Rust in peace: pathogens versus cereal varieties

With extreme diversity locked into yellow rust and brown rust populations, the latest UKCPVS event (1 March 2023) examined how highly adaptable pathogens affect UK wheat and barley varieties.

With a focus on wheat, Jason Pole, who leads AHDB’s crop disease communications, provides an overview of some of the key developments.

Rust diversity

Cereal rusts often grab the headlines at the annual UK Cereal Pathogen Virulence Survey (UKCVPS) stakeholder event, especially wheat yellow rust.

The yellow rust population is diverse and dynamic, which makes disease resistance ratings less stable. So, the high level of interest in these foliar pathogens is unsurprising.

However, from a rust perspective, the current decade has got off to a relatively calm start. It sits in stark contrast to the 2010s, which saw the Warrior yellow rust race start a population take-over.

The UKCPVS 2022 event article (next article) explains why this race was so successful in tumbling the ratings of many high-profile varieties.

At the time, the rapid change shook the Recommended Lists (RL) disease-rating system and put everyone on tenterhooks. Could the 1–9 rating system be trusted?

Based on trial data, the ratings reveal what has happened (in recent seasons), not what will happen – they are not predictive. However, the data does contain clues about what the future might hold.

Yellow rust watch list

To provide predictive power, AHDB looked at RL data in a new way. It culminated in the release of the yellow rust watch list in 2021.

Updated annually, the watch list indicates varieties that performed out of line with their main RL disease rating in some trials.

Varieties that appear to be ‘misbehaving’ can be monitored more closely and treated with rust-active fungicides, where disease pressure merits it.

A recent analysis of the performance of the watch list over its first two seasons of operation (2021–22) suggests that the system is working. It is helping to highlight varieties at the greatest risk of falls in resistance ratings.

The latest yellow rust watch list (released in March 2023) provides little evidence that a dramatic change in fortune is on the cards.

Currently, most RL varieties are performing as predicted from their RL rating. In general, varieties have also performed as expected in the UKCPVS yellow rust field trials.

So, the recent calm appears to be continuing, which is good news for management and plant breeders. Genetics has caught up in the race – the RL now boasts 18 winter wheat varieties with a yellow rust disease rating of 9.

Unfortunately, it doesn’t mean we can relax. Yes, the watch list can spot unusual events happening at the local level, but it will never be a fool-proof crystal ball. For example, it will not predict the arrival of a new aggressive yellow rust strain blown in from overseas.

Brown rust ‘blip’?

This year, AHDB added a brown rust watch list to its growing list of resources.

On the current winter wheat list (RL 2023/24), very few varieties are highly resistant to brown rust – just one variety has a resistance rating of 9, and only two have a rating of 8.

The good news is that, in general, the brown rust watch list suggests there is little change to be concerned about. Like yellow rust, most varieties are performing in line with their RL rating. Once again, this holds true in the UKCPVS trials.

However, one of the 8-rated varieties stood out at an RL trial site in Devon: Theodore, with a relatively high level of brown rust (18.8%). At this site, only Crusoe, brown rust rating of 3, had more disease (25.0%).

This is where the watch list comes into its own. It suggests that a change in the pathogen population has occurred. With implications for commercial varieties, it is a situation that merits closer attention.

Interestingly, UKCPVS received a brown rust sample from Theodore just down the coast (Dorset).

The sampler noted infection levels of 3%, in addition to relatively high disease levels (up to 10%) in some patches (foci). This is much higher than would be expected for a variety with a disease rating of 8.

UKCPVS growth room screening tests, conducted in 2022, suggest that the Dorset isolate may be able to unlock a specific resistance (leaf rust, Lr) gene – Lr24.

Brown rust pathogens able to unpick the Lr24 lock had not been detected by UKCPVS for a little while (since 2017). As a result, this isolate of interest will be included in adult plant trials to help determine its significance to varieties.

It is important to note that the abundance and distribution of isolates in the UK population ebb and flow, increasing and decreasing over seasons. Disease ratings can go up as well as down. This isolate could fizzle out in the population once more. Time will tell.

In general, Theodore has a reputation for being a strong performer against cereal rusts. In addition to its main ‘adult plant’ yellow rust rating of 9, it also resists yellow rust and brown rust at the young plant stage. The latter is a particular achievement – being the only variety on the current (RL 2023/24) list to possess this trait. It also appears to resist wheat stem rust.

Wheat stem rust

Over the past 25 years, UK conditions have become more conducive to stem rust infection. Ten years ago (2013), stem rust was recorded in UK crops for the first time in over 60 years. Since then, the disease has been observed for several years at several sites.

Although not routinely screened, the UKCPVS team did test two stem rust isolates, sampled from UK fields in 2022, on the full set of recommended and candidate varieties.

Symptom development photos provide a clear picture of Theodore’s ability to check for infection (Figure 1).

