AHDB has issued the Recommended Lists (RL) 2022/23 septoria tritici ratings early following high levels of the disease in the 2020/21 growing season and concerns about the breaking of resistance.
Disease ratings have declined for many winter wheat varieties, with the largest reductions in varieties with Cougar in their parentages.
The ratings have been prepared using a standard three-year dataset, in addition to a one-year dataset, to help reveal the influence of the 2021 disease season.
Paul Gosling, who leads the RL project at AHDB said: “Following subdued septoria levels in RL trials and commercial crops during early spring, the disease increased rapidly in June.
"It looked as if there would be a downward shift in disease ratings for those varieties with the old Cougar variety in their parentages.
"Analysis of disease data has translated to lower ratings for many varieties, with ‘Cougar-types’, as expected, suffering the largest falls.”
Introduced in 2013, Cougar had the highest septoria tritici rating on the RL (7), but by 2015, its resistance declined.
Subsequent AHDB-funded investigations found that a septoria tritici variant was able to cause disease exclusively on Cougar.
Although Cougar was removed from the RL in the 2016/17 edition, the use of the variety in breeding programmes means the RL's current edition features 8 varieties with Cougar genes in their backgrounds.
Research from Irish sites published this summer revealed a similar shift in resistance associated with the descendants of Cougar.
Analysis of UK RL results shows that not all the varieties with Cougar in their parentage have been affected equally.
This is because resistance is based on the cumulative impact of many minor disease resistance genes, in addition to the ‘Cougar resistance’.
The results also show that problematic septoria variants are not distributed uniformly across the UK – with resistance in these varieties holding up better in Scotland.
Paul Gosling said: “It is impossible to know how the septoria population will evolve. The Cougar-virulent isolates could become more common in Scotland, but their frequency could also reduce – we simply don’t know.
"Disease ratings are only able to reflect recent pathogen populations, they can’t predict the future.”
