THIS YEAR'S unusually high levels of wheat yellow rust have resulted in an unprecedented and extensive revision to disease ratings for some varieties.

The AHDB Recommended Lists (RL) trials during 2016 recorded massive levels of rust in some crops and the new ratings issued this week sees some badly affected Groups 3 and 4 winter varieties having their resistance ratings reduced by as much as four points.

While disease ratings for yellow rust are normally calculated from data covering a period of three years, major changes in varietal resistance in 2016 across several winter wheat varieties has prompted the change 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.”

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).

No change in ratings were recorded for 15 varieties and 14 varieties retained a strong resistance rating of 8 or 9. New candidates also performed well, with most (11 out of 19) achieving a rating of 8 or 9 and no varieties achieving a rating below 6.

Gavin Dick, AHDB Cereals and Oilseeds knowledge exchange manager for Scotland, added: “The situation in Scotland is really same as rest of UK as regards yellow rust. Farmers really need to be vigilant, especially as the UKCPVS has now confirmed it also detected the Kranich race in a sample collected from East Lothian in 2015.

“I would advise all Scottish growers to take notice of the new ratings and adjust their crop protection plans for 2017 accordingly. Variety selection is becoming more and more critical as treatment options reduce.”

'Kranich' joined by 'Invicta'

In February, UKCPVS announced that the ‘Kranich’ yellow rust race had been detected in the UK for the first time and has 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, it also revealed a group of yellow rust isolates, sampled in 2015, appeared to be similar to the old UK Solstice race. Further testing 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."