RGT Wolverine was also able to limit pustule development.

Unfortunately, it appears that many UK wheat varieties are highly susceptible to the disease. However, good control levels can be achieved with rust-active azole fungicides, especially tebuconazole.


Figure 1. Limited symptom development in two winter wheat varieties following inoculation with stem rust (isolate 1 or 2). Typical symptoms shown for comparison

Breeding wheat to beat yellow rust

Over the last decade, major changes to the UK’s yellow rust pathogen population have added complexity to the plant-breeding puzzle. Jason Pole, who leads on AHDB’s crop disease communications, outlines key points from a recent presentation on the topic.

Our UK Cereal Pathogen Virulence Survey (UKCPVS) project is essential to plant breeders, according to Rachel Goddard of plant breeding company Limagrain.

Rachel presented at a the recent UKCPVS eventand said that information, hard work, investment and time are required to ensure that winter wheat variety developments match the pace set by adaptable yellow rust populations.

The UKCPVS monitors cereal rusts and mildews. Through disease observations (phenotypic work) and analysis of pathogen genetics (pathogenomics), UKCPVS results help the plant-breeding pipeline deliver strong disease resistance to Recommended Lists (RL) varieties.

Yellow rust evolution

Rachel’s presentation centred on the challenges of breeding for yellow rust resistance.

Over the last 50 years, major breakdowns in yellow rust resistance have occurred relatively frequently (Figure 1) – in cycles of around 5 to 10 years.

When new pathogen variants arrive, they can spread rapidly (in just a few seasons).


Figure 1. A timeline showing key years for wheat yellow rust population change in the UK

In 2011, the presence of the Warrior yellow rust race in the UK was confirmed. Compared to previous changes, Warrior was highly unusual for several reasons, including:

  • It was derived from a sexual recombination (outside of Europe)
  • It was first identified in many countries in the same year
  • It caused yellow rust on many wheat varieties
  • It was complex and varied
  • Compared to the previous population, it was highly adaptable. It:
    • Tolerated a greater range of temperatures
    • Had a shorter time from infection to sporulation
    • Produced a greater number of spores
    • Developed black telia relatively late in the season
    • Broke many resistance genes and gene combinations

In fact, Rachel said that post-Warrior “yellow rust was like a new disease”.

A major reason why Warrior affected so many varieties was because it unpicked a single, major adult plant resistance gene – YRClaire – one that had been extensively used in plant breeding since 1997.

The post-Warrior explosion in the diversity of the yellow rust population was so large it demanded a change to the way new variants were named. Today, new races are assigned to a genetic colour group and given a sequential number – unique to the varieties on which they cause disease (pathotype).

Since the incursion of Warrior, the red group of isolates has dominated the population – with it featuring over 50 pathotypes.

Frequencies of these pathotypes vary over time and space: even across a short distance in a field, the pathotypes present can vary substantially.

The three most dominant pathotypes represent around a third of the population, according to the most recent UKCPVS results.

Post-Warrior, the yellow rust population has changed so much that its ability to unlock resistance in some old varieties (from the 1990s) may have, in essence, been forgotten. This includes Brigadier (Figure 1), which features genes known to counter the Warrior population of races. Therefore, historic genetics may offer solutions for the varieties of tomorrow.

Plant breeding challenges

A major challenge for plant breeding is that a traditional wheat breeding cycle takes around 10 years. Despite early promise in pre-breeding and National Lists trials, varieties may no longer be resistant (or may be less resistant) by the time they reach RL trials.

However, UKCPVS pathotype information (presence and frequency) guides plant breeding efforts. The provision of representative pathotypes to breeders – for artificial infection (inoculation) of varieties in disease nurseries – also helps to maximise the chance of successful variety selection.

Disease nurseries are also used to evaluate resistance (R) gene combinations. There are at least eight R genes across the current set of RL winter wheat varieties. Wheat breeders look to incorporate (stack) as many effective R genes as possible to deliver durable disease resistance – providing any major yield penalties can be avoided.

Most RL winter wheat varieties have two or three R genes for yellow rust, and three have four. However, several only contain one R gene – not only increasing the vulnerability in these varieties, but to others too. Plant breeding companies, such as Limagrain, now aim to avoid deploying single R genes in commercial varieties.

There is a continual hunger to introduce new sources of resistance, as the elite wheat gene pool is narrow. Breeders turn to wheat’s wild relatives, direct ancestors, and landraces to help expand it. However, this route takes as much as three times longer than elite crosses due to the need to flush out undesirable traits.

Genetic technologies, such as marker-assisted selection, are helping to speed up the plant breeding process. Despite the changes to the yellow rust population, many RL varieties have strong resistance to the disease at the adult-plant and young-plant stages. It shows the ingenuity of plant breeders.

Adult plant disease ratingNumber of winter wheat varieties (RL 2022/23)
5 or less7