The imminent loss of chlorothalonil, combined with regulatory and resistance pressure on other actives, heightens the need for farmers to explore alternatives at the important flag leaf spray timing.

Before chlorothalonil is lost from the fungicide armoury, growers are being advised to try out replacement T2 strategies designed around their own varieties, disease pressure and farming system.

The flag leaf spray is arguably the most important wheat fungicide timing, as it protects the top two leaves, which contribute around two-thirds of final yield.

Chlorothalonil is a core component of an increasing number of T2 programmes, and will remain so for this season at least, but Hutchinsons technical development director David Ellerton says growers should try alternatives in preparation for when it is no longer available.

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The challenge is further complicated by the mounting threat that resistance poses to the efficacy of azole and SDHI chemistry, while some key actives, such as epoxiconazole and fenpropimorph, also face the regulatory axe, he notes.

A small, informal, farm “trial” area can provide valuable information about how different products and spray strategies perform under local conditions.

“It could be whole fields or part of a field; the important thing is to start trying some of the alternatives and experiment with spray strategies to prepare for future seasons.”

Chlorothalonil alternatives

The two main alternatives are folpet and mancozeb, both of which have multisite protectant activity against the number one wheat disease, Septoria tritici.

“Folpet isn’t generally as effective as chlorothalonil against septoria, but it does offer an extra benefit against rust and there’s an indication it doesn’t hinder the uptake of azoles as chlorothalonil can,” Dr Ellerton says.

“Recent work also shows that including a protectant multisite such as chlorothalonil or folpet, can improve persistence of septoria control and is an essential part of an anti-resistance strategy.

A lot of growers probably haven’t used folpet much before, so it’s worth trying to see how it performs as an alternative.

“Mancozeb is also worth trying both at T2 and possibly as part of later T3 strategies.”

Mildew control options

For growers used to applying fenpropimorph for mildew control, there are two main alternatives to consider in cyflufenamid and proquinazid.

Cyflufenamid offers the most effective eradicant control, but cannot be used for sequential applications, so cannot be used at T2 if it has already been applied at the T1 timing, Dr Ellerton says.

Proquinazid is a powerful protectant product, so must be applied before disease has developed, he adds.

© Tim Scrivener

Try different strategies

As the fungicide armoury diminishes, Dr Ellerton says there is even more need to tailor treatments to variety and disease pressure in individual fields, rather than taking a blanket approach at key timings.

“It’s not just about finding alternative fungicides, but using any product in a much more targeted way, not as routine. This benefits disease control, resistance management and makes financial sense.”

Azoles, in combination with SDHI and multisite chemistry will feature in many T2 programmes, but product choice and rate must be tailored to variety and disease risk, with priority given to wheats rated 6 or less against septoria on the AHDB Recommended Lists.

But he reminds growers that varietal resistance is only ever a guide, particularly for rust, where changing strains of yellow rust can overcome certain varietal resistance genes.

Disease must not be allowed to establish, as curative situations are becoming harder to manage with triazole and SDHI chemistry, and also exacerbate resistance pressure.

Applying sprays at exactly the right growth stage is critical, especially at T2, where application must not go beyond the stage where most flag leaves on main tillers have emerged (growth stage 37-39), usually in mid to late May.

“Go by growth stage not calendar date though.”

If it looks like the gap since the last spray (T1) is likely to exceed the recommended 3-4 weeks, he suggests trying an interim T1.5 fungicide at GS 33-37 to protect leaf 2, rather than apply the T2 too early and leaving the later-emerging flag leaf vulnerable to infection, or waiting too long and letting disease establish on the upper leaves.

New products coming

The cereal fungicide armoury should get a welcome boost next spring with the expected arrival of two new products for septoria control, Dr Ellerton notes.

Mefentrifluconazole is a different type of azole to those currently available, so promises better control of septoria strains that are not controlled as well by existing azoles.

The other option, fenpicoxamid, is a “Qi inhibitor” from the picolinamide group of fungicides, which offers a new mode of action in cereals that will help with resistance management. It too promises good efficacy against septoria.

“There are also some new SDHI products in the pipeline, but these won’t be available next spring, so for now we must look after what we’ve got.”