Triazole troubles will bring disease control headaches

Future fungicide sprays may need to be based on a greater number of active ingredients due to the steady fall in effectiveness of the dominant disease-controlling chemicals, the triazoles.

Mixes of two triazoles, the addition of new SDHI fungicides and protective products could be the way forward, according to Bill Clark, commercial technical director at consultant NIAB TAG.

“In an eradicant situation against septoria, the two best performing triazoles – prothioconazole and epoxiconazole – now only give 40% control at best. That’s come right down from 80%,” Mr Clark said.

Many growers use fungicide products such as Adexar, Seguris and Aviator – which are mixes of well-tried triazoles and newer SDHI – although Mr Clark believes wider mixes may be needed for optimum control.

Co-formulated triazole mixtures, such as Brutus (epoxiconazole + metconazole), are useful and could help extend the lives of individual ingredients, he predicts. “They are very effective, as has been shown in trials and the field in the past. And it’s a better strategy than alternating triazoles,” he says.

SDHIs have helped to improve the eradicant activity of sprays, Mr Clark says, but they have to be used at high doses to be effective.

He believes that future spray treatments may be based on a mix of two triazoles, one SDHIs and a protective multi-site fungicide such as chlorothalonil.

“That would be the standard spray. It has cost implications over our standard practice, but it could become necessary,” he says.

For now, Mr Clark suggests growers keep the most effective triazoles – prothioconazole and epoxiconazole – for septoria control, using others for early rust control. “It may help to prevent early selection if you make use of something like tebuconazole early on, keeping the big guns back for T2 and T3,” he adds.

His other point is to include chlorothalonil at the earlier timings, and possibly throughout the season. “Multi-site actives are an important part of what we do.”

The decline in the eradicant activity of the triazoles may lead some growers to need an extra, pre-flag leaf spray as the norm.

“If you can’t get sufficient eradicant activity from the flag leaf spray unless you use very high doses of an SDHI/triazole combination, the solution could be to include a GS33/37 spray to protect leaf 2,” Mr Clark says.

Such a pre-flag leaf spray would probably be a triazole/chlorothalonil combination.

Multi-site protectants prove their worth in septoria season

Multi-site protectants in a fungicides spray programme last year proved useful for septoria control, reports Mark Hemmant of adviser Agrovista.

“Whether it was Bravo (chlorothalonil) or Phoenix (folpet) doesn’t really matter. Both have a place and are important components of any good disease control strategy,” he says. This was emphasised by the results of the HGCA-funded fungicide response work, which included folpet for the first time in 2012.

While chlorothalonil is cheaper at almost half the price, it raises some issues.

“It can really mess up partner products and there might be timing constraints. So you need to choose the right material for the field situation,” he says.

The two obvious timings for these materials are T0 and T1, advises Mr Hemmant. “At T2, their inclusion can be more difficult. It isn’t recommended to use chlorothalonil with the SDHI Aviator, for example.”

If there’s a need for eradicant rust control, folpet should be the multi-site protectant of choice, he recommends. “Folpet can actually improve rust control. Unlike chlorothalonil, it doesn’t inhibit the uptake of the triazole.”

In barley, there’s a case for using multi-site protectants at T1 and T2, he says.

“We’ve seen better control of leaf spotting, or ramularia, from the T2 timing if folpet is included with this spray. It’s another example of where folpet is a better choice than chlorothalonil.”

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