Yellow rust threat has not gone away

Commercial wheat crops may have escaped yellow rust this year, but the less fortunate untreated trial plots tell the real story. Louise Impey reports

A combination of luck and good planning. That’s how Rosemary Bayles of NIAB TAG summarises the way that wheat growers managed to avoid yellow rust in 2010.

“All credit to growers and agronomists,” she says. “They were aware of the risks, knew which varieties to watch for and then used the appropriate spray programme.

“Fortunately, there were no breakdowns in the remaining resistant varieties. We certainly had plenty of aggressive yellow rust in all of our untreated plots, so the threat was there.”

The cold winter also gave growers a helping hand, she continues. “It meant that yellow rust got a slow start this year. It came in between three and four weeks later than usual.”

That was in stark contrast to the previous year, when the disease got going early and spray programmes weren’t completed on time or able to cope.

“This time, everyone was on the lookout and the homework had been done.”

Paul Fenwick of Nickerson agrees. “There weren’t many problems with yellow rust in commercial crops because the disease got delayed and farmers were able to spray in time. The surprise element wasn’t there.

“When it finally did put in an appearance, it was late and growers could get on top of it. We had favourable weather at spraying time, which also helped.”

But both point to high levels of the disease in untreated crops and trial plots on a nationwide basis. “Yellow rust was everywhere, which shows that the problem hasn’t gone away,” adds Mr Fenwick. “Getting sprays on at the optimum time proved to be crucial.”

The dry spring and summer conditions haven’t deterred it either, remarks Dr Bayles. “Rust needs short periods of high humidity for the spores to germinate and get into plants. A heavy dew is enough, there doesn’t have to be lots of rainfall.”

Looking ahead to next year, there will be inoculum around, both experts stress. “Don’t let your guard down,” they advise. “The same guidance will apply again – limit the area of susceptible varieties that you grow and consider the use of seed treatments to buy some time.

“Don’t grow any more of a variety than you can spray in one day. Everyone has managed yellow rust well this year, so we’ve learned some valuable lessons. Variety choice has proved to be important.”

A mild winter and early spring could see a repeat of the 2009 situation, warns Dr Fenwick. “Those are the conditions which allow the disease to come in early. And farmers are still growing varieties with rock bottom resistance.”

Broom’s Barn View

The cold winter was only part of the yellow rust story, says Bill Clark, director of Broom’s Barn.

The disease’s ability to survive low temperatures is well recognised, he adds. “But it also needs cool, moist weather in the spring to grow and produce sporulating lesions. This year’s dry spring months slowed and then stopped it.”

Mr Clark adds that the fungus is inhibited by temperatures of over 20C, so the weather in June and July also prevented a potential late epidemic.

“But any areas that were wetter and cooler in the spring saw very aggressive rust attacking untreated parts of the field. It’s a warning for next season – the disease was only stopped for the time being.”

Fluquinconazole-based seed treatments can be a good first line of defence, he adds. “They prevent autumn and early spring infection, allowing a planned approach. That’s important because in the right conditions, the disease can cycle in seven days.”

Brown Rust

Brown rust has also put in an appearance this summer, but only on a patchy basis.

And with wheat crops ripening early in the south – due to the hot, dry weather – the disease hasn’t been able to make an impact.

“Brown rust has come in too late in the season to have much of an effect,” notes Dr Bayles. “But it can come in much earlier. It was another disease slowed up by the hard winter.”

Dr Fenwick adds that rain would have made it worse. “It is going well in our untreated plots, so it hasn’t dried up completely.”

• Bill Clark takes a more indepth look at yellow rust and how to avoid it in our online yellow rust academy, including a test-yourself section.

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