The identification of the Warrior yellow rust race in the UK and mainland Europe in 2011 marked a shift in the complexity of the disease.
Pre-Warrior, the UK yellow rust population was clonal and there would be one dominant race at any given time.
In response to wheat varieties being grown over a larger area, a new variant would arise and then over time this new variant would become the new dominant race.
Warrior was unusual in a number of ways.
Firstly it was identified in multiple countries throughout Europe in the same year.
Secondly, Warrior broke a number of effective resistances in European winter wheat germplasm, so many of these resistances were no longer effective.
And thirdly, compared to the previous clonal yellow rust populations in the UK, Warrior appears to have been derived from a sexually recombining population outside of Europe.
A major reason why Warrior affected so many varieties was because it broke a single, major adult plant resistance gene – YRClaire – one that had been extensively used in plant breeding since 1997.
Limagrain cereal pathologist Rachel Goddard says: “Since 2011, yellow rust has remained very complex in the UK; we have a new naming system to reflect this complexity and pathogenomics work has identified that there are several genetic groups present in the UK.”
Incursion 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, which comprises more than 50 pathotypes, has dominated the population.
Warrior race yellow rust is essentially a new disease, says Ms Goddard.
Since it has become prevalent, yellow rust has become much more adaptable and able to withstand greater changes in the environment.
The Warrior race can tolerate a greater range of temperatures, it has a shorter infection time, which means there is potential for the disease cycle to repeat several times during a season, and it is much more aggressive, producing more spores than the ‘old’ yellow rust.
“So yellow rust post-Warrior is very different to the old yellow rust and we can essentially consider it to be a new disease.
This has changed the way we have been able to breed for resistance,” says Ms Goddard.
From a plant breeder’s perspective, the diversity of the UK yellow rust population means it is difficult to predict how the population will change between a cross being made and a variety reaching the market 10 years later.
It is also difficult to predict which host resistances are durable.
“Durability is resistance which remains effective during prolonged and widespread use in an environment which is favourable to disease.
We know single genes which we deploy on their own are unlikely to be durable, so we know we are looking at combinations of genes together.
However, it is difficult to predict which combinations are likely to be durable, so there is still some uncertainty.”
Breeding for resistance to yellow rust post-Warrior means using all the data available, says Ms Goddard.
United Kingdom Cereal Pathogen Virulence Survey (UKCPVS) data helps provide a greater understanding of the pathogen population.
Ms Goddard adds: “As wheat plant breeders, we work on host plant genetics, not the pathogen and the work of the UKCPVS allows us to understand if there are any race changes, so it is really an early warning system and it is also providing us with validation of new races.
Vulnerable “We also take a lot of time to understand the resistance which is already present in elite UK wheats.
AHDB now provides a yellow rust watch list and this is giving an indication of which varieties might be more vulnerable to a ratings fall in the coming season.
“There are at least eight effective resistance genes for yellow rust in Recommended List varieties currently.
However, these are found in different frequencies and not all our gene combinations are effective against Warrior.
“We also take care to monitor these resistances in our disease nurseries.
By monitoring these resistances year-by-year, we can see which gene combinations are potentially losing efficacy.”
Other techniques deployed include the introduction of new resistances into breeding programmes via pre-breeding, utilising resistances from wheat ancestors, landraces and wild relatives.
But these sources can be tall and prone to lodging and are often lower yielding.
“While there is a wealth of diversity in terms of resistance, they don’t solve all our problems,” says Ms Goddard.
Once resistance genes have been introduced into a breeding programme, they need protecting and this is done by stacking genes together.
New technology is helping here, with genetic markers enabling gene inheritance to be tracked.
Challenges The arrival of the Warrior race may have changed the face of yellow rust in the UK but despite the challenges it presents, many Recommended List varieties have strong resistance to the disease at the adult plant and young plant stages – a testament to the efforts of plant breeders.