The yellow rust picture is complex this year with experts unclear as to whether new strains have emerged.
Yellow rust infection levels in untreated wheat trails have been exceptionally high this year with varieties having high scores for resistance to the disease succumbing and some supposedly susceptible varieties doing better than expected.
The picture surrounding variants of the disease is complex, said NIAB plant pathologist Dr Sarah Holdgate, who heads up the UK Cereal Pathogen Virulence Survey. “We are seeing more on certain varieties. Reflection, Britannia, Myriad and Zulu do look a little bit worse than their [Recommended List] resistance ratings would suggest.
“There has almost certainly been a change in the population of yellow rust this year. Is this because of some new variants we saw towards the end of last year or something we’ve not seen before? There’s almost certainly been a change in race.
“Initial testing of Reflection has shown multiple races on the same leaf; we need to work to tease out the different races.”
However, just because a variety has shown susceptibility in one particular year does not mean it necessarily will the next, said Dr Holdgate. “Some have eroded from the top [resistance score] and some varieties at the bottom are not so dirty. With such a disease population anything is possible.”
Yorkshire-based agronomist James Gaines of Agronomy Supplies said he has seen untreated trials where 80% of varieties were affected. “Varieties with risk scores of 8 are covered in yellow rust. I’ve never seen it in as many different varieties. I’ve never seen it in Claire until this year.
“A mutation must have occurred fairly recently leading to more varieties becoming susceptible. You can throw the Recommended List rating out of the window. It bears no relation to what’s happening on farm.”
However, in commercial rather than trial situations, control has been good, he said. “Most people have controlled it very well. Use of a good triazole and strobilurin chemistry has kept crops very clean. It is relatively easy to control if spray windows keep to a three-week interval.”
After a mild winter, spraying early was the key to yellow rust control this year, believes Procam East Anglia-based agronomist Christina Lankford. “Quite a few farms ended up with it over winter. I said to one of my growers in mid-March – you’re going to have to spray.
“Some people may drop varieties but there’s no reason why they won’t yield well if they’ve been looked after. The answer is to put a seed dressing on to eliminate winter carry over.”