The multisite fungicide chlorothalonil has been a mainstay of both wheat and barley fungicide programmes since it was launched in 1964, however, UK farmers may soon be unable to use it to control key cereal diseases.

Sold under various brand names, the most familiar of which is Bravo, the active ingredient has become a key component of spray programmes aimed at septoria and is the only mode of action left with efficacy against ramularia in barley.

Also used as a cost-effective mixture partner to protect higher-risk single site-acting fungicides such as the azoles and SDHIs, its resistance management role has become increasingly important as pathogens have evolved and become less sensitive to widely used chemistry. 

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Alternatives to chlorothalonil do exist, but they mostly come with a higher price tag and lower disease efficacy. Ramularia control will be a challenge without it, as the pathogen has developed resistance to SDHIs, strobilurins and azoles. 

Continued supplies

The good news for cereal growers is that uninterrupted use of chlorothalonil throughout the forthcoming 2019 growing season is in no doubt at all, says Dave Ranner, technical support manager with Syngenta, who is also hopeful that there will be supplies of the product in 2020 as well.

“There is a good chance that the product will go completely and it would be wrong to give the industry false hope,” he confirms.

“But supplies are not going to dry up overnight. As we’ve seen with other bans, there will be a phased implementation of any market withdrawal.”

Both regulatory and classification hurdles are standing in the way of chlorothalonil’s future, which is expected to be decided by an EU Member State vote in March.

There is a small, outside chance that it will stay for resistance-management purposes, but that means it will be subject to restrictions, with both permitted timings and rates likely to change.

“We are actively involved in work looking at replacements and alternatives, as well as how to get the best from them,” confirms Mr Ranner. “We also continue to invest in new products and innovations.”

Regarding its use in wheat, there are the twin challenges of septoria control and cost to overcome, he acknowledges. “In the short term, we will be able to maintain control of the disease without chlorothalonil, but it will be more expensive.

“Luckily we have new varieties with better disease resistance coming along all the time, so growers will be able to exploit those higher septoria ratings.”

In barley, there’s currently no satisfactory answer to ramularia control without chlorothalonil, he confirms.

Track record

With resistant and less-sensitive pathogen strains on the rise and other chemistry declining in efficacy, the multisites haven’t slipped at all in terms of performance and there are no recorded instances of resistance in any of the cereal foliar diseases.

Chlorothalonil is one in a group of three multisite fungicides that can be used in the UK, explains Jonathan Blake, principal research scientist with ADAS, none of which are as inherently active as newer chemistry, but all of which are important.

“Chlorothalonil, mancozeb and folpet all come under this banner,” he says. “They matter because they are much less prone to resistance. The reason for this is that they have multisite modes of action.”

Of these, only chlorothalonil is under review, meaning that there are alternatives if the EU votes to remove it, he points out.

“They are all protectant materials, so their use must be timed carefully,” he advises.

Basing fungicide programmes on products that don’t develop resistance is important, as it slows the development of the problem and helps to maintain other chemistry for future use.

As such, advice from Fungicide Futures – the joint AHDB and Fungicide Resistance Action Group initiative – is to consider the use of a multisite at T0, T1, T2 and T3, so that maximum efficacy and protection of other chemistry is ensured.

What’s the problem?

© Tim Scrivener

All active ingredients have to be re-approved every 10 years and new assessment criteria used by the EU is making it harder and harder for old chemistry to get through, reports Emma Hamer, senior plant health adviser with the NFU.

A multisite fungicide with more than 50 years under its belt is bound to have areas of concern, but that doesn’t mean that it’s not safe to use providing guidelines are adhered to, she adds

If a ban is put in force, there is usually a six-month grace period for sales and distribution, with a following 12-month use-up period – although these timescales can vary.

“There tends to be a degree of pragmatism with the arrangements, so that growing seasons aren’t interrupted.”

One of the issues for chlorothalonil is that it has already been given a classification as a carcinogen by the European Food Safety Authority (EFSA), even though it is the role of ECHA (European Chemicals Agency) to produce this information.

