All of the canola seeded in Canada has quantitative resistance to blackleg, a serious disease that affects canola growers worldwide.
Quantitative is a fancy way of saying the plant has background defence mechanisms, comparable to an immune system, that fight off and limit the spread of the disease.
The problem, though, is scientists don’t really understand how the defences work or what genes are responsible for the quantitative resistance.
“We find it in all of our varieties but we can’t measure it,” said Justine Cornelson, Canola Council of Canada agronomist and blackleg expert.
“It’s there in all your varieties. It’s a mix of all these different genes.”
Research in Australia may have changed the game when it comes to understanding quantitative resistance.
In a paper published earlier in May, Australian scientists identified sections of the canola genome that provide quantitative resistance to blackleg.
“Eight regions on the canola genome with resistance to blackleg were detected across diverse environments,” said Harsh Raman, a molecular scientist with the New South Wales government.
“These identified genomic regions could be targeted to breed elite canola varieties with durable or long-term blackleg resistance and global application.”
That’s an improvement from existing breeding methods, where quantitative resistance is achieved blindly through hybrid production.
“It’s not like they were selective breeding to have really good quantitative resistance,” Cornelson said.
Quantitative resistance actually allows a pathogen to enter and attack the plant, but it limits the spread and severity of the infection, similar to the white blood cells in a human body.
That’s different from qualitative resistance, which acts more like a roadblock.
“The major (resistance) gene is very specific. It has to match a specific race of blackleg in the field,” Cornelson said. “That (gene) stops the infection right at the site. Doesn’t allow the pathogen to spread throughout the plant.”
The weakness of major genes is that blackleg evolves, and mutations allow the disease to overcome the genetic resistance.
Quantitative resistance is longer lasting, so breeding for it should benefit canola growers, Raman said.
“Given that quantitative resistance is difficult to select and is complicated by environmental factors, molecular markers linked with (it) could be used to enhance blackleg resistance in canola germplasm.”
That doesn’t mean canola breeders can abandon major resistant genes. Ideally, canola hybrids should have a major gene, supported by hardy quantitative resistance to blackleg, Cornelson said.
“We want both. You want a good major gene … and when you have a few of those aggressive strains, (that) overcome it, you still have that quantitative background to minimize the infection.”