How Canadian researchers are trying to win the battle against a stubborn wheat disease

Researchers in southern Alberta are trying to get the upper hand in the battle against a stubborn disease that wipes out up to 10 per cent of Canada's wheat crop annually and costs farmers millions of dollars.

It's called tan spot disease, a fungal infection named for the small tan-coloured lesions it creates on the green leaves of wheat plants. While it may not threaten to wipe out entire fields like others might, its persistence makes it a regular headache for growers.

Dr. Reem Aboukhaddour, chair of Agriculture and Agri-Food Canada's research lab in Lethbridge, Alta., is leading a global collaboration to find out more about how the disease operates. 

"[Tan spot] is not necessarily the most damaging … but it is widespread, that's for sure. In a commercial field, I always can find tan spot no matter what," Aboukhaddour said.

"So it's up to you how you look at it. Are we privileged to ignore the five to 10 per cent damage every year?"

The damage caused by the disease can add up in a hurry.

In 2022, wheat generated over $50 billion in revenue in Western Canada, according to Cereals Canada, and is responsible for thousands of jobs. Saskatchewan is the biggest producer, followed by Alberta and Manitoba.

Like any cultivated plant, wheat is susceptible to blights. While tan spot disease may be obscure, it accounts for millions of dollars in losses annually.

Aboukhaddour's lab in Lethbridge has, in recent years, made new research breakthroughs that could help shed more light on how the disease spreads and infects plants. These findings, Aboukhaddour says, will hopefully aid the creation of more disease-resistant forms of wheat. 

"The fundamental question for us has not changed, which is how to protect our food, basically. That's what it comes down to for me," she said. 

"To build effective genetic resistance, you have to build tolerance to the pathogen itself." 

Understanding how tan spot works

The tools a pathogen uses to get around or hijack a plant's defence system are known in the scientific community as a disease's "virulence factors." 

Aboukhaddour says there isn't a broad understanding of tan spot's virulence factors, but there are a few things known for certain. 

Tan spot spreads via spores, and can overwinter on the stubble of crops — wheat, but also barley. This means it is more threatening to growers who practise monocropping, or the repeated cultivation of one crop on the same field. 

Aboukhaddour also knows that tan spot thrives in temperate wet weather, particularly in the early spring, and in fields that use irrigation systems. 

Another thing researchers are well aware of is that tan spot is resilient, and it's present everywhere in the world where wheat is cultivated. 

Aboukhaddour's team found evidence for this when they were able to sequence the disease's genome based on Agriculture and Agri-Food Canada's vast collection of tan-spot samples from around the world.  

"[We were able to] show that this fungus has the ability to have a very dynamic or flexible genome," she said

"[It has] evolved and adapted through time to share the same weapons to kill the plant." 

Another thing impacting farmer's bottom line

Tan spot disease was identified about 50 years ago, which, in scientific terms, means it's relatively new in its emergence. 

Aboukhaddour says losses in the 1970s due to tan-spot disease were particularly bad. It was before farmers in Canada began to routinely use fungicide. 

Dr. Kelly Turkington is a plant pathologist with AAFC's research lab in Lacombe, Alta., who has collaborated with Aboukhaddour's team on various projects. 

He says tan spot is part of a group of diseases called the "leaf spot complex," which are the main yield robbers for wheat farmers. 

Tan spot can be challenging for producers to manage, not only because it can survive over the winter, but also because there isn't much information on what wheat strains are less susceptible, says Turkington. 

It's known to infect all major strains of wheat grown in Canada, including durum wheat. 

While crop rotation is an effective way to combat tan spot, many factors are at play when a producer is deciding where to plant what crop, disease management being just one, says Turkington.

Other aspects, such as commodity prices and knowledge of alternative crops, are also considerations. 

Another tool farmers have in their tool chest are fungicides. But, depending on the size of the farm, Turkington says it can cost producers thousands to hundreds of thousands of dollars. 

Stephen Vandervalk, a farmer and Alberta vice-president of the Wheat Growers Association, told CBC News the impact of tan spot disease on farmers in the province fluctuates from year to year.

"Because it's so variable, one year you'll have it bad — and it could be three years before you see it again," Vandervalk said. "When you do get it bad, it can be totally devastating."

He added farmers often use fungicide as a one-size-fits-all approach when dealing with diseases such as tan spot. But there is a better way to prevent the blight that he's hopeful will develop.

"The best way to do it is to put it into the genetics, so it's resistant to the tan spot right when it's a seed." 

Though Aboukhaddour thinks the use of fungicide is important, she says it may have given farmers the impression tan spot is less damaging than it really is. She's also worried about the disease becoming resistant to certain fungicides. 

Turkington says tan spot isn't on his radar as something that could outwit fungicides, but he wants the research and agricultural community to stay vigilant. 

"It's so important in terms of being prepared to address particular issues that start to arise, at an earlier stage, versus letting things get to a point where it's more difficult to effectively manage [them]."