Major revisions to wheat yellow rust disease ratings

Unusually high levels of wheat yellow rust, observed in AHDB Recommended Lists (RL) trials during 2016, have resulted in an early and major revision to disease ratings.

The new ratings affect Groups 3 and 4 winter wheat varieties, with some ratings reduced by as much as four points.

A new annual report, by the AHDB-funded UK Cereal Pathogen Virulence Survey (UKCPVS), also published today, has confirmed the presence of the ‘Kranich’ race and a potential new race – provisionally named ‘Invicta’ – in the UK yellow rust population in 2015.

With the pathogen population changing so rapidly, people are urged to monitor crops, including those with a high disease rating, and report unexpected findings to the UKCPVS.

Ratings revised

Disease ratings for yellow rust are normally calculated from data covering a period of three years. As major changes in varietal resistance were observed in 2016, across several winter wheat varieties and UK trials, AHDB Cereals & Oilseeds has taken the unprecedented decision to base yellow rust disease ratings on 2016 trials’ data alone.

Dr Jenna Watts, who manages disease research at AHDB, said: “We made the decision to calculate the disease ratings on a single year’s trials because the 2016 yellow rust assessment data was so different to what had been seen before. We have to make sure that the RL reflects varietal performance in the field the best it can.

“We’ve seen disease ratings for seven varieties drop by two or more points. We consider anything more than a one-point change as highly unusual.”

Varieties affected

Of the 32 recommended varieties in trial, the following varieties’ disease ratings dropped by two points or more.

Group 3s – Britannia (8 to 4), Spyder (8 to 6), Zulu (9 to 5) and RGT Conversion (8 to 6)

Group 4s – Myriad (8 to 4), Reflection (6 to 3) and JB Diego (7 to 5)

Table: Changes in wheat yellow rust disease ratings – AHDB Recommended Lists 2016/17 versus 2017/18. 

No change in disease rating, however, was recorded for 15 varieties and 14 varieties retained a strong resistance rating of 8 or 9.

Candidate varieties also performed well, with most (11/19) achieving a rating of 8 or 9 and no varieties achieving a rating below 6.

Yellow rust races

In February, UKCPVS announced that the ‘Kranich’ yellow rust race had been detected in the UK for the first time (from a field sample taken in 2014).

UKCPVS has now confirmed it also detected Kranich in a mixed-race sample collected from East Lothian in 2015.

Dr Sarah Holdgate, UKCPVS project manager based at NIAB, said: “Multiple races can be present on the same leaf and this is becoming more common.

“This can make initial diagnosis difficult, so we adjusted some of our testing techniques and now know that Kranich was present in the UK in both 2014 and 2015.”

In March, UKCPVS also revealed that a group of yellow rust isolates, sampled in 2015, appeared to be similar to the old UK Solstice race.

Further testing, however, showed these isolates had a distinct pattern of virulence on a wider set of varieties and were genetically unrelated to the old race.

Dr Holdgate continued: “Given these differences, we think it’s highly likely we have detected a new race. The provisional name ‘Invicta’ has been given to it, as it was first detected on the wheat variety with the same name.

“We are continuing to work with this new race to assess the potential impact in adult plant tests.

“We are also working with the global rust community to establish how this race fits into the wider European population.

“It is highly possible that Kranich, Invicta, or even an as yet undetected race, could have been behind the unexpected levels of yellow rust seen in 2016.”

The UKCPVS is currently investigating isolates collected during 2016 and further information will be presented at the annual stakeholder event on 8 March 2017 in Cambridgeshire.

Seedling resistance status

Revised disease ratings and seedling susceptibility information has been published online, in advance of the Recommended Lists launch (28 November 2016), to help the industry respond to the changes.

Dr Watts concluded: “Many more varieties are now susceptible at the seedling stage, but it is the adult stage that really counts.

“It’s good news that many varieties, both established and new, continue to show solid resistance to yellow rust during the critical adult stages.

“But it’s a turbulent time and we must continue to expect the unexpected. The genetic defence of our crops is fragile and we all need to monitor crops and report unexpected findings to the UKCPVS.”