Yellow rust has become the bane of both growers and plant breeders over the past decade. At last month’s UK Cereal Pathogen Virulence Survey (UKCPVS) meeting, delegates heard why the disease has become so unpredictable when it comes to the robustness of varietal resistance. CPM reports.

Yellow rust pathotypes present at the beginning of the season could be very different those present at the end, even in the same area of the field.

By Lucy de la Pasture

There was a time when yellow rust was predictable. Every five to ten years a new variant would arise and then become dominant, often in response to the area of a particular wheat variety being grown at the time.

Disease outbreaks followed certain weather patterns and resistance bred into varieties was both dependable and durable, that is until there was a step-change in the yellow rust population and an adjustment in variety choice became necessary. But then everything changed with the arrival of the Warrior race.

The UK yellow rust population used to be clonal, with one dominant race at any given time, explains Rachel Goddard, cereal pathologist at Limagrain. “New variants would arise and over time become dominant and this gave step-wise changes in virulence every few years.”

But in 2011, a new race was identified in the UK and Europe which led to a complete change in the yellow rust population, she says. “This was exceptional for a number of reasons. Unusually, the change was identified in a number of countries in the same year, and it also broke a number of effective resistance genes in UK wheat germplasm (not just one as used to occur previously).

“Compared with the old clonal population, this new Warrior race incursion came from sexually recombining populations outside Europe. Effectively yellow rust had become a different disease.”

Since 2011 yellow rust has remained very complex in the UK and a new naming system was derived to reflect this – with genotypes classifying the pathotypes within Red, Blue, Purple and Blue groups. While there have been some fluctuations in the yellow rust population over the past few years, the Red group has remained dominant – with Red 37, Red 36 and Red 41 pathotypes the most common in 2021, she explains.

“The Red group is also very diverse, with more than 50 pathotypes identified, and it’s possible to see this diversity in the field. In work undertaken by Chris Judge at NIAB, he discovered that different pathotypes could be found in the same region of the field. He also noted that the frequency of different pathotypes could change throughout the growing season.

“That means that the yellow rust pathotypes present at the beginning of the season could be very different those present at the end, even in the same area of the field. So it’s quite difficult to predict pathogen population change because the situation is so fluid,” says Rachel.

Back in the day, when a clonal population of yellow rust presided, the disease was more predictable as it preferred mild winters – which enabled it to overwinter – but frosts and cold weather would stop it in its tracks. In spring warmer weather was tolerated, but if it became too warm yellow rust would slow down, she explains.

“Since Warrior has become prevalent, yellow rust is more adaptable and able to withstand changes in the environment without being inconvenienced. It can tolerate a greater range of temperatures and this means it can keep going at 20⁰C and above, which is when older pathotypes would have given up.”

Warrior-types are also much more aggressive, adds Rachel. “The Warrior group pathotypes have a shorter time from infection to sporulation, which gives the potential for the disease cycle to repeat several times within the season. They also produce more spores.”

As well as acting differently, the disease can look very different in the field too. “We often see black telia on the leaves later in the season instead of the usual yellow pustules in stripes on the leaf. That’s also an indication that these newer pathotypes evolved from a sexually recombinant population.”

Because the disease has changed, the way breeders look for and incorporate resistance has also changed, says Rachel. To illustrate this, she highlights two complete shifts in yellow rust resistance breeding in the UK.

“Following the breakdown of Brigadier in 1997, Claire was used extensively in breeding programmes. It had a single adult plant resistance gene – YrClaire – which provided complete protection against yellow rust.

“For ten years the YrClaire gene was very effective, however, following the appearance of Warrior in 2011 there was a marked increase in its susceptibility. In more recent years in the Limagrain nurseries (2019-2020), Claire has an infected leaf area of 60% or more, indicating it’s still very susceptible to yellow rust.”

But for Brigadier, which broke down in 1995-1996 before the ingress of Warrior, the story has a very different ending. “In the Limagrain nurseries, Brigadier was susceptible to yellow rust after its resistance broke, but from 2017 onwards it’s become very resistant to the disease. This means Brigadier has Yr genes which were defeated by the old clonal lineage of yellow rust but now provide resistance against the Warrior population,” she explains.

So, what’s next for plant breeders? Breeding for durable resistance is the Holy Grail but the unpredictable nature of the yellow rust population makes it very difficult to anticipate which host resistances genes will continue to work long-term, says Rachel.

“We know single genes are unlikely to endure so we’re looking at combinations of genes together, but it’s difficult to forecast which combination will prove to have the most longevity.”

Trying to breed for resistance post-Warrior means plant breeders have to use all the data available to try and make the best and most-informed choices, she believes. That’s where the data collected by the UKCVPS comes in useful.

“As plant breeders we work with the wheat host genetics and not the pathogen, so the work of the UKCVPS helps us understand if there any significant changes in the yellow rust population – providing an early warning system to potential problems, as well as validating new races.”

The survey work also identifies new pathotypes and provides breeders with information on the virulences these have and at what frequency they’re present, she adds.

There are at least eight effective resistance genes for yellow rust present in the varieties currently on AHDB’s Recommended List. Those that have three or four Yr genes have greater resistance than those with less, says Rachel.

But not all resistance genes are effective against Warrior. To establish which genes are working well together and which are losing efficacy, Limagrain closely monitors its nurseries. “By monitoring resistances year-on-year, we can see which gene combinations are shifting.”

Pre-breeding also provides an added opportunity to introduce resistances, particularly because the elite wheat gene pool is fairly narrow, she says.

“If you cross an elite wheat with another elite wheat then you produce a limited number of Yr gene combinations. So, it may be necessary to introduce Yr genes from elsewhere.”

Approximately 75% of yellow rust resistance genes are native to Triticum aestivum but others, such as Yr5, Yr8, Yr9 and Yr17, are non-native.

“There are a number of additional resources we can exploit. Wheat has three gene pools, with the primary gene pool from elite wheats, landraces and genetic stocks; the secondary gene pool from wheat progenitors; and a tertiary gene pool made up of crop wild relatives. The secondary and tertiary sources can bring more diversity and new resistances.”

But as always with plant breeding, there’s no magic wand and new traits are often accompanied by unwanted ones. “Often these genetics produce plants that are very tall and prone to lodging, they can have lower yields and are difficult to cross, with sterility all too commonly encountered.

“Because of these problems, introduction of new resistances into breeding programmes takes 2-3 times as long compared with using elite sources.”

And that isn’t good news because traditional plant breeding already takes about ten years to progress from the initial cross to nursery stock selection, through to yield trials and eventually National List and RL trials.

“A further downside is that new breeding lines derived from crosses made 4-5 years ago may be less resistant by the time they reach NL trials due to the diversity and speed of change in the yellow rust population.”

Rachel says that for these reasons breeders have to be selective with resistances and the use of DNA markers is essential to help pick those combinations they believe will be most useful, taking into account changes in the pathogen population and how quickly they’re occurring.

“But it’s no easy task! To protect resistance genes we have to stack them together as this provides more durability, but it requires 1000s of lines to be screened with markers in order to create effective gene stacks with four or more resistance genes.”