The Fungicide Resistance Action Group (FRAG-UK): The use of multisites

In response to the withdrawal of the multisite chlorothalonil on 20 May 2020, FRAG-UK has updated its fungicide resistance stewardship advice.

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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 on the 20 May 2020 removes 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.

The core principles of resistance management are:

  • Use cultural and agronomic measures (e.g. resistant varieties and late drilling) to reduce disease risk and reliance on fungicides in programmes
  • Alternate and diversify the chemistry used in programmes
  • Use balanced mixtures of chemistry in programmes
  • Minimise the use of high resistance risk chemistry and maximise the use of lower-risk options in programmes
  • Include multisite fungicides, which are categorised as low-resistance risk

The use of multisite fungicides – such as folpet and mancozeb (within label recommendations on maximum dose, application number and timing) – will reduce pressure on higher-risk chemistry.

Given lower levels of efficacy compared to chlorothalonil, the rates of folpet or mancozeb should be kept high to maximise their efficacy.

Note: Mancozeb is not approved for use in barley. Take care to stay within label limits for both actives.

Fungicide resistance risk explained

Fungicides vary in their inherent risk of resistance development. All fungicides are assessed prior to launch.

Multisite fungicides, which act at many points in the metabolism of a target pathogen, are harder for the pathogen to overcome and are classed as low risk.

Single site fungicides are at greater risk. SDHI fungicides are classed as medium-to-high risk and azole fungicides are classed as medium risk.

An example of a fungicide strategy with a high risk of resistance would include multiple applications to a crop of a high-risk fungicide unsupported by mixture partners.

Fungicide strategies that reduce and minimise the use of high-risk actives and only use them in balanced mixtures are at reduced risk of resistance.

Programmes that include the use of low-resistance risk choices, such as multisite fungicides, to support mixes are at the lowest risk of resistance development.