Wheat rust concerns

by Chris Lyddon
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European wheat producers are dealing with increasing rust problems as a new type of the disease, one of the most devastating for wheat, has started to affect what were previously resistant varieties. The disease’s ability to change could pose a threat to world wheat production.

At a meeting sponsored by the Home Grown Cereals Authority in Cambridge in February, experts described how the development of the disease is being tracked, while a Pakistani scientist described how the Himalaya region is proving to be the source of new and more threatening strains of rust. These could pose a much more worrying threat to more varieties than the Warrior race, named after the variety it has affected most on British farms.

The discovery that yellow rust can reproduce sexually as well as by cloning raises the worrying prospect of more wheat varieties becoming susceptible, delegates at the annual meeting of the U.K. Cereal Pathogen Virulence Survey (UKCPVS) were told.

UKCPVS has monitored cereal rusts and mildews in the U.K. for more than 40 years. Each year, the survey tests 25 isolates of wheat yellow rust and 25 of wheat brown rust. Yellow rust races are often named after a variety on which it is virulent, for example Warrior or Solstice. These are characterized by virulence for the differential variety Spaldings Prolific, though not all of the variants are virulent on Warrior itself.

“You all know why we study yellow rust,” said Dr. Diane Saunders of the Sainsbury Laboratory. “It significantly reduces grain quality and yield in susceptible cultivars.”

Over the last five years Britain has seen the emergence of a succession of isolates that have overcome key wheat varieties. She noted that the resistance in wheat varieties was based on a “very narrow genetic range.”

“In 2013, up to 75% of the U.K. wheat acreage was susceptible to the newly emergent race Warrior,” she said. “Despite the success of the UKCPVS and our understanding of the phenotypic diversity of wheat yellow rust, we know almost nothing about its genetic diversity in the U.K.”

Dr. Saunders’ research is looking again at samples collected in the survey over a number of years. Her team is investigating the genetic sequence of yellow rust to draw a genetic “family tree” to see how the different races are related to each other. The results seem to show a lot of difference between “Warrior types” and other races of yellow rust in the U.K., raising the possibility that the “Warrior type” may have been brought into the U.K. rather than evolving from a race already present.

Britain’s mild wet winter means that 2014 is likely to be what researchers call “a good year” for yellow rust.

“We have a bit of a yellow rust bonanza,” Professor James Brown of the John Innes Centre told journalists before the meeting. “It’s been such a mild winter. Yellow rust will surely take off.”

He noted concern that what have become known as the Warrior type races, which can beat the resistance bred into certain wheat varieties, would take off in Europe. “The take-home message is that they are from outside the U.K.,” he said. “Over the last 12 years there have been three or four epidemics of yellow rust worldwide. It has become a problem in places where it didn’t exist before.”

New types of rust were coming from the Himalaya region where it’s changing fast.

“The pathogen has a sexual stage,” he said. “It can keep developing new genotypes. What we have been used to over the last 100 years is step-wise changes. What we’re now seeing is worldwide dispersal of every disease and waves of yellow rust.”

He considered the reasons. “Greatly increased international travel is one,” he said, but a further factor was the “use of the same types of resistance over a wide area.”

Dr. Sajid Ali of the University of Agriculture in Peshawar looked at how diseases have moved around the world. “We have to consider the global picture if we want to have a strategy for disease resistance,” he said. “One thing is the selection of resistance genes that are durable. The other is how to deploy these resistances. This is something coming from outside Europe.”

“Breeders in Britain have been successful in breeding for resistance to the yellow rust we have,” said Brown. “Not that this is a panic situation. The purpose is to give plant breeders information to consider that the strategies they are using are adequate.

“Historically, yellow rust has been the biggest disease of wheat and we want to make sure it doesn’t go back to being the biggest disease of wheat.”

Because of achievements with breeding yellow rust, septoria has been the big problem for the last 20 years.

“The sexual stage was identified four years ago,” he said. “Sajid’s paper published (in January) shows that it is important in agriculture. Our native European yellow rust has got a very low capacity for this sexual stage. It’s something to be aware of and adapt plant breeding strategies.

“You’ve got this quite clear evidence that the sexual stage is important in re-establishing populations in southern Pakistan.”

New type of rust in U.K.

He explained that what is seen in the U.K. is a new type of rust that is more complicated.

“There isn’t a Warrior race,” he said. “It’s quite diverse.”

Reporting on the survey work being done on yellow rust, Amelia Hubbard of NIAB warned delegates at the meeting that growers need to “be on the watch for yellow rust on varieties with high disease resistance ratings,” pointing out that the mild winter meant that lots of spores will have successfully overwintered.

In 2013, the researchers had found five main pathogen types. The presence of the Spaldings Prolific isolate was deemed to be the key differential for Warrior type pathotypes. In 2014, the researchers have already received samples from autumn sown crops.

Brown rust has also already been observed in some crops, her colleague Emma Coventry said.

Ali explained findings of a multinational project to map out diversity in yellow rust populations. Yellow rust in Northern Europe has less genetic diversity than other regions of the world, in particular the Himalayan region where the pathogen is able to reproduce sexually which can result in novel types emerging and an increase in population diversity. With increased global travel, there is a concern that new races could be spread more quickly than in the past where the wind was the main method of dispersal.

“In the past two centuries there has been more and more human mobility across the world,” he said. “That has resulted in the emergence and re-emergence of, for example, late potato blight.”

He described a series of movements of pathogens. Chestnut blight crossed Japan to the U.S. in 1904. Oilseed stem canker moved from the U.S. to Australia in 1950. Ug99 wheat stem rust moved from eastern Africa to Asia in 1999. Perhaps most devastatingly, he pointed out how potato late blight had invaded Ireland from Central America in 1845.

More recently, what are known as the Warrior-Kranich strains of yellow rust have invaded Europe.

“If we have invasions, the question is how to manage it,” he said. “We need to study the population structure of the pathogen worldwide. Until 2005 or 2006 we were considering only the asexual cycle of the pathogen. It was considered to be cloned. But then in 2010, a U.S. group found a sexual lifecycle.

“We know the pathogen is moving around the world. Can we identify the sources of these invasions, which will help us to identify future ones? We have six genetic groups worldwide. Mostly we have geographically spaced genetic groups.

“This is the zone of diversity of the pathogen, the Himalayan region. We have these important populations in China and Pakistan.”

He explained that berberis, an alternate host for rust, is common around the fields in Pakistan.

“This is not your average European Yellow Rust,” said NIAB’s Bill Clark. “It’s going to be more difficult to control.”

Chris Lyddon is World Grain’s European editor. He may be contacted at: chris.lyddon@ntlworld.com.