Unexpected levels of yellow rust on some commercial varieties in 2015 triggered investigations by the UK Cereal Pathogen Virulence Survey (UKCPVS) to determine the underlying causes.

The initial findings show that multiple factors are at play, adding further complexity to the already diverse UK yellow rust pathogen population.

Earlier this year, the UKCPVS announced that it had identified the ‘Kranich’ race in the UK for the first time from isolates collected in 2014.

However, no further Kranich isolates were collected in 2015 in the UK. Therefore, this race is considered highly unlikely to be behind the unusual rust observations in the field last season.

At the UKCPVS event on 8 March in Cambridgeshire, delegates heard the latest findings, including some clear investigative leads to help shed light on the mystery.

Targeted at breeders, crop scientists and technical agronomists, the event reported on recent pathotyping test results (using samples received during 2015) and adult plant nursery tests (using samples received during 2014).

Unique yellow rust pathotype identified

The UKCPVS 2015 pathotyping results revealed a unique yellow rust pathotype not observed before in the UK.

The new pathotype was identified in samples taken from sites with unexpected levels of yellow rust – on varieties with high Recommended List (RL) disease ratings – in Essex, North Yorkshire and Scotland in 2015. Some of these sites were RL untreated trials.

Dr Sarah Holdgate, UKCPVS project manager based at NIAB, said: “It’s too early to tell if this new pathotype caused the high yellow rust levels but our adult plant tests this summer will help provide an answer.” 

Race complexities

The samples received by the UKCPVS in 2015 continued to be dominated by Warrior-type isolates (approximately 70% of the isolates tested to date).

Warrior-type isolates can be subdivided into groups based on genetic sequence data and their ability to cause disease.

Using the latest information, a revised classification system, with three pathotype groups – Warrior 1, Warrior 3 and Warrior 4 – was unveiled at the event.

Dr Sarah Holdgate said: “When it was detected in 2011, Warrior 1 was dominant. Since 2012, the Warrior 4 group has increased in frequency.”

Another group of isolates, sampled in 2015, appeared to be similar to the Solstice race, dominant in the UK population before the Warrior-type races, based on their reaction to a selection of varieties in UKCPVS tests.

However, the severity of symptoms observed at some of the sites from which these isolates were sourced suggested that the story may not be that simple.

Dr Sarah Holdgate said, “This group of isolates may also be behind some of the unusual yellow rust findings in 2015. Once again, we will investigate this in 2016 to provide a clearer picture.

“We might even find that these isolates fit better within the new Warrior 3 grouping, rather than the old UK Solstice race group.”

Brown rust update

Last year, the UKCPVS reported that the wheat brown rust population has changed over the last few years with tests showing that isolates are becoming more complex.

In 2006, most isolates tested carried two virulence genes. Now the population is dominated by the Glasgow pathotype which has six virulence genes.

Although lower levels of brown rust were reported in 2015, UKCPVS still received reports of relatively high levels of disease on Crusoe.

The high levels of disease on this variety in 2015 resulted in a reduction in the RL brown rust disease rating – from 4 in 2015/16 to 3 in 2016/17.

The current dominance of the Glasgow pathotype in the UK brown rust population could explain the high levels of disease and associated drop in RL disease ratings.

However, further adult plant tests are required to confirm the susceptibility of Crusoe to this specific pathotype. 

Mildews

Both wheat and barley powdery mildew were surveyed in 2015. Based on seedling tests, virulence frequencies were broadly similar to previous years.

RL and disease resistance

As the pathogen population is so diverse, RL disease ratings provide growers with a relatively simple way to assess potential disease susceptibility risks for commercial varieties in the UK.

Encouragingly, 19 out of 36 of the current RL varieties display good yellow rust resistance to all isolates tested by UKCPVS.

Dr Jenna Watts, AHDB Research Manager, said: “The strength behind our disease ratings is that they provide a good indication of the potential risk a disease poses to individual varieties.

“As a result, growers need not worry about the details of pathogen population complexity.

“However, the ability of populations to change rapidly does mean that the season can throw up surprises.

“As a result, it is really important to monitor all crops for disease and alert UKCPVS to any unexpected observations.

“Once aware of potential changes in populations, the team will assess the implications on disease ratings and communicate any change to the industry as soon as possible.”

The year ahead

Both yellow rust and brown rust have been active in crops during the autumn and winter, creating the potential for a significant epidemic development this spring.

Close monitoring of crops, applying a suitable protectant product and maintaining protection throughout the season will be important to help realise yields.

Funded by AHDB and Defra (via the Animal & Plant Health Agency) and managed by NIAB, the UKCPVS receives infected cereal leaf samples from agronomists, trials officers and researchers.

RL untreated trials, which are inspected regularly throughout the season, are an important source of rust samples.  

Dr Watts said: “This year, we will conduct additional disease monitoring at a subset of RL untreated trial sites and results will be made available on the AHDB Cereals & Oilseeds website.”

If you would like to get involved in the survey, sampling instructions are available from Amelia Hubbard (amelia.hubbard@niab.com) or via cereals.ahdb.org.uk/ukcpvs

The UKCPVS Stakeholder Event 2016 also featured several related technical papers based on pathogen virulence experience from across the globe.

The papers will be published on cereals.ahdb.org.uk/UKCPVSevent in the next few days.