Fungicide Resistance Action Group (FRAG-UK) is a forum that looks at fungicide resistance issues and publishes information and advice relevant to the UK. The group combines the expertise of industry with the independent sector to produce up-to-date information on the resistance status of important diseases in UK agriculture and to suggest ways to combat resistance.