The discovery in south-central Manitoba of a new clubroot strain not controlled by traditional resistant canola varieties underscores the need to be proactive in keeping clubroot spore numbers low enough so they don’t damage canola crops.
The 3A clubroot pathotype was found in a field in the RM of Pembina, Manitoba Agriculture posted on its website Sept. 6, and stated it “is able to overcome some first-generation sources of genetic resistance in commercial canola cultivars.”
Why it matters: While clubroot starts off slowly if left unchecked the soil-borne disease can multiply and spread if not detected and controlled early, cutting canola yields by up to 75 per cent.
“Varieties that contain resistance specifically to pathotype 3A are limited,” Manitoba Agriculture’s oilseed specialist Dane Froese said in an interview Sept. 11. “There are only two varieties that I am aware that claim specific resistance to 3A that are commercially available. There are another three that contain second generation-type resistance meaning… they don’t specify (that they have resistance) to 3A… so you need to take it with a grain of salt if you choose to grow it in a field with that pathogen.”
So far the new 3A pathotype has only been found in one field and there’s no reason to believe it is widespread in Manitoba, Froese said.
While it’s suspected clubroot spores are present in many Manitoba fields, levels are are still low enough that crops aren’t showing systems. That means it’s still possible to keep the soil-borne disease caused by a fungus-like protist called Plasmodiophora brassicae in check.
When clubroot spores build they infect canola roots and produce galls that prevent plants for taking up moisture and nutrients.
“The number one recommendation (to keep clubroot down) is crop rotation — having at least two different crops between a canola crop,” Froese said. “That’s a one-in-three-year rotation.”
Greenhouse tests show half of clubroot spores in soil die over four to six years, he added. A field with a million spores will drop to 500,000 after four to six years of being canola-free.
“Once you drop below 100,000 (spores) based on those same greenhouse studies we tend to see less infection of plant root material,” Froese said. “That’s the aim — to draw down the spore load so it’s less aggressively affecting plant roots so you might not notice field loss or visual infection.”
A three-year canola rotation also helps control other pests, including blackleg, a fungus disease that can also decimate canola yields.
Manitoba’s first case of clubroot in canola was found in 2013.
In 2014 there were 13 cases, but 11 were based just on soil samples showing spores, with no visible signs of disease in canola.
So far this year 40 fields with visible clubroot infection have been confirmed in seven Manitoba municipalities, Froese said.
A suspected case in an eighth municipality is currently being investigated.
Clubroot is not a reportable disease in Manitoba, unlike in Saskatchewan and Alberta. But Manitoba Agriculture encourages farmers who suspect a clubroot infection to let the department know.
“We keep track of that and work one-on-one with growers to proactively have a plan in place to be able to proactively manage it,” Froese said. “And we concentrate extension efforts in municipalities we know are higher-risk, so growers and agronomists are aware and know how to deal with it.”
Farmers are also encouraged to soil test fields for clubroot, even if canola crops aren’t showing signs of infection.
“What we really want is proactive clubroot testing,” he said. “Once we have galls (on canola plant roots) in the field it’s not as useful because if we can see symptoms we know the spore load is high enough to cause that.”
If spores are present, even in small numbers, farmers should plant clubroot-resistant varieties, Froese said. That, along with crop rotation, will keep spore loads down and reduce the possibility of selecting pathotypes that can infect normally resistant canola varieties.
Farmers should scout for clubroot too, he said. Be on the lookout for dying patches of canola while spraying, or dead patches when swathing.
Other measures to mitigate clubroot include controlling volunteer canola and other host plants, and cleaning equipment between fields to avoid spreading clubroot spores.
Farmers with an infected field should seed and harvest it last, Froese said.
Cleaning equipment takes time, but is worth the effort while clubroot still isn’t widespread, he said.
“If the soil is dry… you can remove about 90 per cent of the soil and that removes 90 per cent of the risk from moving infected soil into a non-infected field,” Froese said. “If you use the pressure washer or air gun to clean it off completely, 99 per cent of the problem is removed. The more we can be proactive with sanitation and managing patches — clubroot usually appears as patches in a field — that really does reduce the spread and the risk overall.”