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Considering clubroot

Photo courtesy of Clint Jurke.

Clubroot is a major disease of canola in Alberta and a worry for growers in Saskatchewan and Manitoba.
What are the chances that it could become a problem in Ontario canola? And what advances in clubroot management could help Ontario growers if the disease does threaten their canola crops?

Clubroot is caused by a soil-borne organism called Plasmodiophora brassicae. It affects cruciferous crops, such as canola, mustard, cauliflower and cabbage, and cruciferous weeds like stinkweed. Ten years ago, clubroot was discovered in a canola field near Edmonton. That was the first time the disease had been found in a commercial canola field in Canada. At that time, a small survey found the pathogen in a handful of canola fields in the Edmonton area. Within a few years, clubroot had become a serious problem in Alberta canola, especially in the region around Edmonton, and caused devastating yield losses in severely infested fields.

The clubroot pathogen has also been detected in Saskatchewan and Manitoba, but so far it has not caused serious disease problems in canola in those provinces. In Quebec, clubroot first appeared in canola fields in 1997. According to information on the Ontario Ministry of Agriculture and Food’s website, there have been no reported cases of clubroot in Ontario canola.

To infect a crop, the pathogen requires moist conditions. It likes temperatures between approximately 20 and 25 C; temperatures below 17 C inhibit its development. It does better in acidic soils than in alkaline soils. The earlier an infection occurs during the growth of a susceptible plant, the greater the yield loss will be.

The pathogen infects plant roots, causing irregular swellings. “The pathogen hijacks the plants’ ability to use their roots correctly,” explains Clint Jurke, the Canola Council of Canada’s agronomy specialist for western Saskatchewan. “It uses their roots to reproduce, so the roots can’t move nutrients and water into the top parts of the plant. As a result, the plant ends up having yield loss and looking drought stricken.”

Ontario situation
In Ontario, clubroot has been found in localized areas in cruciferous vegetable crops. “For vegetable growers, clubroot is found in areas in and around the Holland Marsh, areas south of Guelph, and many other small pockets throughout the province,” explains Dr. Mary Ruth McDonald, a plant pathologist at the University of Guelph.  

However, she adds, “no one has really done a survey looking for clubroot in Ontario since the 1960s and 1970s. So we only know where it is because it has shown up as a problem on a susceptible crop.”

Why isn’t clubroot causing problems in Ontario canola? “In the last five or six years as I’ve been doing work on clubroot on canola, in conjunction with my research on cruciferous vegetable crops, I’ve been asking myself that same question,” notes McDonald.  

She thinks three factors in combination are helping to keep clubroot at bay in Ontario canola.
One factor is that the strain, or pathotype, of Plasmodiophora brassicae that appears to be most common in Ontario isn’t a fan of canola. Pathotypes are types of a pathogen that are identified based on which particular crops they attack.

At least five clubroot pathotypes occur in Alberta – pathotypes 2, 3, 5, 6 and 8. “The pathotype causing all of the problems in Alberta is pathotype 3. It is very virulent and causes high levels of disease on canola. The pathotype found in Ontario in surveys in the late 1960s is pathotype 6. There have been two or three surveys since then looking at small numbers of infected crops, and on vegetables they always find pathotype 6,” says McDonald.

She has tested canola cultivars in soils infested with pathotype 6. “We found that many of the cultivars, even ones that are highly susceptible to pathotype 3 in Alberta, are either resistant or partially resistant to pathotype 6. If we get extremely high concentrations of pathotype 6 in the soil, then we get some disease on canola, but it’s much less than a grower in Alberta would see.”

McDonald notes, “You’d only get those extremely high levels of the pathogen if you grew a susceptible crop year after year in the same field.

“So I think another reason we’re not seeing clubroot as a problem in Ontario canola, is that canola is rotated quite a bit. Canola acreage in Ontario is increasing, but it is nothing compared to canola acreage in Alberta. And Alberta growers sometimes are tempted to grow canola after canola or they have very short canola rotations. In Ontario, because canola is not a major crop, it is a lot easier to rotate out of it.”

