Clubroot gets aggressive

September 6, 2016

by Jay Whetter, Canola Watch Editor

Moist conditions in 2016 may have helped to fuel an aggressive advancement of clubroot pathotypes, many of which can quickly overcome current clubroot resistance traits. Check dead, thin and prematurely ripening patches for clubroot galls.

Dan Orchard already knew seeding and tillage equipment did a great job moving clubroot spores but the scene in one Alberta field this summer made it crystal clear. Looking over a low area in a field, the Canola Council of Canada agronomy specialist for North Central Alberta could see distinct lines of dead plants coming out of a low spot in alternating directions. The lines were exactly the width of the grower’s drill.

The patch would have likely been evident in the low spot the last time canola was grown on that field — two years ago. And when the grower seeded the field this spring, the drill picked up the spore-infested soil and dragged it, the first pass going north, the second south, and so on. By harvest time, the pattern of death was a perfect match.

“The grower told me that a seeing-eye dog could spot that clubroot patch,” Orchard says. “Every plant was affected in the draw, with little or no seed produced.”

How could the grower not notice the patch two years ago? Perhaps conditions were far less conducive for the disease to appear, or maybe the spore load in the soil was considerably lower, or maybe he wasn’t looking for it because he wasn’t expecting a problem. “He knew he had clubroot, which is why he started growing a clubroot-resistant variety,” Orchard says. He thought it would be fine — at least for a few years — but the speed at which new clubroot pathotypes appear can be surprising.

Stephen Strelkov, clubroot researcher at the University of Alberta, has been finding more virulent pathotypes over the past three years in central Alberta. As of now, he has identified more than 10. Clubroot resistant varieties do not have resistance to all the pathotypes.

Pathotypes that infect clubroot-resistant (CR) varieties increase quickly due to sheer numbers and selection pressure. Fields in central Alberta can have 10 million clubroot spores or more per gram of soil. If just one per cent of these soil-borne resting spores are virulent to CR varieties, that could mean 100,000 virulent spores per gram of soil. While the CR variety may protect against most spores, the virulent spores are still enough to cause significant infection and spore build up. After one season, the patch becomes infested with predominantly virulent spores and the CR variety is no longer effective on that field.

“If the grower uses the same clubroot resistance on that same field, the yield effect from that virulent pathotype can be devastating,” Strelkov says.

“We can’t downplay how bad this can be,” Orchard adds.

Management plan

Crop rotation. Three or four years can make a difference in Canada. “After a break from canola for two years or more, most of the spores will not be viable. Recent research suggests more than 90% of the spores will no longer be capable of infecting plants. This doesn’t eliminate the threat but greatly reduces the likelihood of severe yield loss,” Orchard says. “A two-year rotation in a clubroot-infested area seems to be the recipe for clubroot resistance to be overcome quite quickly. It’s nature at work.”

Reduce soil movement. A typical drill or tillage implement has many kilograms of soil clinging to its openers, discs or shanks. If one gram can have millions of spores, any field work that moves soil is also moving clubroot — and lots of it. Use tillage only when necessary and clean off as much soil as possible when moving any machinery from field to field. Reducing tillage to retain more surface residue will decrease wind erosion and infestation of other fields downwind. In wet harvest conditions, cleaning off combine, cart and truck tires between fields may be required to reduce soil tracking. Remember, clubroot spores can be present in all soil, not just fields seeded to canola this year. Take care to reduce soil movement in all fields.

Keep looking. Don’t assume those dead patches in low lying areas were just drowned out. Identifying clubroot patches early will mean growers can implement rotation and other management measures to contain the patch and minimize the economic impact from clubroot. “County fieldmen in Alberta are concerned and becoming very educated and willing to work with the growers,” Orchard says.

Variety selection. Although most current varieties use the same mechanism (genetics) for clubroot resistance, a few new options have been introduced recently and much work is being done to introduce new genetics. “Not over-relying on our current genetic sources is very important to preserve them,” Orchards says, “so rotating out of canola for an extra year or two will help this.”

Set aside the worst patches. Putting severely infested patches into forages for a few years might be the best choice to stop spread of spores through field work. Make the patch 50% larger in area than the infested patch to contain fringes that may have elevated spore levels. In that time, research on management practices, soil amendments, and new genetics may come along that provides an economic management option for these areas.

For more on clubroot management, go to the canolaencyclopedia.ca and look for the clubroot chapter in the Diseases section or visit clubroot.ca.

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