By Carolyn King
Last year, Ontario had its first-ever detection of clubroot symptoms in canola. On the heels of that discovery came an even more unsettling surprise – a survey found the pathogen scattered across the province’s main canola-growing areas and this year, the symptoms are showing up in more fields.
Now that clubroot has been confirmed, canola specialists advise growers to take steps to stop the spread of the pathogen. The disease is manageable, but if growers wait too long to act, spore numbers can skyrocket and the disease will be much harder to deal with, as Alberta growers have found.
The clubroot pathogen, Plasmodiophora brassicae, infects Brassica species including crops such as canola, mustard, cabbage and cauliflower. The pathogen lives in the soil as microscopic resting spores. The roots of Brassica plants secrete substances that stimulate the resting spores to germinate and transform into zoospores. The zoospores swim through the soil water to the plant’s root hairs and begin the infection process.
The disease causes galls to form on the plant’s roots, preventing water and nutrients from moving up into the rest of the plant, so it withers and dies prematurely. With susceptible canola cultivars, yield loss can reach 100 per cent in severely infested fields.
“The galls on a single infected root can produce billions of resting spores. One diseased plant can infect a large area when spread around,” notes Dan Orchard, agronomy specialist for north central Alberta with the Canola Council of Canada (CCC).
Clubroot is spread through the movement of infested soil, mainly by soil clinging to field equipment. As a result, the disease almost always starts at a field’s entrance. But, clubroot can also be spread with contaminated soil on vehicles, tools, boots or animals, and soil moved by water or wind erosion.
The pathogen has been in Ontario since at least the 1960s, but the province’s main strain, called pathotype 6, prefers cruciferous vegetable hosts so the disease hasn’t been a problem in canola until now.
“In Ontario canola, clubroot was first found in a field in the West Nipissing area in 2016,” says Meghan Moran, canola and edible bean specialist with the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA).
After that initial discovery, OMAFRA surveyed for clubroot in many of the province’s canola-growing regions. “We collected soil samples from 96 fields. The pathogen was detected in samples from 11 fields in the areas of Temiskaming Shores/New Liskeard, West Nipissing, Bruce Peninsula and Dufferin County. So we’re finding it across Ontario,” Moran says.
This year, OMAFRA sampled 25 fields in areas not covered by the 2016 survey and didn’t detect the pathogen in any of those soil samples. Moran notes, “We sampled in the Peterborough area and other parts of eastern Ontario, as well as Huron County and Wellington County. They are not big canola-growing regions, so it’s not a surprise that we didn’t find any clubroot.”
However, the problem definitely hasn’t gone away. “We’ve had growers this growing season say they have clubroot in their canola crop and are going to have yield loss. A lot of those farmers who are seeing symptoms in their canola this year are farmers who had a positive test last year in a different field,” she says. “One of the differences from last year is that 2017 has been a really wet year with a wet spring. In wet conditions, the zoospores can move more easily in the soil so they can infect earlier and to a greater degree. I suspect that is one of the reasons why we are seeing more clubroot this year.”
The University of Guelph is determining the clubroot pathotypes of selected samples from 2016 and 2017. The field where the initial discovery was made has pathotype 2. That pathotype has been found in Ontario in the past, and it is one of the main pathotypes in Quebec. Clubroot pathotyping is a lengthy, labour-intensive process so it will take time to get the results for the rest of the samples.
Why you need to catch it early
Clubroot symptoms in a few canola fields may not sound like a serious problem. But the experience in central Alberta – the epicentre of the clubroot infestation in that province – underlines that taking action early could save Ontario growers a world of worries.
“Clubroot was first found in 2003 in a handful of fields in central Alberta,” Orchard says. “In 2004, an intense survey in that general area showed nothing. At the time, we thought maybe there were only a handful of fields. So we still weren’t doing anything about the disease except alerting people that it was there. But actually it was the very dry conditions in 2004 that made the disease impossible to find. Since then, more fields with clubroot have been found every year.”
These days the disease is widespread in most central Alberta counties. In the surrounding “fringe” counties, new infestations are discovered each year. Surveys in 2016 detected 289 new clubroot-infested fields, making a total of 2,443 infested fields found since 2003.
One of the contributing factors in this almost relentless spread is that some soil infestation levels are extremely high. “It’s hard to fathom that one gram of soil, which could be the size of a loonie, could contain a billion clubroot spores, but that has been tracked. It is common to have millions of spores in a gram. That amount of spores can infect many, many plants,” Orchard says.
In 2017, a field in one of the fringe counties forcefully demonstrated that clubroot can take hold shockingly fast. “This county just recently discovered that it has clubroot; only four or five fields had been found. Many farmers weren’t really growing resistant varieties because they thought the disease wasn’t there yet,” Orchard says. “However, the county does a clubroot survey every year, checking every single canola field in the county. They go to the field entrances, which are the hot spots for infestation, and sample by pulling up canola plants and looking for clubbed roots.”
The county’s clubroot inspector discovered an infestation in a field that had never had canola before. “The field had been in hayland and was broken two years ago. The first crop was barley and this year was canola, and there was an alarming amount of clubroot,” Orchard says. “Before we saw this field, we didn’t realize the pathogen could get established so quickly.”
It turned out that the grower had an unnoticed, seriously infested patch in another field. “The heavily infested patch is small, about the size of a double garage or a little bigger,” Orchard explains. “It’s a low spot that is compacted and wet all the time, where nothing much would be expected to grow anyway. In that patch, there wasn’t a canola plant alive, and the clubbed roots were sticking out of the ground. The patch is right at the field’s entrance, so every time equipment left the field it went through that patch and moved the infested soil to another field.”
