Wind seems to be the clubroot culprit in Manitoba

Researcher urges producers to watch for early infestations and get on them quickly

Clubroot galls are the sure sign the disease is present, but dead patches in the field should also be easy to spot at swathing time. 
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It looks like clubroot is moving a bit differently on the east side of the Prairies.

In Alberta, clubroot is showing up a lot at field entrances (in 90 per cent of the infected fields, according to one study), which suggests equipment is the main culprit in its spread.

It’s a similar pattern in Saskatchewan — field entrances and in areas where water pools temporarily around grain bins and also access routes to power lines or oil sites.

But in Manitoba, clubroot is showing up in low spots or along drainages in fields, said Bruce Gossen, a research scientist with Agriculture and Agri-Food Canada.

“This probably reflects the fact that, at this point, most of the spores are coming in as airborne spores, and being washed into and concentrated in some areas, where you are getting hot spots of infection,” said Gossen.

As the disease becomes more established in more fields the transmission pattern will likely be more similar to Alberta, when it will be transported on equipment and vehicles moving from field to field.

But in the meanwhile, producers might be able to help slow its spread.

“Most of the strategies for reducing spore concentrations are extremely expensive to do on a field basis, but if you catch it early there’s an opportunity for growers to minimize the movement of soil out of those patches and then fix the patch in place,” said Gossen.

Clubroot is spreading fast in Alberta and Saskatchewan, largely because a heavily infected canola field can produce quadrillions of spores.

He urged the farmers in his audience to remember that no single approach is effective on its own.

“You can’t just use a resistant cultivar and call it a day,” said Gossen.

There are some good resistant canola varieties available that work well until there is high disease pressure which is beginning to break down resistance.

“Breeders are developing new resistant cultivars, but 12 new pathotypes have been identified so far, so it’s hard for the breeders to keep up with the range of pathogens out there,” said Gossen.

Seed treatments are ineffective against clubroot, but there are some alternatives to genetic resistance that look promising, such as liming, fumigation and solarization.

But they are expensive and still at the experimental stage.

Generally, clubroot favours acidic soils (with a pH lower than 6.5). A pH of 7.2 or higher inhibits spore germination and disease development, but not always.

“When you look at it under controlled conditions, the relationship, although strong, in terms of disease is not immutable, so even at a very high pH like eight, when the conditions are good in terms of moisture and temperature, infection still happens at a high rate,” said Gossen.

Experiments have been done in Alberta, with surface applications of hydrated lime to try and raise pH. It does appear to work, but at $1,000 per ton, liming is not cheap and is only feasible for small patches.

Gossen’s research hasn’t found much connection between soil type or organic matter content in terms of clubroot development, but compaction is a big factor.

“Where soil is compacted, moisture stays in the soil surface a lot longer and lets the zoospores swim,” he said. “In a field situation, sandy soil will drain faster and so likely is a little bit less at risk but even in a dry area, a sandy soil can get a three-day rain and you don’t even need three days for this pathogen to get going.”

Top tips for clubroot management

Federal researcher Bruce Gossen has seven tips for dealing with a small patch of clubroot in a field:

  • Identify and mark the infested area with GPS co-ordinates and also mark a bigger area in every direction because it’s not likely the only patch in the field.
  • Don’t dig and transport plants away from the area as this will potentially spread hundreds of billions of spores. Remove plants, getting as much of the root as possible and burn the roots at the spot.
  • Keep out of that field until you have dealt with the patch and avoid spreading it — for example, do not cultivate.
  • Treat with lime — aim for a target pH of 7.5, which is about seven tons of lime per acre.
  • Get a grass growing that has lots of roots to keep the soil in place and is less conducive to the disease. There is some evidence that the roots stimulate germination of resting spores and because of the life cycle on grasses it will bring down the number of resting spores in the area.
  • Leave in grass until the levels are reduced and you have a resistant canola cultivar that you can go back in with. If you don’t use a resistant cultivar, you will end up with more disease.
  • Put in a new field exit and a sanitation zone to clean equipment before leaving the field to protect other fields.

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