The presence of yet another significant wheat yellow rust race has been confirmed in the UK by the UK Cereal Pathogen Virulence Survey (UKCPVS). This disease is increasingly of significance to Irish growers also and it created significant control challenges in 2017.

The genetics of yellow rust and plant resistance mechanisms has become increasingly complex and diverse and this has forced the introduction of a new naming system for races discovered in the UK. This was introduced by the UKCPVS earlier this year and led to the renaming of some previously discovered races, as well as the most recent newly discovered race.

In the past, a resistant race tended to be named after the variety in which it was first detected, but scientists state that this system no longer copes adequately with the complex genotypic and pathology data available. The new nomenclature is more complex and has commonalities with other disease race identification, such as potato blight. For example, the Invicta race is now renamed as WYR Blue 7 in the new system.

These changes led to a revision of variety yellow rust resistance ratings on recent UK AHDB recommended lists. And the recently identified new race is to be named as Red 24. It joins the previous new race, called Blue 7, which was announced by the UKCPVS last March.

The UKCPVS said that both of these races were present in UK populations in 2016 and it would seem that both races contributed to changes to the variety disease resistance ratings last year.

Based at NIAB, UKCPVS project manager Dr Sarah Holdgate said: “Tests reveal many varieties are affected by Red 24 and Blue 7, so these races are significant. We also know some varieties are more susceptible to one race over the other.” This might help explain regional variation in commercial crops.

Seedling resistance

Many Irish growers will also have experienced the very aggressive seedling or juvenile plant phase of yellow rust.

Seedling susceptibility information is also published online by the AHDB and it notes that many varieties became susceptible at the seedling stage in 2016.

“All candidate varieties in UK winter wheat recommended list trials have been classified as susceptible to yellow rust at the seedling stage,” according to Catherine Garman, who manages disease research at AHDB.

She went on to say that the only recommended varieties in the UK that show seedling-stage resistance to yellow rust are Costello, KWS Crispin and KWS Siskin.

Costello has become one of the important varieties for Irish growers, but the yellow rust resistance status of any variety cannot be taken as permanent.