Yellow rust in wheat

Early wheat fungicide spray timings should be driven more by disease levels in the crop coming out of the winter rather than by any specific growth stages.

The first fungicide spray of the season, referred to as T0, is normally applied towards the end of March or into April, but signs of early yellow rust are a better spray trigger.

See also: Wheat growers swing to varieties with better disease resistance

Bill Clark, commercial technical director at crop consultant Niab Tag, says the T0 spray timing tends to be less critical than later timings and should be determined by disease levels.

“Generally, yellow rust is driving the T0 timing. If you can find the disease in a crop then you should be looking to spray,” he tells the Farmer Weekly.

In a wet spring, wheat’s biggest disease threat, Septoria tritici, will normally be present. But it will be difficult to eradicate from the lower leaves.

Unless leaf four is visible at the classic T0 timing, Mr Clark argues that there is little point in trying to control septoria on lower leaves such as leaves five and six.

The T0 timing coincides with GS30-31, or the end of tillering to first node detectable on the main stem, and is aimed at protecting leaf four as leaf three will not yet have emerged.

With plenty of yellow rust and septoria present in many crops after a relatively mild winter, a T0 spray is likely to be essential this spring unless the weather turns very dry.

If growers are able to delay their T0 spray as long as possible then this will reduce the gap between the T0 and the later T1 timing, when leaf three will have emerged, and hence give better disease control.

The choice of fungicide products at T0 will depend on the risk of septoria, based on the winter wheat variety’s resistance to the disease and if the weather is wet, as septoria thrives in wet conditions in March/April/May.

In a high-risk septoria situation, Mr Clark advises using the most effective and expensive azoles, prothioconazole or epoxiconazole, along with the multi-site protective product chlorothalonil.

Whereas in situations of lower septoria risk, he would select cheaper azoles with good knock-down activity, such as tebuconazole or cyproconazole, and again adding in chorothalonil.

Chlorothalonil is often used at this stage as it gives good protectant activity against septoria and in wet weather it moves up on to the emerging leaf three.

For the later T1 timing, normally in late April/early May, Mr Clark says growers need to target septoria and suggests an azole/SDHI/chlorothalonil approach.

“Growers cannot rely on just an azole at T1, they need the SDHI to give better control,” he adds.

However, the newer SDHI fungicides need protecting from resistance developing to these fungicides to avoid or delay the fall in efficacy seen with the azoles.

Mr Clark advises keeping the rate of the azole at two-third or three-quarter dose rate to give good disease control and also protect the SDHI.

“Anything below a half rate of azole is simply a waste of time. You need the higher dose rate for disease control and to protect the SDHI,” says Mr Clark.