There are growing concerns that new races of yellow and brown rust have overcome varietal resistance in wheat, with high levels of both diseases being seen on varieties with good resistance ratings.
This could have huge implications for wheat growers, as downgrading of disease ratings may force farmers to change their varietal choices this coming autumn.
Visitors to Cereals were told that experts had started to investigate the issue and they had called on farmers to send samples of suspected problems to help determine whether new races are developing.
What farmers should do
Farmers with any suspicious or unexplained rust samples are being urged to send them along with a brief field history to Sarah Holdgate and the team at UKCPVS. They simply address the sample to Freepost, UKCPVS.
There are as many as 22 commercial winter wheat varieties showing susceptibility to yellow rust this year, including new Recommended List candidates still to be assessed for recommendation.
The same applies to brown rust, which is common on susceptible varieties, but is also being found on a new Group 3 winter wheat with good resistance.
Most of the varieties being scrutinised have existing resistance ratings of 7, 8 or 9 to yellow rust, with the higher numbers on the 1-9 scale used on the Recommended List signalling the best resistance to the disease.
As ratings are an indicator of variety performance in previous years, they can change within a season if new fungal races occur, which is why growers and agronomists must be vigilant, explained Sarah Holdgate of the UK Cereal Pathogen Virulence Survey (UKCPVS).
“We know that fungal populations are capable of changing very quickly so it’s important that you treat what you see in the field,” she said.
“Sending samples into us for testing is also crucial – it’s too early to say whether these are new races or not, but we are working with our partners across Europe to test rust samples and provide further information as soon as we can.”
The last time new races were identified was in 2016, when two new yellow rust strains were confirmed, prompting a revision of wheat variety resistance scores and seeing sales of popular varieties plummet.
Wheat varieties can be susceptible to yellow rust at the seedling stage but will recover if they have high levels of adult plant resistance, which kicks in during the spring, added Dr Holdgate.
“By this stage of the season, we are well into the adult plant resistance stage, so you can’t expect plants to shrug it off now. There’s certainly more rust than we would like to see on Zyatt, Gleam and Firefly, but others are being looked at too.”
For growers hoping to make their variety choices for this autumn at the event, uncertainty over susceptibility to yellow and brown rust is not helpful, said Lee Bennett of Openfield.
“Telling farmers that we have the chemistry to control yellow rust is not what they want to hear,” he said.
“We’re entering a new era – growers are keen to avoid both the financial and environmental cost of spraying.”
He added that some varieties always take a bit of rust, so are less of a concern.
“This year, we’re seeing other leading varieties hammered by it, so that suggests that something is changing.”
The UK situation reflects what’s happening across Europe, with severe yellow rust being reported in several countries and an unusual pattern of varieties affected, report collaborators in the EU RustWatch project.
Spain, Belgium, Northern France and Denmark are all on high alert – with early testing done in Denmark suggesting that new races are involved.
“That doesn’t mean that the same is happening in the UK, but it does give an advance warning of what we might see at the end of the year and into next season,” explained Sarah Holdgate.
“There’s always a risk of aerial spread of rust spores to other countries, which is why Niab is a partner in RustWatch. It also means that we will be able to issue timely updates that help farmers make better variety choices.”