EDMONTON – Clubroot continues to spread in north-central Alberta and has now been found in canola fields southeast of Calgary.
While scientists seek genetically resistant lines and new management strategies to minimize producer losses, municipal governments are enforcing a no-canola rule for infected fields.
First discovered in a Sturgeon County farm field near Edmonton in 2003, researchers say clubroot is one of the most serious emerging threats to canola production in Western Canada.
“Mainly because we don’t have any direct methods of control,” Canola Council of Canada agronomist Doug Moisey told producers during Crop Production Week in Saskatoon last month.
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“You can’t spray for it or use soil applied products to get rid of it.”
The fungal disease was first found in Alberta gardens in the 1970s but failed to become a serious dryland crop pest until the past few years.
So far University of Alberta and Alberta Agriculture staff have found clubroot in 171 fields in 12 counties and municipal districts, mainly in the Edmonton area. In 2007, municipal field agronomists identified an additional 79 fields in those counties, bringing the total number of fields to 250, with 137 new cases in 2007.
However, researchers point out that not every field in every infected county has been tested and not every county in Alberta has been inspected, so the increased incidence may also be due to increased surveillance and awareness.
In 2007 the pest, known in scientific language as Plasmodiophora brassicae Woronin, also showed up in southern Alberta, in Newell County.
In half of the fields infected since 2005, researchers have found that the disease is mild to moderate, creating limited amounts of yield or quality reductions. In Quebec, where the disease has a longer history, producers often experience 35 percent yield reductions and 10 percent drops in oil content when clubroot is present in canola fields.
In slightly less than half of the fields containing clubroot, damage was moderate with significant yield reductions.
But in severe infestations, which were found in 10 percent of the fields, crop damage and losses exceeded 80 percent and were total in the most severe cases, said University of Alberta researcher Stephen Strelkov.
“It becomes a management issue once you have it,” Strelkov told producers at the FarmTech 2008 conference in Edmonton Feb. 1.
“You can’t really ever expect to get rid of it, but you can manage the problem and keep losses to a minimum.”
Strelkov has been a lead researcher in Western Canada, observing the spread of the disease and attempting to address methods of control that will limit producer losses.
Moisey said clubroot has become such a significant problem in the Edmonton area primarily because of the region’s crop rotation history.
“Economics of the past few years dictated that canola was one of the only crops you could grow at a profit. For lots of canola growers around Edmonton, this meant a short rotation of canola, snow, canola, snow,” he said.
“Or at best canola, cereal, canola.”
Strelkov said the short rotations created the perfect environment for clubroot.
Longer rotations keep the spores from building up and allow economically viable production even with mild infections.
The organism is an obligate plant parasite, one that requires a living host to grow and reproduce. That susceptible host crop must come from the cabbage family, which unfortunately for canola and mustard growers, includes their crops.
Spores can remain dormant in the soil for up to 28 years, waiting to be triggered by root exudates that leak from canola and other cruciferous species of plants. In this way the susceptible plants are responsible for signaling to the spores that they are present and ready to be infected.
The disease can be spread by wind and water runoff, and researchers are examining accumulated dust at seed cleaning plants and elevators for DNA evidence that spores may also be transferred on the surface of canola and crop residue.
However, it is primarily spread by farm equipment that tracks soil from infected to uninfected fields.
Moisey said agronomists sampling fields repeatedly find the highest concentration of the disease in fields at road approaches. It then fans out from the field entrance, matching the routes that farm equipment and field support vehicles take as they enter.
“Field infection maps confirm this every time,” Strelkov said.
“It’s coming into the fields from dirt attached to farm equipment. Getting that dirt off before it enters the field is key to keeping clubroot from spreading any faster than necessary.”
The canola council is recommending producers create a grassed area at field entrances where equipment can be cleaned.
Soil can be knocked off and swept on the grass. Seed, spray and fertilizer tender trucks can remain on the grassed areas, where they pose limited risk to the rest of the field. Grain trucks can be parked on the grass during harvest, which avoids tracking dirt from field to field when moving grain to bins in other fields.
Ultimately the best way to suppress clubroot spread might be to use small portable water tanks, high pressure washers and possibly bleach in these areas to remove soil and kill spores.
Moisey said identifying a specific area for clean-out is particularly crucial now that so many producers rely on custom operators for at least some of their field operations.
Spores landing on the grass won’t have the root exudates they need to reproduce.
Strelkov said the soil fungicide Terraclor 75 W applied at a rate of more than 85 pounds per acre has improved plant growth and yield.
Soil amendments such as calcium carbonate and wood ash applied at more than three tonnes per acre also provide positive results but have failed to control the fungus.
“It’s very costly, very inefficient and not practical at this point,” Strelkov said. “But it might be possible to provide some control in a small area of a field.”
Clubroot appears to prefer acidic soil but has been repeatedly found to thrive in Alberta soil as high as 7.5 pH.
Strelkov said work this winter has found four main genetic strains of the fungus in Alberta of the nine that occur worldwide.
“Luckily the main ones, we have some brassica with resistant genes and are working with those,” he said.
Researchers at the U of A and Agriculture Canada in Saskatoon and Beaverlodge, Alta., are adding resistant genes to spring canola germplasm, crossing resistant winter canola and rutabaga with spring canola and attempting to resynthesize Brassica napus from its resistant Brassica oleracea and rapa parents.
However, Strelkov said clubroot-resistant canola won’t stop the disease’s spread. As a result, management will always be key to a successful control program.
“They have it in Europe and Sweden and they work with it,” he said.
Moisey said municipal law prohibits producers from growing canola in a field for four years after clubroot is found.
He said the best way to avoid a problem is to stick to rotations that only plant canola in a field once every three or preferably four years.
