Watch out for disease in extended spray interval

Growers that applied their first wheat fungicide early must monitor crops closely and prepare to take a tougher approach at T1 if the gap between sprays extends beyond the recommended three to four weeks, leading agronomy firm Hutchinsons says.

Northumberland-based agronomist Tom Whitfield says warm, settled weather in late March meant many forward crops received the growth stage 30 (T0) fungicide around a fortnight earlier than last year. But with a return to cooler conditions slowing crop development and growth stage largely determined by day length, it could be a month before leaf three emerges in some crops.

“You can’t go completely on calendar date, but it is a good guide. In this region, the wheat flag leaf usually emerges around the 25th May, up to early June in the latest crops, so working backwards, that puts the GS 32 fungicide (T1) at the end of April to beginning of May and the T0 around the start of April.”

While he does not think an intermediary “holding spray” between T0 and T1 is necessary – unless crops come under abnormally high disease pressure – stretching the spray interval to four weeks or more could let disease in, requiring a more robust curative and protectant approach at T1.

Mr Whitfield says the stronger curative action of SDHI chemistry will therefore underpin many fungicide programmes, both at T1 and T2 (flag leaf emergence).

At T1, he favours a combined approach of SDHI, plus triazole and multisite, such as bixafen, prothioconazole and chlorothalonil, to give a range of modes of action and broad spectrum of curative and protectant activity against the main diseases; notably septoria and rust.

“Applying an SDHI at T1 is generally the way to go, although if you’re growing a variety with very strong resistance to the main diseases, there may be scope not to use one.”

Hutchinsons technical development director David Ellerton says that as a minimum, the T1 should combine a high triazole dose with a multisite, and agrees an SDHI is worthwhile where greater disease pressure needs more curative action (e.g. earlier-sown crops, extended spray interval).

“While recent resistance testing has shown increased resistance to the established actives bixafen and fluxapyroxad, they generally still offer good Septoria control, while the more recent additions benzovindiflupyr and fluopyram (in combination with bixafen), have both shown good activity in trials. Wherever possible, try to avoid relying on curative control.”

The choice of exactly which actives to use depends on individual variety and disease risk, so growers are advised to monitor crops closely over coming weeks to identify issues early and ensure sprays are applied at the optimum growth stage.

Where rust control is needed, Dr Ellerton recommends the triazoles; epoxiconazole, tebuconazole, cyproconazole and metconazole, while strobilurins including azoxystrobin and pyraclostrobin offer an alternative mode of action.

Stem base and root diseases such as eyespot and take-all must also be considered, particularly in early sown second wheats, where boscalid or prothioconazole should be included for eyespot control, he says.

Certain strobilurin fungicides (e.g. azoxystrobin and fluoxastrobin) can help reduce take-all, and promote rooting in more backward crops; which may improve nutrient scavenging, or help overcome drought later in the season, he adds.

The mild winter has allowed winter barley to establish and develop well, with six-row hybrid varieties showing particularly strong vigour compared with conventional types, Mr Whitfield says.

“There’s perhaps more variation in barley growth stages than wheat, which is mostly due to the hybrids moving on so quickly in the autumn and the spring.

“A lot of barley T0 fungicides were applied at the end of March and most crops will receive their all important T1 in approximately 7-10 days’ time, which is only slightly earlier than normal.”

Mildew and rhynchosporium have been present in many crops given the mild winter, so his favoured approach is a programme based on fluxapyroxad, plus epoxiconazole + fenpropimorph.

“Although there is some disease present given the mild winter and forward crops, pressure isn’t necessarily any higher than normal. Wheat and barley crops generally look very well at the moment, with good yield potential, so we need to protect that as best we can over coming weeks.”