A major resistance gene used to protect winter wheat from yellow rust disease has been overcome by evolving strains of the pathogen, according to the AHDB and its research partners.

It follows unusually high levels of the disease reported this spring in both Recommended List (RL) variety trials and commercial winter wheat crops.

AHDB first issued an early warning in April advising growers not to rely on the current RL disease resistance ratings for yellow rust.

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Early warnings

Unusual activity was first seen in late March in an RL fungicide-untreated trial near Sunderland.

At this site, several varieties had yellow rust symptoms, despite being classified as resistant to the disease at the young plant stage on the current RL.

AHDB senior crop production systems scientist Paul Gosling, who manages the RL, says that following discussions with plant breeders, it was suspected an important yellow rust resistance gene – Yr15 – had potentially been overcome.

“We subsequently observed similar resistance-breakdown patterns more widely in RL trials, starting in southern Scotland, before spreading down the eastern coast of England, down as far as East Anglia,” he says.

Resistance breakdown

Analysis by the AHDB-funded UK Cereal Pathogen Virulence Survey (UKCPVS) confirmed that Yr15 had failed.

Not only did the pathogen infect test varieties carrying Yr15, but it also sporulated profusely – clear evidence of a resistance breakdown, he explains.

Molecular markers further confirmed the presence of the Yr15 gene in several of the varieties affected at the young-plant stage, with this genetic testing funded by Niab.

The initial tests prioritised varieties with a strong level of adult-plant stage resistance (disease rating 8 or 9) on the current RL.

Hard Group 4 varieties – such as Dawsum, Typhoon, and Champion – were among those found to carry the now-compromised gene.

Niab is currently testing a wider range of varieties, with further genetic screening also being conducted by the John Innes Centre in Norwich, and the Global Rust Reference Centre in Denmark.

Paul adds: “Adult plant resistance has clearly taken a knock, but the full impact will depend on the other resistance genes, which vary from variety to variety.

“Some varieties appear to be fighting back, whereas others are recording unusually high disease levels.”

Paul points out that while the unusual symptoms haven’t been seen in the South or the West, it appears to be spreading fast.

“No doubt it will impact on variety choice and disease management across the UK next season,” he adds.

AHDB is currently analysing yellow rust data from its UK network of variety trials and will issue revised disease resistance information on the RL web page as soon as possible.