The discovery of low levels of clubroot DNA in two unrelated Manitoba fields is a wake-up call for the province’s canola producers.
The good news is none of the canola in those fields showed any symptoms of the disease that can cut yields in half or more. As a result Manitoba is still considered “clubroot free,” said Holly Derksen, a plant pathologist with Manitoba Agriculture, Food and Rural Initiatives (MAFRI).
But officials say farmers need to prepare for the likelihood that club-root (Plasmodiophora brassicai), a soil-based pathogen, will eventually infect Manitoba canola crops.
Prevention is the best way to manage clubroot, known as a protist — an organism with plant, animal, and fungal characteristics. Since clubroot moves with soil, equipment from infected areas should be cleaned.
Clubroot spores can survive in the soil up to 20 years. Once a field is infected the pathogen can’t be eradicated.
While tight canola rotations do not cause clubroot, if the disease is already present, it accelerates its buildup, which makes it harder to manage, Derksen said.
“It may be present in fields but undetected and so without knowing it, it could be building up,” she said.
First infections often show up at the entrance to a field. In another season there may be an infected patch and then the next time canola is seeded the entire field might be hit.
Data from the Manitoba Agricultural Service Corporation (MASC) shows Manitoba canola growers have been tightening their canola rotations. In 2000, about a third of Manitoba canola fields were seeded back to back to canola or had just a one-year break. By 2010 almost two-thirds of canola fields had either no break or just one year off from canola.
Clubroot, as its name implies, produces galls on canola plant roots, resulting in wilting and premature ripening, said Derwyn Hammond, resource manager with the Canola Council of Canada.
The council’s website says yield losses are about half of the percentage of infected stems. If almost 100 per cent of plants are infected, expect yields to be cut in half.
“Severe field infestations in Alberta have caused total yield loss — not worth harvesting — in a few cases.”
When it comes to tight canola rotations, Hammond says the bigger immediate threat to Manitoba canola is blackleg, a fungal disease that can also cut canola yields.
Clubroot was first discovered near Edmonton in 2003 and has since been found in more than 600 fields in that province, some as far south as Lethbridge. Alberta was caught unawares and as a result, infection levels were high in many fields.
In 2008, soil samples from a field in west-central Saskatchewan tested positive for clubroot, Derksen said. No symptoms were observed in the field, but a plant bioassay conducted under greenhouse conditions showed low severity clubroot symptoms. However, last year two cases of clubroot were found in north-central Saskatchewan.
Minor clubroot symptoms were observed in a Manitoba canola nursery in 2005, but it hasn’t turned up in the soil or plants since.
“I don’t think Manitoba farmers should overreact,” Hammond said. “Technically we’re still negative for clubroot. Odds are we’ll eventually get it here. Sanitation and scouting will be the key things.”
Cleaning equipment isn’t something busy farmers like to do, but it should be done when bringing in machines from Alberta, Hammond said.
Using a wire brush and/or compressed air will remove approximately 90 per cent of the soil from equipment, Derksen said. Following that with a pressure washer will remove about 99 per cent of the soil. To do an even better job follow that by misting equipment with a weak disinfectant such as a one per cent bleach solution.
Farmers should scout their canola fields for wilted or prematurely ripening plants, Hammond said. Don’t assume those symptoms are from sclerotinia or blackleg. Examine the roots of suspicious plants. If clubroot is suspected send the plants to MAFRI for testing.
Once a field is infected longer rotations and growing clubroot-resistant canola varieties can help keep the disease in check, Hammond said. Longer rotation is important for preventing clubroot from overcoming the resistant varieties, he said.
Planting clubroot-resistant varieties in uninfected fields will not undermine resistance since the crop isn’t exposed to the pathogen. If a field becomes infected growing a resistant variety will prevent the infection from spreading. The more infected plants there are, the more spores produced making management tougher.
The soil samples that contained the clubroot DNA were collected and tested through the Manitoba Canola Disease Survey, funded by the Manitoba Canola Growers Association, MAFRI said in a news release. The last three years 209 soil samples were collected from across the province.
The 2012 disease survey will include 150 to 175 fields with 70 to 90 fields having soil samples collected and analyzed for clubroot DNA.
The fields found to have clubroot DNA in 2011 will be monitored and tested.
Clubroot prefers warm, moist soils and is generally associated with more acidic soils, but has been identified in soils that range from pH 4.5 to 8.1. Derksen said she doesn’t know the pH of the fields where the clubroot DNA was found.