A third factor is that many soils in southern Ontario tend to be somewhat alkaline. She says, “Our research over the last few years confirms that if the soil has high pH, less clubroot will develop on a susceptible crop. However, even with a high pH, if you have good soil moisture and the temperatures that are best for disease development, you can still get reasonable amounts of disease.

So the general high pH is probably playing a role, although it might be a smaller role, in keeping clubroot at lower levels.” She concludes, “As long as Ontario canola growers continue to follow long crop rotations, they probably have many years ahead where they don’t have to worry about clubroot.”

Learnings from research and experience
The 2003 finding of clubroot in Alberta canola spurred Canadian research on this disease. “Clubroot researchers in Canada have now moved into a leadership role internationally. We are doing way more high quality clubroot research than any other jurisdiction in the world,” says Jurke.

Recent research and breeding advances, plus the experiences of agronomists and growers on the Prairies, have helped the Canola Council develop guidelines for managing this disease.

“Plasmodiophora brassicae is kind of like a fungus but not really. It’s the ‘not really’ that makes it tougher to control than most of the fungi because topical fungicides don’t work with this disease,” explains Jurke.

“But what makes clubroot especially tough to control is that the resting spore phase of P. brassicae allows the pathogen to survive for a long time. Studies in other countries indicate the resting spore can survive up to 20 years in the soil. So unless canola growers are on a 20-year crop rotation, once clubroot is present in the soil, they probably aren’t going to be getting rid of it anytime soon.”

If you do find clubroot in canola
Fortunately, clubroot-resistant canola varieties are now available. “Clubroot resistance is by far our single best weapon for managing this disease in Western Canada. I’ve seen fields where the disease completely wiped out a canola crop because it was a susceptible variety. Then a couple of years later, the farmer planted a resistant variety on that same soil and got a 55-bushel yield off of it,” says Jurke.

“But we want to protect that resistance so we’re strongly encouraging growers to use rotation. The resistant varieties can break down if we overuse them. For instance, if you choose one variety and use it in a tight rotation, you are putting pressure on that resistance to be overcome by the pathogen.” He suggests using a four-year rotation with resistant varieties.

“The clubroot-resistant canola varieties are resistant to that very dangerous pathotype 3,” says McDonald. “From our trials, those varieties are also resistant to pathotype 6. So if the worst-case scenario developed and the organism in Ontario mutated to become pathotype 3 or that pathotype got introduced or something, then Ontario growers could use clubroot-resistant canola lines.” As well, she says some seed companies have developed clubroot-resistant vegetables, such as cabbage, cauliflower and Brussels sprouts.

Another valuable finding from McDonald’s research is that pathotype 3 is able to outcompete and overwhelm pathotype 6. Under contained, controlled environment conditions, she tested canola with various combinations of pathotypes 3 and 6. “Basically, pathotype 3 just takes over and causes as much disease as if it were there all by itself. So it is important that pathotype 3 is not introduced into our soils in Ontario.”

Several other practices can also help manage clubroot in canola. For instance, early seeding helps delay the disease.
“In colder soils the pathogen is not as active. If the plants can get to the three- or four-leaf stage before the pathogen becomes active, then you’ve saved yourself three or four weeks of potential yield loss.

So early seeding won’t eliminate infection, but it could reduce it if you’re growing a susceptible variety on infested soil,” explains Jurke.Sanitizing also helps. “If farmers have clubroot on their farm or it’s in their area, then we encourage them to think about how to sanitize within their farming operation. Perhaps for their high-risk fields, they might sanitize their equipment before leaving the field,” he says. “However, it’s a tough one to do because the cost of sanitation is huge. To properly sanitize a tractor and a cultivator takes a minimum of six hours work, and in that six hours, a farmer could have seeded or sprayed one or two fields.”

