Septoria disease rating dip revealed by early RL dataset release

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). 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 current (2021/22) edition of the RL features eight 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 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.”

The early release of the septoria tritici disease ratings includes the standard dataset, which is based on three years’ data (each year weighted equally), and ratings based on 2021 data alone (see table). The largest downward shifts in the single-season dataset are associated with varieties with Cougar in their parentages.

Although the smaller one-year dataset introduces volatility into the ratings, it may help indicate the level of resistance in the coming season and guide the tailoring of fungicide programmes.

Bracketed data () is limited.

¹ LG Sundance is no longer in trials, as a result, there is no data for 2021 and the three-year data is based on two-year data only.

*Indicates varieties with Cougar in their parentages.

**Information on Candidate varieties added to the RL 2022/23 will be published on 6 December 2021 on the RL home page.

How septoria tritici disease resistance ratings have changed in the RL 2022/23 (2021 one-year data set), relative to RL 2021/22 (2017–20 three-year data set). The dark blue bars indicate known Cougar descendants. New varieties to the RL 2022/23 are not shown.