Conditions favour sclerotinia spread

Wet weather across much of the Prairies could create ideal conditions for the spread of sclerotinia, a fungal disease that affects canola and pulse crops and thrives under moist, humid conditions.

With frequent rain across much of the West and ample soil moisture, conditions are favourable for the development of spore producing apothecia, small mushroom-like fungi that signal the potential for the yield-robbing infection.

Abundant soil moisture before and during flowering are key factors in determining sclerotinia risk.

An unusually wet June combined with intermittent thunderstorms throughout early July could result in higher-than-normal infection rates.

“It’s a really weather dependent disease,” said Faye Dokken Bouchard, plant disease expert with Saskatchewan Agriculture.

“We almost always see a little bit of it but definitely in hotter, drier years we see a lot less of it than we do in wet years.”

According to Dokken Bouchard, drying weather over the next week or two could limit infection rates and reduce the need for fungicide applications.

A dry soil surface and a moisture-free environment on the lower leaves and stems of canola plants should eliminate the risk of infection.

To infect plants, sclerotia bodies must have enough moisture to produce apothecia, small, tan-coloured growths shaped like golf tees.

Wet conditions in the two weeks leading up to flowering are usually conducive to apothecia growth and spore production.

Later, when canola plants begin flowering and dropping petals, wet conditions on the lower canopy will allow the falling petals to stick to the damp lower leaves and stems of the canola plant.

Those decaying petals, combined with the presence of spore producing apothecia and wet humid conditions in the plant canopy, create an ideal environment for sclerotinia infection.

According to Derwyn Hammond, resource manager with the Canola Council of Canada, farmers in high risk areas should consider fungicide treatment if the lower leaves in flowering canola crop remain wet for most of the day.

“I heard a really good analogy last week … that if (the legs of) your pants are wet at 9 a.m. when you’re walking through your canola, there’s probably some risk of infection,” said Hammond.

“But if they’re still wet in the middle of the afternoon, then you’re almost certainly at higher risk of sclerotinia infection occurring because that means that those lower leaves are staying wet throughout most of the day….”

In addition to weather, production history is important when assessing sclerotinia risk.

Prairie producers, particularly those in parts of Manitoba and eastern Saskatchewan, are coming off two wet production years where sclerotinia infection was heavier than normal.

In many regions, infection rates were very high in 2010 and above normal again in 2011.

“We probably have had enough moisture in some areas so … it’s important for people to get out there and scout…,” Dokken Bouchard said.

According to Hammond, producers should assess soil moisture conditions before plants flower, monitor the ground for apothecia, and determine whether canopy conditions are conducive to infection after the crop begins to flower.

“You’re not really looking for the disease at this point, you’re just looking to assess your crop for the risk of infection at a field level,” Hammond said.

“The thing that you can scout for is the presence of those apothecia….”

A field that has not produced canola for five or more years is much less likely to become heavily infected than a field that has produced canola in the past year or two.

Calculating an economic threshold for fungicide application can also be a challenge.

But with canola prices at their current levels, the economic threshold for applying fungicides will be relatively low this year, added Hammond.

A potential yield loss of one or two bushels per acre might be enough to warrant a fungicide application.

In heavily infected canola crops, sclerotinia can easily reduce yield potential by 10 to 20 percent, he added.

If growers are uncertain about potential yield losses, they can consider split or half-rate fungicide applications, where fungicide is applied in two passes at a lower application rate

According to Hammond, split rate applications can be effective under the right circumstances:

  • the risk of infection is only moderate at the early flowering stage and growers want the chance to spray again if the risk of infection increases as the crop approaches 50 percent bloom
  • if crop development is uneven and the flowering period is likely to last longer than usual.

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