As Canada’s prairies continue to churn out more and more canola, there’s mounting concern about the rapid spread of a soil-borne disease that’s threatening to crimp yields.

Clubroot is taking hold in parts of Canada’s vast prairies. It’s an invasive disease that grows like a cancer on canola roots, preventing the plant from taking up water and nutrients, cutting yields and potentially killing plants.

While the disease first appeared in Alberta in 2003 in only a handful of fields, there are now hundreds of new cases identified each year and new strains are becoming resistant to some of the only tools available to limit its spread. The threat has sparked concerns that farmers won’t be able to grow the oilseed profitably if it continues to spread.

“We’re bracing for a record year in terms of the number of clubroot fields found,” said Dan Orchard, an agronomist with the Canola Council of Canada in Wetaskiwin, Alberta. “People are finding wilted, dead patches in their field.”

Canada is the world’s top grower of canola, an oilseed invented by Canadian scientists in the 1970s by breeding out undesirable traits from the rapeseed plant. Since then, it’s become the go-to cash crop for farmers as global demand exploded for use in everything from salad dressing to french fries. The number of acres planted has increased 40 per cent in the past two decades, surpassing spring wheat as the country’s most planted crop. Canola prices, little changed in the past year, are down about 35 per cent from a 2008 peak.

While the surge in production has helped create a domestic industry worth more than C$27 billion ($20 billion), tight rotations are helping to facilitate the spread of clubroot. The disease releases spores that can live in soil and spread by the wind or hitching a ride on farm equipment. Planting canola every year or every other year on fields increases the risk that clubroot could take hold.

There’s now as many as 300 new fields a year in Alberta and cases have started to spring up in Manitoba and Saskatchewan, the country’s largest canola grower, Orchard said. There are nearly 3,000 confirmed fields with clubroot, though the actual number is probably much higher as there are still limited resources to survey and inspect fields, he said.

While there is no hard data on exactly how much of the country’s total canola crop is affected by clubroot, it may be at least two per cent of all acres as not all cases have been reported, said Stephen Strelkov, professor of plant pathology at University of Alberta.

“It’s a devastating pest and it’s very difficult to control,” said Sebastien Dutrisac, an agricultural fieldman at Northern Sunrise County in Alberta’s Peace River region. “We only have a very small amount of options genetically and if we don’t take care of it we’re going to be ruining the tool that we have.”

Dutrisac’s county, like other Alberta municipalities, has zero-tolerance policies to stave off clubroot’s creep. In Peace River, a northern area where canola accounts for about 40 per cent of all crops, municipal officials don’t allow farmers to plant the oilseed in fields where clubroot is found for as many as five years, Dutrisac said.

Even that may not be enough.

The first canola seeds with genetic resistance to known strains of clubroot were introduced in 2009 and became the most widely-used method for managing the disease, said plant pathologist Strelkov. Since then, researchers have been finding increasing number of fields where new strains of clubroot have emerged and have overcome that resistance, he said. There were more than 100 known fields with resistance issues identified as of last fall, Strelkov said.

Companies such as Bayer AG are working to tap clubroot resistant traits from vegetables such as rutabagas to breed into new canola seed varieties, said Jed Christianson, the canola pipeline lead at Bayer Canada in Winnipeg. Still, the next generation of clubroot resistant seeds are potentially a year or two away from release, he said.

“It’s a significant issue because of the fact that it’s continued to spread and once it’s there it’s very difficult to get rid of and manage properly,” Strelkov said by telephone, noting as much as 15 per cent of fields with clubroot experience severe infestation. “If it does become severe in a field it can have devastating effects.”