Vigilance needed as clubroot overcomes resistant canola varieties

While scientists and plant breeders have been able to slow the advance of clubroot in canola fields across Western Canada, producer vigilance is still the best weapon against the disease as it continues to advance.

“The most problematic issue has been that some of the fields we have been finding in the last few years were planted with clubroot-resistant canola, what we call first-generation clubroot resistance. That’s definitely disconcerting,” said University of Alberta Stephen Strelkov.

Strelkov was speaking to about 280 delegates at the annual Canola Industry Meeting and Canola Innovation Day in early December presented by Ag-West Bio in Saskatoon. The event drew researchers, plant breeders, seed growers and producer groups from Canada and worldwide.

Strelkov said they detected the first strains of clubroot that could overcome canola’s resistance in 2013 and these instances have been spreading. Resistance aside, since the disease was first detected in 12 Canadian canola fields near Edmonton in 2003, it has spread to more than 3,300 fields as of the fall of 2019. It has now appeared in Alberta’s Peace River area, and there are cases in Saskatchewan and Manitoba.

The issue is on producers’ minds in both provinces. Dan Froese of Manitoba Canola Growers reported clubroot-tolerant varieties are proving popular in that province.

“We’ve had five of those varieties that are clubroot tolerant,” he said. “We’re seeing an increase in market share for clubroot-tolerant varieties.”

Carter Peru from Saskatchewan Agriculture said the ministry partnered with SaskCanola to enlist farmers’ help to determine the extent of clubroot in the province, encouraging them to take samples of their soil and send them in for testing. Results of the clubroot survey are being assessed and are expected to be put onto a provincial map to be released in early 2020.

When it comes to clubroot, canola may be a victim of its own success. Producers looking at narrow profit margins can be tempted to shorten rotations from, for example, canola-pulse-cereal to “canola-snow-canola” as one delegate quipped.

“Nearly all these fields where we have these issues tend to be in very short rotations, like one in two,” Strelkov said.

Once in a field, clubroot can become extremely difficult to control. Clubroot spores can persist in the soil for 20 years. Depriving the pathogen of a host ideally means no canola, mustard, or any other brassica crop should be grown.

Crop rotation remains one of the best defences against clubroot. Research shows the heaviest infestations occur near field entry points, so careful and thorough cleaning of equipment is needed when moving between fields. As well, producers should work fields with known infestations last.

Strelkov explained that defensive strategies remain essential because scientists are still struggling to understand the disease. While they know clubroot is caused by Plasmodiophora brassicae, the soil micro-organism comes in many disease-causing varieties, or pathotypes. Originally, they knew of 17 pathotypes, but another 19 have been identified.

“We’ve had a real issue with pathotype identification,” he said. “Unfortunately, we’re not as advanced as the blackleg system. We’re probably 15 to 20 years behind.”

From a practical standpoint, Strelkov said they had no way to tell whether the clubroot strain found in a particular field was the resistance-breaking variety. To make matters more complex, several pathotypes can be found in a single gall (the “club” in clubroot that disrupts a plant’s circulatory system, weakening or killing it).

With so many pathotypes to deal with, Strelkov and his colleagues are focusing their efforts on the most common and virulent strains.

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