Canola has its share of enemies. Several diseases, such as blackleg and sclerotinia, eat up yields and cause headaches for farmers. But canola’s newest foe is proving much tougher to fight than first thought. “Clubroot is a soil-borne disease, not air-borne like many other canola diseases,” explains Murray Hartman, oilseed specialist with Alberta Agriculture. Soilborne diseases are typically easier to contain, but clubroot is proving otherwise. “I’ve been surprised by just how fast this disease has spread and is still spreading,” he says.
A lack of registered fungicides and resistant varieties to use against clubroot has left farmers with few management options once clubroot is suspected in a canola field. That’s why containing the disease so important.
“We know that soil transfer is the number one way the disease spreads. New infection and highest levels of infection occur at field entry ways. If seed were a major carrier, we’d see the disease more uniformly through the field,” Hartman says. That’s not to say seed isn’t a source of infection, because it is, but it’s not the primary source. Still, seed harvested from an infected field — or even from a following crop — shouldn’t be used as seed.
Soil movement by the energy sector has been flagged as a significant cause in spreading clubroot. Hartman notes that farmers have been diligent in putting “cleaning clauses” in their land lease agreements and uptake within the energy sector has been good. It’s a good first step in minimizing soil contamination, but Hartman says farmers have to be just as diligent if not more in keeping drills and other equipment cleaned out between fields. “It is tough, because soil transfer happens most often in wet, muddy conditions, and those also happen to be the conditions when farmers are most behind and time is of the essence.” Stopping to spray down all equipment can be time consuming at the worst times, he says.
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Clubroot forms galls on growing plants. You can see how the disease gets its name.
“I can’t blame a farmer for pushing canola rotations — it’s one of the few crops that consistently delivers decent returns — but eventually, especially with this disease, pushing rotation limits may mean not being able to grow canola at all.”
The Canola Council of Canada has a great online resource for clubroot management at www.clubroot.ca.The site has tips and photos to identify, prevent and control clubroot. Click “resources” at the top to find links to factsheets from Agriculture and Agri-Food Canada and from provincial ag
HOW TO CLEAN EQUIPMENT
Is spraying down with a pressure washer really enough? Hartman says a pilot project launched this year will compare the benefits of disinfecting equipment versus a power spray-off. In the meantime, spraying with plain old water and removing all visible soil is the minimum required to slow the spread of clubroot.
Determining clubroot infection is best done by field scouting, Hartman says. “If you’re seeing the galls on the plant, the premature ripening and wilting indicative of clubroot, that’s three quarters of your diagnosis,” he says. There is a lab test available to confirm clubroot, but Hartman cautions that it’s not necessarily the most accurate diagnosis. “I know that we like to send away a sample and get a definitive yes or no answer, but the test for clubroot is very sensitive and therefore vulnerable to contamination between samples. We had several false positives last year as well as false negatives. The lab test needs validation under commercial conditions. Field evidence is your best indicator at this time,” Hartman says.
That’s not to say that the lab test isn’t worthwhile, but it’s not the ultimate diagnostic. Three labs have teamed up with the Canola Council of Canada, Alberta Agriculture and the University of Alberta to establish a quality standard for the diagnostic test. “We’re hoping this project will let us know what the capabilities of the test truly are,” he says.
RESISTANT VARIETY IN THE WORKS
As with any disease, a variety with resistance is usually the first line of defense in preventing or managing an infection. To that end, Pioneer Hi-Bred has fast tracked a variety for registration with improved tolerance. The Roundup Ready hybrid may be available as early as 2010, which Hartman agrees is a great thing. “Any new tool in the management of this disease is welcome,” he says. But he cautions against depending heavily on plant resistance for management. “Planting only one resistant variety on infected fields or across several fields, over and over means that the disease will overcome the resistance that much quicker. Plant resistance isn’t a replacement for cleaning equipment and a one-in-four-year canola rotation.”
Perhaps clubroot’s most troubling feature is its persistence in the soil. “We’ve seen fields that had low levels of the disease in 2003 be seeded back to canola in 2008 and the disease was still there. At Leduc, a three-year gap between crops did nothing to reduce infection levels,” Hartman says. “The longevity of resting spores can be as long as two decades.” It’s this persistence that makes avoiding a build up of spores absolutely critical. “At low levels, you may still be able to grow canola once every four years, but at higher rates of infection, it’s just not going to be an option.”
“Pushing the crop rotation window is like speeding to a meeting you’re always late for. You know there are risks and that you might get a ticket or worse in an accident, but you
take the risk instead of figuring out why you’re always late for the meeting,” Hartman says. “I can’t blame a farmer for pushing canola rotations — it’s one of the few crops that consistently delivers decent returns — but eventually, especially with this disease, pushing rotation limits may mean not being able to grow canola at all.”
Lyndsey Smith is field editor with Grainews and Farm Business Communications.
Here is where clubroot has been found in Alberta. Counties in red have at least one confirmed case of clubroot. Counties in white were not surveyed.