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  • Eric Gussin

Chlorothalonil: Unforeseen Consequences

Updated: Dec 30, 2019

Most arable farmers and advisers will not have been surprised

by the EU’s decision to revoke the approval for the use of the fungicide Chlorothalonil. This particular fungicide has a proven track record of both safety and efficacy on a range of crops, most notably on cereals, and has been used for over 50 years in the UK.

One feature of this material which may well have escaped the attention of the Expert in Pesticides Committee is that as it acts as a multisite active. It ‘protected’ a significant number of other fungicides from a shift of resistant strains of pathogens. This effect is derived primarily from the way it attacks fungi in several different ways meaning that single mutations are rarely if ever able to grow in its presence.

Most notably Septoria tritici (Mycosphaerella graminola) which is considered by many plant pathologists as the single most damaging fungal pathogen of wheat and Ramularia (Ramularia collo-cygni) a particularly damaging disease in barley and especially spring barley. This effect when combined with one or more fungicides with other modes of action has proven to be a remarkably effective and consistent treatment. As such this active ingredient was a significant tool to the fight against fungicide resistance development and its loss may have unforeseen consequences.

From 20th May 2020 Chlorothalonil may not be used and although most wheat crops will probably have been treated by then many barley crops and particularly spring barley crops will be awaiting their T2 treatment and will need a preventative fungicide against Ramularia.

There is a very short list of possible alternatives which offer some protection when used prophylactically.

Folpet: A multi-site protectant fungicide from Adama: this has approval for use on wheat and barley. FRAC Code M4.

Mancozeb*: A multi-site protectant fungicide from several manufacturers: this has approval for use in wheat and some other crops but currently NOT barley but see footnote. FRAC Code M3

The situation has been even more crucial as a result of the poor autumn which is almost certain to lead to a significant increase in the area of spring barley grown and this crop is by some way the most susceptible to Ramularia. The consequence of this is that disease control and management in barley has become very much more complex, something Optima Excel are actively helping growers with.

AHDB publish a very useful on fungicide performance:

* Optima Excel are aware that an application for an emergency approval for the use of Mancozeb on barley has been made but refused by the HSE despite there now being effectively only one active that can be used with good efficacy on barley.

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