Spray timing vital to beat winter wheat disease

The timing of a key fungicide spring spray is set to be crucial this season with winter wheat crops very forward and plenty of disease 
lurking in fields.

A mild winter has seen widespread levels of septoria – wheat’s most yield-sapping disease – so timing of the so-called T1 fungicide treatment will be important.

See also: Disease pressure alert for winter wheat

Bill Clark, commercial technical director at crop consultants Niab Tag, says identifying this T1 timing will be vital in combating the disease, especially as this can vary by two to three weeks depending on the season.

Mr Clark advises that growers need to look at leaf emergence on wheat plants rather than at growth stages or calendar dates for the T1 spray, which is usually applied in late April or early May.

“Timing is more important than product choice or dose, and there is no substitute for looking at leaf emergence,” he tells Farmers Weekly.

The T1 spray needs to be applied when most of leaf three is emerged on the main stem of the wheat plant, but the emergence of this leaf is not easy to determine.

“Spraying too early can be as bad as spraying too late. There is no substitute for looking at leaf emergence,” he adds.

Many regard T1 as relating to growth stages 31-32 (GS 31-32), but there is a big difference between these two growth stages, and T1 sprays should not be going on at GS 31 when leaf three is not present.

Bill-Clark“Timing is more important than product choice or dose, and there is no substitute for looking at leaf emergence” – Bill Clark

In winter wheat, GS 31 is when one node can be detected on the main stem; and GS 32 when the main stem has two nodes some 2cm apart.

The T1 timing often coincides with GS 32, but Mr Clark emphasises that counting nodes is only a rough guide, while spraying by a particular date is to be discouraged.

He advises to cut wheat stems open to get a true picture of the number of nodes, while splitting open the stem will also give a good idea of leaf emergence.

However, the emergence of leaf three is not always easy to determine. Mr Clark suggests that the best way to be sure is to cut out the top three leaves. If leaf three is emerged then leaf two and leaf one, or the flag leaf, will be furled up inside the main stem.

This dissection needs practice as the flag leaf is less than 1cm long at this stage and can easily be missed.

He suggests the T1 fungicide should be applied when leaf three is two-thirds to three-quarters emerged on the main stem, while tillers will be further behind in growth stage, perhaps a third or half emerged.

Correctly timed, the T1 spray can give a high level of disease control on leaves three and four, both of which can affect whether or not the 
disease spreads to the yet-to-emerge upper two leaves.

Growers with large areas to spray usually start before and end after the optimum timing; so crops at a high risk from disease should be targeted near the optimal spray timing, while crops at a low risk have more flexibility.

T0    GS 30-31
T1    GS 31-32
T2    GS 37-39
T3    GS 61-65

Beating blackgrass

With many crops drilled later in the autumn this season to counter blackgrass, the difficulty in identifying the T1 timing increases as the emergence of leaf three often coincides not with GS 32, but GS 31.

The main target for T1 sprays is septoria tritici, but other diseases such as yellow and brown rust and eyespot can be 
controlled with a broad-spectrum fungicide.

GS 30    Ear at 1cm (pseudostem erect)
GS 31    First node detectable
GS 32    Second node detectable
GS 37    Flag leaf just visible
GS 39    Flag leaf blade all visible
GS 61    Start of flowering
GS 65    Flowering half way complete

Mr Clark says that growers who spray T1 can no longer rely on just an azole fungicide, due to the decline in the effectiveness of this group against septoria.

He suggests growers need an azole mixed with a newer SDHI fungicide and a protectant product, such as chlorothalonil, at the 
T1 timing.

It would have to be a very dry spring and generally low disease year to get away without this three-way mix, he adds.

His message is to try and prevent disease resistance building up by keeping dose rates high and to use sensible mix partners, such as azoles with SDHIs.

Due to the downward drift of efficacy of the azoles, such as epoxiconazole and prothioconazole, rates need to be high as the lower azole rates that were effective 15 years ago are simply not applicable now.

Mr Clark says that growers should use an azole rate of at least two-thirds to three-quarters, and stresses that anything below a half dose is a waste of time.

The higher dose is needed to control disease and also to protect resistance building up to the SDHIs.

“A low dose of azole would put the SDHI at risk and also lessen the control of septoria,” he says.

Chart of T1 spray timings

All illustrations are from Agri-tech Register and Training for Innovation and Skills (ARTIS), which is an industry-led, accredited training initiative with funding from the UK Commission for Employment and Skills. Its training courses are developed by Niab Tag with input from agronomists, experts and consultants.

1. A T1 fungicide should be applied when leaf three is almost fully emerged on the main wheat stem, with leaves four and five already fully emerged

2. Wheat plant at the T1 stage with leaf threes almost fully emerged.

3. Wheat plant with leaf threes coloured red.

4. What happens if you get T1 wrong. Septoria is well established on leaf three with the flag leaf fully emerged (GS 39). This is a good indicator of the need for a robust T2 spray.

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