Cereal disease ratings to rein in the rust (article)

Technical article

In recent years, rapid changes in wheat brown and yellow rust pathogen populations have caused a fair few management headaches. At the top of the snagging list is unexpectedly high levels of disease on some varieties with strong disease ratings in the Recommended Lists (RL).

With pressure from all directions to reduce fungicide inputs, it is essential that disease ratings are as robust and reliable as possible. At the end of June, our team took part in a series of RL 'ultimate guide’ webinars. Archived on YouTube, these videos take an in-depth look at the trial system, describe the season to date and provide insight into where the RL is heading next.

In the wheat video, AHDB’s Catherine Harries explains the twists and turns in the rust story, with our RL field trials manager Mark Bollebakker providing the latest observations from the plots.

With pathogen populations changing at an unprecedented rate, it has become clear that the rating system also needs to adapt to survive.

The rating system

The RL uses a three-year dataset to calculate disease ratings. In 2016, the RL assessment system experienced a major shock. Disease levels in trials (and in commercial crops) were frequently – and quite consistently – higher than expected. As a result, in the 2017/18 RL, disease ratings were based on the 2016 dataset alone to best reflect varietal performance.

Since 2016, more rust has been observed, on average, with a greater difference in disease levels between varieties. In 2019, disease observations rang alarm bells once more. However, unlike 2016, these unexpected disease spikes were highly localised. As a precaution, trial data was subject to additional quality control checks. This concluded that varietal resistance remained broadly in line with recent years. Consequently, the ratings in the 2020/21 RL were based on the standard three-year dataset, which saw only a few small changes in ratings. Although, warnings accompanied the ratings to encourage extra-vigilant crop inspections.

With the unexpected now expected, it is unsurprising that we received reports of disease-rating defying levels of rust in some 2020 wheat crops. With the rust revolution akin to a runaway train, the RL Project Board is considering how best to approach ratings in this fast-moving world.

The simplest approach is to limit the analysis to the most recent season’s data (as we did in 2016). However, such limited data is highly prone to external seasonal influences (e.g. weather and disease pressure) and does not produce a fair test of varietal performance. It is a technique for ‘emergency use’ only.

Whatever the solution, it needs to align with the main purpose of the RL: to aid variety selection. The RL must clearly show the potential disease risks associated with each variety.

At present, we are looking at a range of options, including the potential to:

1. Provide information on varietal disease-resistance trends over time
2. Give the most recent results more influence (weight) in disease ratings
3. Highlight varietal performance in the highest disease scenarios
4. Conduct extra data analyses to give early warning of potential changes to resistance
5. Flag the lowest-risk varieties with the most stable ratings, along with the varieties of most concern

However, whatever system is used, with highly localised and dynamic rust populations seemingly the new normal, it is highly unlikely RL trials will detect every twist and turn in the rust story. Even varieties with high rust ratings need to be monitored and treated according to the disease present.

How can you help?

AHDB funds the UK Cereal Pathogen Virulence Survey (UKCPVS) to analyse infected cereal leaf samples. UKCPVS has received a good supply of material in 2020. However, more samples are always required. If you have observed unexpected levels of disease in your wheat or barley crops, submit a sample for testing. Sampling instructions are on the UKCPVS page.

Later this month, the AHDB Harvest Results service will return to show how cereal and oilseed varieties have performed in RL trials this year. Regular email updates, throughout the harvest period, provide information on yield, grain quality and agronomic data, as well as information on disease levels. The dataset includes variety-specific information on yellow rust percentages at specific regional RL trial sites.