Clubroot is a serious soil-borne disease. If conditions are right, it can cause up to 100 per cent yield loss in canola. And while yield loss can be low when conditions aren’t favourable, the pathogen is hardy and easily overwinters in the tough Canadian climate.
Come spring, resting spores not only germinate where they overwintered, but they are also capable of swimming short distances in search of root hairs to infect. Once it’s present, there are no control measures that remove the pathogen from the field. It is possible, however, to curb spread and reduce incidence and severity.
Why it matters: While there are no economical control measures for clubroot once it’s in a field, it is possible to prevent further spread and to reduce its severity.
While there are no economical control measures for clubroot once it’s in the field, it is possible to prevent further spread and to reduce its severity.
One cannot know what’s in the field without scouting, so the first step to preventing clubroot is to scout. Many farmers misunderstand the timing around scouting, says Canola Council of Canada (CCC) clubroot specialist Dan Orchard. They wait until they see a dead patch and then investigate. However, farmers should be scouting well before they see anything strange in the field, he says.
Select hotspots at random, says Orchard, pointing to entrance areas, lowlands, high-traffic spots and water runs where runoff occurs.
The best time to start scouting is in July, although it can be difficult to access fields. Right after swathing is much easier and still effective, says Orchard.
“Pull up as many plants as you can,” he says. “Look at the roots carefully, and shake off the dirt.”
While highly infected plants will have galls that are impossible to miss, some galls will be the size of a pea seed. When spore levels are low in the soil, galls will often be found on the lateral roots.
Growers who wait too late to scout may find root systems that appear like mushy sawdust, says Orchard. This is because the roots have already begun decomposing, making detection all the more difficult.
“Really late into the fall they are much harder to recognize,” says Orchard. “By the spring, most of the roots will already have decomposed.”
It’s not always easy to tell if you have clubroot. In fact, some of the best experts out there sometimes get confused. Herbicide drift and injury and hybridization nodules can look like clubroot too, warns Orchard.
One way to be certain is to send in soil and/or plant samples. Plant samples are more accurate, says Orchard, as clubroot is extremely variable within a field and even within a patch, so it can be missed with a soil sample.
Once you do know where you have clubroot, Orchard suggests keeping a record to avoid further spread. This suggestion came from Alberta farmer Curtis Henkelmann, says Orchard. Situated in the heart of Alberta’s clubroot hotspot, Henkelmann has been able to keep his fields relatively clubroot-free. He has done so by following a stringent plan and through strict record-keeping.
“If you try to just remember everything, you can’t,” says Orchard.
Minimizing risk starts with good hygiene. This means reducing soil movement and implementing a sanitation plan. Of utmost importance is incoming equipment from unknown areas needs to be extremely clean. This includes custom work and used or rented equipment. Oilfield exploration can be another source of contamination. Farmers can assess what level of risk they want to assume.
Clubroot risk mitigation also means having a sanitation plan. This involves making sure everyone who comes onto your farm is free from soil. Orchard suggests having a grassed area where large lumps of soil can be easily and safely knocked off. In fact, this is the first step to good sanitation, he says. The second step involves stopping at the edge of the field to minimize the movement of soil. Equipment should also be cleaned off between fields.
Another means for mitigating risk is to control host weeds and volunteers where and when possible. Host weeds include stinkweed, shepherd’s purse, wild mustard and flixweed.
“Make sure you don’t have these host weeds growing in your other crops or it’s like you didn’t take a break from canola,” says Orchard.
Choosing the right varieties before you have clubroot is also a crucial mitigation strategy, as is rotating resistance genetics.
“Because we’re really not good at identifying clubroot early, by default, the industry has agreed that using clubroot-resistant varieties before it gets established is a really key factor to keeping spore loads at manageable low levels,” says Orchard. “It’s that one susceptible crop too many that causes big explosions in spore loads that become really difficult to manage. And then that big spore load puts pressure on our resistant varieties and they won’t be as durable.”
You found clubroot? Here’s what to do next
If you do have infested areas in the field, CCC recommends using patch management techniques. This includes everything from telling farm workers not to drag equipment through areas to fencing off bad patches.
Grassing bad patches also helps, as it holds soil in place. Research shows some grasses have the ability to coax spores out of dormancy quicker, which helps reduce spore loads in soil rapidly. Researchers are continuing to investigate these findings.
“Time spent managing and making plans is better than time spent figuring out who brought clubroot to the farm,” says Orchard. “It doesn’t really matter how bad clubroot is around you, and it’s doubtful it would affect you enough to cause you hardship if you do these management things.”