The word on clubroot in canola: worrisome

Mention “clubroot” to a canola grower in northeastern Ontario, and the reaction is likely to be anything but positive. The disease has been the largest threat to canola production in that region since swede midge appeared roughly 10 years ago. Unless growers take a break from canola for two years or longer, the disease could worsen, dropping production further.

Clubroot was first confirmed in Ontario canola fields in 2016 in Nipissing District. The pathogen has existed in southern Ontario vegetable crops since the 1920s, although that pathotype is not one that easily infects canola.

Since then, it’s been identified as “clubroot positive” in fields in Temiskaming and Algoma Districts and Grey, Dufferin, Bruce, and Simcoe counties.

The disease pathogen is one of the things that makes clubroot so confounding. It is a protist, meaning it has plant, animal, and fungal characteristics and is from the same genus—Plasmodiophora—as malaria. Its spores are mobile compared to other pathogens, and its resting spores are hardy and can overwinter in soil.

The number of pathotypes is another factor that makes the disease difficult to manage. According to Meghan Moran, a 2018 report stated there were 17 pathotypes of clubroot identified in Western Canada, a number she believes has grown since.

“In the limited sample analyses through Dr. Mary Ruth McDonald’s lab (at the University of Guelph), there are about five or six found in Ontario canola,” adds Moran, the canola specialist for the Ontario Ministry of Agriculture, Food and Agribusiness (OMAFA). “We know it’s only a matter of time until populations shift and we start to see symptoms farther south.”

SIZE IS IMPORTANT

Although clubroot-resistant hybrids are available, most are grown by Western Canadian growers, given the extent of the disease and the fact there are roughly 20 million acres of canola grown on the Prairies. That leaves few resistant spring hybrids available to Ontario producers. On the winter canola side, there is little interest in breeding for the market or registering hybrids. The only registered winter hybrid has no resistance to clubroot.

“Nobody really breeds canola for the Ontario market as far as I can tell,” says Moran, despite recent production increases in winter canola in southern regions in the past three years. “Given the scale of the issue and acres in Western Canada, breeding spring canola hybrids for clubroot resistance is a high priority.”

The primary motivation for Moran is to spread the word on clubroot: she wants all growers with canola on their farms to “get familiar” with the disease and be proactive about scouting. During a Clubroot Question and Answer webinar last December, industry representatives offered their recommendations for managing clubroot in canola: scouting, adjusting soil pH, cleaning equipment, and opting out of canola in the rotation for at least two years. Growing a resistant hybrid and managing brassica weeds (and volunteers) were others.

Scouting cannot be underestimated for its impact. Aside from visual evidence, the best way to determine if a field has clubroot is to pull the plants to examine the roots.

Adjusting soil pH is another recommendation, although it’s a lengthy process, and some products can be hard to source. Raising it to 7.2 or above can reduce the effect of clubroot, although a pH of 8 can still see 40 per cent disease severity.

Washing and cleaning equipment can be beneficial, particularly with soil movement on tractors and combines moving from one farm to another or on units that have been sold.

A two-year break from canola in the rotation is one of the more important management recommendations, and extending that to four, six, or seven would be better.

For growers in southern Ontario, whether they’re planting winter or spring canola, the options for switching to other crops are greater than in the “Near North.” For those in Nipissing and Temiskaming, their choices of crop alternatives are scarce; corn isn’t an option (except for a handful who start their corn (for feed) under plastic), soybeans, spring cereals, and forages.

Managing volunteer canola and other brassica weeds was also mentioned.

ALTERNATIVES

The lack of viable alternatives is a limiting factor in dealing with clubroot. Will Runnalls, a grower from New Liskeard, acknowledges the importance of the recommendations listed during last December’s webinar, but most have differing layers of complexity. Opting out of canola in favour of two, four, or six years of soybeans isn’t something he wants to consider, yet the idea of switching to barley or oats is less appealing because of the pricing.

The problem is northeastern Ontario is mired in perception issues: it isn’t similar to southern Manitoba, be it in terms of weather or soils. But growers in Temiskaming or Nipissing can’t source the same registered products as those in Western Canada. The same is true with soybean varieties, where Runnalls doesn’t always have access to 00 or 000 varieties.

“If you’re trying to sell 1,000 or 1,500 tons of spring barley, there’s no one you can sell that volume to,” says Runnalls, who farms roughly 1,000 acres with a spring wheat- soybean-canola rotation. “Canola’s in a really good spot now—there’s virtually unlimited capacity for what could ever be grown in Ontario. But it’s small grains—oats and barley—where there isn’t enough capacity. They’re becoming illiquid—you cannot turn them into cash quickly.”

With clubroot in canola, the default that what can work in Western Canada doesn’t translate to the east: soils in Temiskaming and Nipissing receive more moisture than the west, which is a bigger help to the movement of clubroot spores.

“Clubroot’s like nothing we’ve ever dealt with,” adds Runnalls. “When you see a field with no canola, and people ask why, that’s starting to educate them. But even starting with the basics—what it is, how we can’t control it with chemicals, how it’s in your soils for a long time—people are more receptive, and not that it’s too late, but the education is happening because you can see the problem.” •