Clubroot is on the move. After the first discovery near Edmonton in 2003, the deadly canola disease has spread in Alberta, and last summer it moved into northwestern Saskatchewan and jumped across hundreds of kilometres of bush to the Peace Country area of Alberta.
Doug Moisey, a DuPont Pioneer area agronomist in central Alberta, says he has watched the spread firsthand. “Within the county of St. Paul, they have gone from four or five known fields to 25 known fields with clubroot in the last year.”
That means farmers across the Prairies should start taking precautions. Step one is minimizing soil movement between fields, including from equipment, vehicles, footwear, wind and water. “Most of the places we see the early signs are in the entrances to the field. With the initial spread of clubroot you can see which way the planter turned to start seeding in the field — the affected areas spread out from there as the seeding equipment is dragging in the diseased soil.”
Race shifts and Breeding sources
Step two is rotation. Moisey says canola is a one-in three-year rotation on most acres and he ideally recommends a one in-four rotation, but recognizes many producers grow canola one year in two because of economics. “Knowing this we need to make sure growers use a clubroot-resistant hybrid as part of their rotation and now should include products with new sources of clubroot resistance as well.”
Aaron Miller, a DuPont Pioneer agronomist for northwestern and west-central Saskatchewan, says the resistant hybrids are effective. “Usually you see over 90 per cent reduction in clubroot… the issue is when you get into tight rotations and you start selecting for pathotypes that can overcome the disease.”
The challenge for plant breeders is that there are many strains of clubroot, so they are developing different resistance backgrounds that need to be included in the rotation cycle. The resistance genes can come from other brassica plants such as cauliflower or cabbage. From each new source of clubroot resistance, they will look to a different plant that has resistance to known races that will become a different source of resistance other than what is being utilized at this time.
“The bottom line is that there are only so many sources of resistance. We are now offering a new source of resistance to provide a different option for growers,” says Miller. More hybrids with different sources are needed to provide growers with different clubroot resistance options, but it takes years to bring new sources of resistance to market. “We now have canola hybrids available with different sources of clubroot resistance and the goal is to develop more.”
This means that a grower on a three- or four-year rotation growing base genetics with clubroot resistance A in year one and then comes back three or four years later with base genetics with clubroot resistance B is actually on a six- or eight-year rotation for clubroot resistance, which is good.
In the past few years, resistance packages have been quite good at managing clubroot races 2, 3, 5, 6 and 8.
“However, we have seen a race shift, especially in the Edmonton area,” says Moisey. “Because of the selection pressure we have put on the current clubroot pathogen (with a two-year canola rotation). The clubroot pathogen has made a natural selection for a phenotype that can attack canola. Our new canola hybrid Pioneer® hybrid 45CM36 is still effective against 2, 3, 5, 6 and 8 but also is resistant against variants of clubroot race 2 and 3. With the shorten canola rotations in some areas we have seen a change in the clubroot pathogen population. ”
The volunteer problem
Controlling canola volunteers in subsequent crops is critical part of clubroot management. “When using the resistance package effectively, the spores do germinate, they do attack the canola but the resistance doesn’t allow them to re-infect, and doesn’t allow for spore production — which means you actually achieve a net reduction of spore load after growing a clubroot-resistant product.”
Moisey says the key is to have a multifaceted resistance strategy. “Use different hybrids that contain different resistances packages to clubroot which allow growers to manage the disease with the rotations they are using — often two years instead of three is what happens in the field which we do not recommend or encourage as we want to have a greater break between canola crops.”
Moisey says with the shift of clubroot races over the last two years and with more fields being discovered rotation is even more important.
“I recommend a three-year rotation, but the trick is to find a third crop that is economically viable on your farm, some crops may be peas or soybeans that have the geographic ability to grow the crop or something else for the third year that is not susceptible to clubroot. That’s where we really see the reduction in clubroot spore loads.”
He says he knows growers who have moved back to a threeyear rotation using clubrootresistant hybrids, and as a result are not seeing the clubroot pressures on plants even though they have the disease in their field.
Resistant hybrids are not completely immune and under heavy disease pressure some galls may show up on the root. As well, there will be some non-resistant plants in any population and if you push canola rotations that strain of clubroot will multiply and new clubroot variations will build up.
Miller says weed control is also important. “Any brassica species can carry clubroot, so if you are growing wheat you can get volunteer canola come in and be a host for the clubroot disease. Stinkweed and shepherds purse are other weeds that can keep the life cycle of clubroot going so we want to make sure that we control any brassica species in a field that might be a host.”
Carry a rubber mallet
“What I say to the growers who don’t have clubroot right now is to start managing as if they do have it on their farm and by managing as if they have clubroot it should never be a big issue,” says Miller. He stresses good sanitation.
“It’s so important to take a rubber mallet and spend a half hour knocking all the dirt off the equipment before moving into the next field; it reduces the risk of moving the disease around by 80-90 per cent says Moisey.”
“We usually tackle clubroot one year too late. It’s there and we ignore it. If we start using clubroot-resistant products early, we potentially minimize any issues.”
Moisey recommends that if you have a known clubroot field(s), plant your new canola field early into cool soil conditions as the clubroot spores germinate in warm wet conditions. By seeding early it allows the young canola seedlings to establish and grow without the disease pressures early in the season to establish a healthier stand.
While the best time to scout for trouble is during swathing, Miller says it is an ongoing process. “Stop the truck if you see a problem area, get out and check the roots for galls.” Drones and other technology can also aid in finding trouble spots.
Some growers seed grass in field entrance ways where all equipment and vehicles get cleaned to avoid soil contamination. “As an agronomist, I suggest that farmers maintain grass fencelines and treelines to prevent spores moving into the field; they create a barrier to soil movement,” he adds.
Avoid purchasing hay from known regions that have clubroot and take extra care in cleaning any purchased equipment from affected areas or when moving equipment from a known clubroot area to other areas.