Collaboration is key in cereal pathogen race

AHDB Senior Crop Protection Scientist Catherine Harries discusses the latest cereal disease data from the RL trials and the UKCPVS project and urges people to get involved in pathogen population monitoring.

From a commercial perspective, there are no positives associated with high disease pressures. From a variety-trialling perspective, however, it is beneficial. It delivers good-quality data that can pull apart the performance of varieties.

Our harvest results service usually starts at the end of July, which releases the latest Recommended Lists (RL) trial results shortly after each trial is cut. It will be interesting to see how the difficult growing season affects how varieties rank.

As part of the service, the RL team issues regular commentary to put the results in context, which will include thoughts about this season’s disease pressures. You will not be surprised to hear that we have seen high levels of disease across the UK network of trials.

It is too early to draw conclusions on changes in the pathogen population based on the septoria tritici, yellow rust and brown rust data received so far.

The RL trials are an excellent indicator of varietal disease resistance. Not only do they allow us to calculate disease resistance ratings, but the trial data also holds clues about early shifts in pathogen populations (as revealed by the watch lists for yellow rust and brown rust, for example).

We need farmers and agronomists to play their part, especially to keep tabs on the changeable wheat brown rust and yellow rust pathogen populations.

Usually, we focus on yellow rust, although brown rust has also been a talking point in recent seasons.

This year, brown rust took hold quite early. We have also recorded some unusual stress-related brown rust symptoms.

Unusual brown rust symptoms in an RL winter wheat trial

In recent years, the UK brown rust population has shifted, with more pathogen isolates able to cause disease (virulence) on varieties with the specific resistance leaf rust (Lr) resistance gene – Lr24.

Virulence to this gene was first detected by the UK Cereal Pathogen Virulence Survey (UKCPVS) in 2011. It disappeared in the population after 2017 before resurging in 2022.

Theodore has the Lr24 gene. Increasing areas of this variety could have selected for brown rust pathogens with the keys to unpick the Lr24 lock.

Although Theodore is not included on RL 2024/25, it is still included as a disease control in the trials. The variety used to have high levels of resistance to brown rust. From initial RL trial data for harvest 2024, the variety now appears to have more brown rust than Crusoe (a variety with a brown rust disease rating of 3). Virulence for Lr24 seems to be firmly embedded in the pathogen population.

However, the UKCPVS cannot confirm this without a good geographic distribution of samples (from any variety). At present, most brown rust samples have come from around Lincolnshire (see map). Although the spread is better for yellow rust, there are still many gaps.

Rust samples received by UKCPVS (28 June 2024)

The UKCPVS first detected yellow rust virulence for the variety Crusoe in 2013. For years, the variety sported a disease resistance rating of 9. In 2022, an increasing proportion (8%) of pathogen isolates had virulence for Crusoe, with more yellow rust observed on this variety in RL trials in 2023, with the disease rating dropping to 8 in the current (2024/25) RL. We use isolates that are virulent on Crusoe in the inoculated RL trials, so the levels of disease were not surprising.

The UKCPVS tested an isolate with virulence for Crusoe against all RL varieties at the young plant (seedling) stage. This isolate also had virulence for KWS Extase, which has also seen a decrease in its RL yellow rust disease rating in recent years (it has a rating of 7 on RL 2024/25).

It isn’t all about wheat and rusts, of course. Barley powdery mildew is also a topic of conversation.

All spring barley varieties on RL 2024/25 have the mlo resistance gene, which is associated with strong resistance to powdery mildew.

We are particularly interested to monitor this pathogen because disease has now been observed on some recommended spring barley varieties.

Your samples will help us capture the best picture of the status of the UK’s pathogen populations.