High disease risk sees many Manitoba canola growers spraying fungicide

A wet spring creates canola fields full of sclerotinia as flowering blooms

This season Manitoba canola growers shouldn’t be asking if they should be spraying fungicides — they should be asking themselves if there’s any possible reason they shouldn’t.

Clinton Jurke, director of agronomy for the Canola Council of Canada, says it’s been a moist spring, yield looks good in much of the province and dense canopies abound. That’s prime conditions for the development of sclerotinia issues in any canola crop. Junke says it’s a case of taking the bad along with the good.

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“If a farmer likes the way the canola crop is looking, unfortunately so will the disease,” Jurke said in a recent interview.

With higher yield potential comes the elevated crop risk and it also means growers have something to lose, making spraying for sclerotinia almost a default position.

“This year I’m telling growers they should be looking if there might be any reasons not to spray, rather than wondering if they should,” he said.

As the crops flower, they are susceptible to infections, and the moist spring conditions means the pathogen is likely present in the fields, Jurke said.

“It’s only if something suggests there’s a reason not to spray — say for some reason they’ve got yield potential below 30 bu./acre or their canopy has dried right out — that they should consider not making an application,” Jurke said. “Otherwise it really looks like a season where sclerotinia will thrive.”

Sclerotinia has a strong correlation with weather, and in particular wet weather. If a field experiences regular rains, high humidity, or both, in the two weeks prior to flowering and through flowering, the Canola Council says infection “will likely occur.” If the conditions persist after flowering the disease severity will be high and there will be “significant” yield loss.

Flowering is a key time for infections because the tiny wind-borne ascospores that move the infection can’t directly infect a healthy plant. They need dead tissue, such as fallen petals sticking to leaves and stems, to continue their life cycle. The decaying petals give the ascospores energy to produce the compounds such as oxalic acid, which makes infecting the living tissue possible.

Once inside the plant, the fungus grows up and down the stem, eventually cutting off moisture and nutrient flow and killing the plant.

When conducive conditions are present and the yield potential is above the 30 bu./acre mark, a fungicide application should be made at 20 to 30 per cent flowering, unless the control product specifically states on the label that it should be applied closer to 50 per cent flowering, according to the council.

Jurke is also cautioning growers to take steps to ensure resistance to fungicides doesn’t become a problem. He said there are some signs it’s becoming an issue, and rotating to a different type of chemistry will help combat this problem.

“If you’ve been using a single brand for a few seasons now, it’s time to change to something else,” Jurke said.

Like weed resistance, resistant diseases will develop after enough selection pressure of a single chemical group is applied to kill all but the very small proportion of naturally resistant members of the population being treated. Using different chemical groups slows or prevents this from happening.

Canola Council guidelines also state that an earlier application is usually better because early infections on the main stem are what tends to cause the most yield loss. However the organization also adds that if conditions are borderline early in the application window, or it starts to rain, a late application may be a better option.

Timing may be an issue for some parts of the province this year with wet conditions. In recent weeks the Manitoba Agriculture crop report has stated that farmers in a number of regions throughout the province have been struggling to complete field operations in a timely manner because of the wet conditions.

This article first appeared in the July 7, 2016 issue of the Manitoba Co-operator

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