“The NFU would prefer to see the due processes being followed, so have argued that any decision should wait until ECHA has issued its classification.”

She describes chlorothalonil as the backbone of many fungicide programmes and the cornerstone of resistance management.

“NFU lobbying work has raised awareness of the importance of chlorothalonil products and explained why the active ingredient is so useful. We have been working to show MPs and MEPs why the loss of chemistry causes difficulties for growers.

“Unfortunately the precautionary risk assessments now being used by the EU mean that very few older molecules will stay. They just don’t meet these new criteria.”

Future optimism

septoria © Blackthorn Arable

There are a number of reasons why growers needn’t worry too much about losing chlorothalonil, providing the timescales of any ban are realistic, believes Bill Clark, technical director of Niab Tag.

There are alternatives, in the form of folpet and mancozeb, which have been looked at in trials and do have some effect, so can be used in programmes instead, he points out.

In addition, new chemistry and low-risk varieties are on their way, making two spray and diverse fungicide programmes a reality and allowing fungicide use to be limited.

“Of the other existing options, neither folpet nor mancozeb should be considered as a direct replacement and they are more expensive,” he says. “But we know what they’re capable of.”

Sulphur, a biological solution, is another choice, but more evidence of its efficacy is required, he adds.

Whatever the choice, chlorothalonil’s inclusion in programmes as an anti-resistance measure will be more difficult to replace, with both mancozeb and folpet being a harder sell for this purpose, he notes.

However, having chlorothalonil still available for 2019 and 2020 takes the pressure off and means that the planned introduction of new chemistry – in the form of Revysol and Inatreq – provides an opportunity to protect the remaining azoles and SDHIs.

“These new products, both of which are expected to be launched in 2020, will allow us to mix and alternate chemistry, so that they are only used once in a season. That will make a huge difference.

“Using completely different chemistry at T1 and T2 will be possible. Both products are genuine mixing partners, as they are eradicant and systemic.”

Mixing chemistry and limiting the number of sprays is important in preventing resistance developing, he adds. “Revysol and Inatreq are a glimmer of hope and could be on the market in time for chlorothalonil’s demise.”

His other reason for optimism is the arrival of new varieties with better disease resistance and lower responses to fungicides.

“We’re starting to see the introduction of low-risk types that only need one highly eradicant spray in a season,” he reports. “These are genuine two spray varieties, which don’t need a T0 spray.”

Growing highly susceptible and very responsive wheat varieties can’t be done in most seasons without chlorothalonil, stresses Mr Clark.

“If you haven’t already started growing more-resistant varieties, it’s definitely the time to start.”

 T0 and T1 replacement options

Trials conducted by Zantra show just how significant the loss of chlorothalonil and other likely pesticide losses will be.

In 2018, a T0 trial that started with seven different treatments was reduced to just two once the active ingredients under threat were discounted.

“It’s not just chlorothalonil that’s going,” points out technical director Chris Bean. “We know that epoxiconazole won’t be with us for much longer, nor will cyproconazole and propiconazole.”

That means popular T0 products such as Cherokee and Alto Elite won’t be available, as well as Cloister and Perseo.

Given those constraints, there was just one treatment in the Zantra T0 trial that gave a 0.4t/ha response over untreated, consisting of a tebuconazole + mancozeb mix.

“It looks like a useful combination at T0,” reports Mr Bean.

A further T0 trial compared the performance of chlorothalonil, folpet and mancozeb, alongside a coded development product. “The results show that nothing quite gets to the heights of chlorothalonil, but mancozeb is trying.”

The company’s T1 work gave similar results, he adds, with mancozeb “doing its best” when added to the spray and showing that there is a replacement if needed.

“We also saw the value of a higher azole dose at T1, as well as the value of a strobilurin over an SDHI in a dry year.”

With an eye on the future, prothioconazole and metconazole were assessed in the place of epoxiconazole at T1, with both performing well.

“The toolbox is diminishing but there are options. We are also looking at other products, such as bio-elicitors, to see whether they have a role in future disease management strategies.”