All cereal pathogens can mutate to overcome plant varieties’ inherent resistance.
Since the UKCPVS began its work, the way in which fungi overcome that resistance, both in seedlings and in adult plants, has become better understood, and ongoing work to unravel the complex way in which they do so underpins the Recommended Lists (RLs).
Dr Charlotte Nellist, pathology programme leader at NIAB Cambridge Crop Research, says: “The overarching aim of the UKCPVS is to provide growers with the most up-to-date information on the virulence of populations of wheat yellow and brown rusts and wheat and barley powdery mildews in the UK.”
As well as monitoring the development of those pathogen populations, the mainly AHDB-funded project* aims to identify new types within them and assess their significance.
We then feed this information into variety testing systems by providing isolates of the fungi involved and through knowledge transfer activities,” says Dr Nellist.
“By understanding the pathogen populations present in the UK, we can screen new candidate varieties against representative isolates to make sure they can resist them.
This makes the resistance ratings relevant for the populations present within the UK.”
In practice, samples of wheat and barley provided by growers, agronomists, trials officers and researchers are registered in a database and incubated to propagate the disease isolates, adds Dr Nellist.
“The spores are then transferred to the leaves of a susceptible variety to multiply.
We put all isolates into long-term storage and we have a yellow rust collection going back to the 1970s.
“We take a subsample of isolates from each pathogen, based on the host variety, disease severity and location for further testing in our seedling differential screens, to identify any population change.
Risk “To identify the risk associated with change for the rusts, we select five isolates displaying previously existing and novel pathotypes [combinations of identifiable virulence genes], to perform adult plant trials and variety seedling tests on the full set of RL and candidate RL varieties.
“We report the pathotypes of each of the isolates and test them to determine which ones are new.”
One of the project’s recent innovations is use of the yellow rust genotyping developed by Dr Diane Saunders at the John Innes Centre.
“We have genotyped a selection of isolates from 2019-2021 and plan to do so on a selection of isolates from this year’s survey, says Dr Nellist.
“Representative isolates of wheat yellow rust and wheat brown rust are chosen to be used in the National List and Recommended List wheat disease trials to ensure varieties are screened against current populations of rust isolates.”
The 2011 outbreak of the Warrior yellow rust race (the so-called Red Group) and its displacement of previous populations was a surprise.
“Through collaboration with our European counterparts, we were able to establish that this was a widespread event affecting many countries.
“Currently the Red Group dominates, but we must continue to survey and monitor the situation to identify the first signs of resistances breaking down.
“Several new virulence combinations have been detected in the past few years, with Yr8 of particular interest as virulence to this resistance gene is still relatively low in the population.
It peaked in 2019 with 15% of isolates but was found in only 8% of isolates tested in 2021.”
The breakdown of KWS Firefly’s resistance was also a surprise.
“It showed that yellow rust continually evolves and adapts so that we now have a highly diverse pathogen population.
“Our tests suggest that Firefly seedlings are more vulnerable to yellow rust in cooler weather.”
Four samples of KWS Siskin (RL rated 9 in 2022/2023) with very low infection levels were received in 2021.
“But none of the isolates have reinfected Siskin at the seedling stage, confirming that this resistance remains stable.”
Limited options Brown rust in wheat is less important than yellow rust in the UK, and at the start of the survey there were only limited options for resistant varieties, says Dr Nellist.
“Over recent years the pathogen populations have remained relatively stable, although an unusual outbreak on KWS Firefly in some parts of the country was identified during 2019.”
The UKCPVS has received no reports of unexpected outbreaks of wheat powdery mildew recently and the survey receives relatively few samples.
“The barley powdery mildew population has remained fairly stable for a decade.
Resistance in many varieties can be attributed to the MLO gene, which has been effective for many years.”