THE PERSISTENCE OF CLUBROOT
Imagine one gram of soil. Pretty insignificant, right? Now, pretend that same gram is infested with clubroot. All of a sudden that teaspoon of dirt went from being relatively harmless to potentially holding billions of clubroot spores ready to devour your canola. The visual is enough to send chills down any farmer’s spine.
This troublesome pathogen continues to spread its devastation across Canadian canola fields every year. It is an incredibly complex disease and particularly frustrating because there is no one universal way to control it.
As of today, clubroot remains strictly a management play with no fungicide solution. That, combined with its 36 known, and counting, pathotypes, and the research challenges it presents to plant pathologists, clubroot shows no signs of slowing down.
So, is there hope? Believe it or not, there is. Quite a lot.
CLUBROOT: A SHORT HISTORY
Clubroot has been known for centuries and is believed to have originated in the Mediterranean region. The disease has been detected in Canada for more than 100 years and many posit its likely journey to this country was made via infected produce, perhaps in fodder turnips. Often present in cole crop vegetables, it wasn’t until 1997, that it first appeared in canola near Lac Saint-Jean, Quebec and, six years later, on the outskirts of Edmonton.
Today, clubroot is prevalent from central Alberta to the province’s northern Peace region, stretching right through the Prairies all the way to Ontario. It has even been found near Swift Current, SK, a mighty concern in a semi-arid region known for longer crop rotations, neither of which is a key driver of clubroot.
“Some diseases come and go, but the problem with clubroot is once it gets established into the soil, it’s difficult, if not impossible, to eradicate it,” says Stephen Strelkov, a University of Alberta plant pathology professor focused on clubroot. That’s partly because resting spores are incredibly long-lived, surviving in the soil for decades.
By 2009, canola varieties resistant to clubroot had come to market. Those plants offered resistance to the handful of pathotypes that had been identified at that time, and colloquially they became known as first-generation resistant varieties.
Farmers were ecstatic and began to grow these cultivars en masse. As everyone predicted, things went well for about five years before more pathotypes were discovered and first-generation resistance began to break down.
Today, second-generation resistant canola varieties are coming, but still do not cover all the known clubroot pathotypes.
THE TOOLBOX OF TODAY
Clubroot galls decomposing releasing resting spores.
Growing canola with no clubroot plan is, at best, slightly risky. At worst, it’s a waste of money as fields with heavy spore loads can cause 50 to 80 per cent yield loss, or simply not even bloom. There are management options, but they must be used as a holistic management package or else it just won’t work.
“Just using rotation or just using genetic resistance is just not enough,” says Autumn Barnes, agronomy specialist with the Canola Council of Canada (CCC), based in Lethbridge. “It’s not adequate to deal with this pathogen. The reality is it can be found anywhere. It’s a big concern.”
A recurring temptation with canola is to grow it as often as possible. As canola cash prices flirted with $18 earlier this year, it is understandable that people make market-driven seeding decisions. The CCC’s recommendation is a minimum one-inthree- year canola rotation, if not longer, to reduce spore populations. Research has shown that a one-in-three can shrink clubroot spore loads by up to 90 per cent.
“A two-year break is really quite valuable,” says Barnes. “Things get complicated when you get these astronomical spore counts. Clubroot is really a numbers game. More spores mean larger galls, more yield loss and then more spores released back into the soil to infect future crops.”
Along with rotation, farmers should employ reduced tillage practices on every acre and seed only clubroot resistant varieties. When clubroot comes into a field for the first time, it will have considerably less ability to multiply spores if it immediately encounters resistance.
University of Alberta graduate students Brittany Henning, left, and Keisha Hollman at field plots with clubroot-infected canola.
Biosecurity and sanitation are increasingly important as equipment coming in or off of field carries a lot of soil. Barnes says even kicking off what you can is a tremendous help. As well, farmers must be proactive with farm traffic guidelines. “Conversations with anyone accessing your land can go a long way,” she says. “Make sure that your sanitation expectations are clear.”
Additional measures include vigilant weed control, specifically of stinkweed, flixweed, shepherd’s purse, mustards and even volunteer canola, all of which are excellent hosts for clubroot.
THE TOOLBOX OF TOMORROW
But what of the future? Why isn’t there a fungicide solution or robust genetic resistance? Future-proof solutions require considerable effort because, as Strelkov says, this pathogen is not straightforward. “You can’t culture the clubroot pathogen,” he says. “You can culture blackleg, but we are not there yet with clubroot. It’s a hard pathogen to work with.”
University of Alberta graduate student Heather Tso in the clubroot greenhouse. Tso is working on the development of molecular methods to identify clubroot pathotypes.
This non-culturable feature is what makes clubroot research such a laborious challenge for scientists like Strelkov, who leads one of the largest clubroot programs in Canada.
Pathotyping clubroot, for instance, is a long, drawn out process compared to other canola diseases. Currently, to determine pathotypes, clubroot galls are sent to Strelkov’s lab where they are puréed in a blender, then inoculated on live canola plants and then grown in a greenhouse. Only once a pathogen has developed into active disease (galls) can a pathotype be identified. This process may take up to eight weeks — far too long for routine diagnostic testing for farmers.
“Right now we have ways to detect and measure the clubroot pathogen with molecular means, but no routine molecular tests are available to distinguish between pathotypes,” he says, adding that labs only provide a simple positive/ negative for the presence of clubroot, alongside spore counts. “We rely mostly on bioassays to determine the pathotypes.”
“Some diseases come and go, but the problem with clubroot is once it gets established into the soil, it’s difficult, if not impossible, to eradicate it”
PROFESSOR OF PLANT PATHOLOGY UNIVERSITY OF ALBERTA.
Research is underway to shorten wait times. Strelkov is leading a study at the U of A with graduate student Heather Tso and research associate Leo Galindo, as well as others at the federal government level and at Université Laval, to develop a rapid molecular assay — essentially a canola version of the COVID-19 PCR test, to determine which pathotype is present.
Once proven, results will seem lightning fast compared to current practices. “Once it’s optimized, you could get results within a week,” says Strelkov. “If we had this molecular test, it could be more cheaply available to growers and industry. There are already promising results. We could have a test ready in the next couple of years.”
In addition, Strelkov also leads research on clubroot pathotyping and surveillance to better understand and categorize the prevalence and distribution of pathotypes, as well as their characteristics, especially as this all relates to the successful development of second-generation resistant canola varieties.
The impetus behind this work is that with a better understanding of new pathotypes, which is an ongoing effort, researchers will continue to generate more reliable screening tests. In turn, more informed plant breeders will then be able to design newly resistant lines with specific focus on which pathotypic characteristics are deemed priorities to fight against virulence.
Knowing which pathotypes are present also helps guide studies to develop molecular tests. Research also continues to further understand the pathogen/host relationship.
While the difficulties of clubroot — in the lab and the field — persist, there are glimpses of a future where individual clubroot pathotypes might be identified quicker and resistant canola varieties developed sooner. As with anything clubroot, it is just a matter of time.