He notes, “If the farmer had caught that really infested patch earlier, he certainly would not have spread the disease like this. But he caught it early in the new field – there were no above ground symptoms and likely no yield loss. There were just some galls below ground that you would only find by randomly pulling up plants.”
To get rid of the heavily infested patch, the farmer is limiting the patch and converting it to a perennial grass area where no traffic will go for four or five years. “That is a small price to pay to keep the disease from spreading further,” Orchard says.
For Orchard, this situation emphasized another reason why clubroot has been such a problem: people haven’t been scouting early enough. “We were told to look for clubroot when you see a dead patch in your field. But we now know that by the time you see a dead patch, the disease is already across your farm, not just in that patch. So, people have been wrongly assuming the disease isn’t on their farm yet, and they are moving infested soil on their equipment, they are not growing resistant varieties, and the disease continues to spread.”
He adds, “If you have a small diseased patch in one year and then grow a susceptible cultivar, that patch will become 50 to 100 times bigger. Then with the next susceptible crop, high disease pressure could be across the field, resulting in a major yield loss.”
Since clubroot-resistant canola varieties became available in Alberta in 2009-10, they have been the key tool for managing the disease. These varieties have been grown repeatedly, usually in short rotations. Also, Orchard notes, “Until recently, all the resistant varieties had the same major ‘Mendel’ resistance gene in them.” As well, the resistant varieties were often grown in fields with high spore loads, increasing the likelihood that the pathogen’s population would include some variants that could defeat the cultivar’s resistance.
All these factors have combined to enable the pathogen to overcome cultivar resistance. Eleven new clubroot strains that are not controlled by the current resistant cultivars have emerged in Alberta since 2013-14, creating new problems for growers.
Prevention and management
Both Moran and Orchard emphasize that you can continue to grow canola if you have clubroot, but you need to take the disease seriously and prevent spore loads from escalating. Keeping spore counts low will minimize yield losses and will make clubroot easier to manage.
1. Know what to look for and start scouting now
Start scouting as soon as you hear that clubroot is in your area. Don’t wait until you see dead patches in your fields.
Ideally, all Ontario canola growers should already be on the lookout for the disease. Moran says, “I would love to know that all canola growers are checking plants in their canola fields; that is an excellent way to stay on top of clubroot. The best place to look is at the field entrance. Just pull up plants, especially if they are maturing early, and look for fattened, galled roots on the plants.”
The clubbed roots are an unmistakable symptom of the disease. The aboveground symptoms – yellowing, wilting, stunting, premature ripening and death – are easy to confuse with problems like drought, nutrient deficiencies or heat stress. Also, mild clubroot infections may not produce any above ground symptoms at all, but their roots will have some clubs – and those clubs will add to the field’s spore load.
2. Use a long crop rotation
“The best defence against clubroot is long rotations,” Moran says. “Grow canola once every four or five years to prevent spore build-up. After about three years, 95 per cent of clubroot resting spores will [not] be viable, which is great if you have a low spore counts in your fields. But if you have really high spore counts, rotation isn’t as effective because five per cent of a billion is still a really high spore count.”
3. Grow resistant varieties
Don’t wait until you have diseased patches and yield losses before growing clubroot-resistant varieties. Start growing them as soon as the disease is found on your farm or in your area.
“We are in a position here in Ontario where spore counts should be fairly low because it’s early in this disease issue. If growers use resistant varieties now they can drive down the spore counts, ” Moran emphasizes.
“If clubroot is in your community, treat your fields as if you have it, and grow resistant varieties,” Orchard advises. “If you’re a little complacent and grow a susceptible variety one time too many, your spore load can become astronomical and you’ll have huge yield losses.”
He notes that many of the clubroot-resistant varieties on the market are resistant to pathotype 2 as well as the other common Canadian pathotypes (3, 5, 6 and 8). He adds, “Some of the best canola varieties are the clubroot-resistant ones, so it’s not a sacrifice to grow a resistant one.”
“The seed for clubroot-resistant varieties is typically the same price as non-resistant varieties,” Moran notes. “Right now, each of the companies that sells canola seed in Ontario has one or two resistant varieties for purchase here. As demand increases in Ontario, we will probably have access to more choices.”
4. If possible, rotate resistance genetics
Resistant varieties will last longer if growers use long crop rotations, keep spore loads low, and, if possible, rotate resistance genetics.
Orchard notes, “A lot of dollars are being invested in research on new clubroot-resistance genetics that would be uniquely different from our current genetics, which would allow growers to rotate resistance genetics.”
5. Limit the spread of infested soil
Do as much as you can to reduce the spread of infested soil. Moran advises, “Work clubroot-infested fields last, so you’re not moving infested soil from one field to the next.”
It can also help to minimize tillage and other field operations in clubroot fields. As well, consider converting heavily invested patches to alfalfa or grass for at least four or five years.
Equipment sanitation is important, but it’s more practical if you are also taking other steps to keep spore loads low. “If a field is only lightly infested, then you’d need a big lump of soil the size of your head to move a million or a billion spores from that field. So at least knock off the large, loose dirt before leaving each field,” Orchard says. “With heavily infested soils you would have millions of spores in just one gram. It’s not practical for a farmer to clean every gram of soil off his equipment before leaving each field.” Such thorough cleaning involves not only removing all the loose soil, but also using a pressure washer to remove all the clinging soil and crop debris from everywhere, and then using a disinfectant.
Moran also reminds people like custom operators and agronomists to take precautions when going from field to field.
“Ontario canola growers have a significant opportunity to not let clubroot impact their farms,” Orchard says. “But if they don’t get on board then it will impact their farm, and it can happen in a hurry.”
Moran agrees, “Now is the time to take action and try to drive spore counts down and be on top of the issue. Scout for the disease, maintain long crop rotations, and start using resistant varieties.”