In addition, the Canola Council recommends using cleaned and treated seed because the cleaning and treating process eliminates the pathogen on the seed. It also suggests reducing tillage, if possible, to reduce the amount of soil that’s moved around. And it encourages growers to develop a clubroot management plan, so they’ll be ready if they do find the disease.

The Canola Council has also found a few things that don’t work very well, at least in Western Canada’s canola crops. “The fungicides used in vegetable crops are not an effective way of managing clubroot in canola out here. It’s just too costly to apply a $1,200/acre product,” says Jurke.

“Seed treatments are good for reducing the pathogen on the seed, but seed treatments don’t have enough horsepower to control the pathogen that’s already in the soil.

“For the pathotypes that we deal with in Western Canada, liming the soil is not an option, because this pathogen seems to do as well in high pH soil as it does in low pH soil,” he adds.

“And, although boron does a really good job of killing the pathogen, it’s only effective at rates that are toxic to canola plants.”

For now, McDonald’s key tips for Ontario canola growers are to use crop rotation. If the disease does become a problem, use clubroot-resistant varieties.

“Whether growers are in Alberta or Ontario, there are many good reasons to follow good crop rotations, with at least two years out of canola and longer would be better,” she emphasizes. “Even if you don’t have clubroot, a longer rotation increases yields and plant health. And if there is a risk of clubroot [or if you’re using clubroot-resistant varieties], it is really important to practise crop rotation and other methods to keep the pathogen levels low.”

Preventing and managing clubroot in canola
A single infected canola root can produce billions of resting spores. Each resting spore is very tiny – about 0.004 millimetres in diameter. “The resting spore is smaller than particles of soil. Essentially, wherever soil travels, this pathogen has been travelling,” says Clint Jurke with the Canola Council of Canada. “Soil movement is the big culprit in the spread of this disease.”

Given the pathogen’s 20-year survival period, preventing the disease from arriving in a field is very important.
“We’re encouraging growers in Western Canada to do risk assessments of where the pathogen might come onto their land. That might be from purchasing used farm equipment that hasn’t been sanitized, or hiring custom operators that haven’t sanitized their equipment, or purchasing manure, seed potatoes or hay. Anything that could be moving soil to their field is a risk, including recreational vehicles, utilities vehicles, construction equipment and even livestock,” says Jurke.

“After growers have done a risk assessment], they can either restrict that type of risk from happening or they can start sanitizing equipment.”

Sanitizing large field equipment is time consuming. He outlines how it’s done: “If you’re in a known clubroot-infested field, then before you leave that field, you park on the side near the entry point. First you physically remove any soil on the equipment, using a scraper or something to knock off the soil clumps. That will remove 90 to 95 per cent of the soil, so you’re removing 90 to 95 per cent of the risk. If you want to get to 99 per cent risk reduction, then you wash the equipment with a pressure washer. That takes a couple of hours to do as well. If you want to get to 99.999 per cent, then after pressure washing, you can spend a couple of hours bleaching the equipment, but bleach is corrosive and it’s not easy to apply.”

Scouting is a key practice for clubroot prevention and management. The easy way to scout for clubroot is to look for patches of wilted, yellowed and stunted plants. If you find such patches, check a few of the affected plants to see if they have clubbed roots.

But that’s not the best way to scout for the disease. Jurke explains, “When there’s a patch like that, it means the pathogen has probably been in that field for a couple of canola crop rotations. In the meantime the farmer has probably been dragging his equipment over that soil and moving the pathogen around.”

So the Canola Council advises growers to start scouting early – before they have infested patches. “The best places to scout are in a field’s entry and exit points because that’s usually where the disease is introduced first. If canola plants are growing there, look for really small galls on their roots; there won’t be any above-ground wilting at that point,” he says.

“Even better would be to take a few soil samples of the top one or two inches of soil within about 10 or 15 metres of the field entry point. You can send the samples to a lab for a polymerase chain reaction (PCR) test to see if the pathogen’s DNA is present in the soil. It costs a little bit of money to do that, but it gives a more precise indication of whether or not the clubroot pathogen is present.”

October 1, 2013
By Carolyn King


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