How to manage sclerotinia stem rot in canola

Disease risk comes down to moisture

Sclerotinia stem rot leaf lesion forming from an infected canola petal.

Sclerotinia stem rot is one of the most destructive diseases in canola. The fungus can cause up to 50 per cent yield loss. From year to year, region to region, and field to field, occurrence and severity are extremely variable. Foliar fungicides remain the main source of control. Canola Council of Canada agronomy director, Clint Jurke, explains how to manage sclerotinia stem rot in canola.

Fungicides remain the most effective tool when dealing with sclerotinia, says Jurke. “On average, they reduce the amount of sclerotinia infection by about 80 per cent, and even the very best resistance on the market doesn’t provide that level of control. In high-risk situations, a fungicide application is certainly the best tool in order to manage the disease.”

A high level of disease can lead to 20 to 30 per cent yield loss, and in some cases more. However, the difficulty with applying a fungicide is it only provides economic value in high-risk situations.

“The difficulty is knowing whether or not it is a high-risk situation for the development of the disease,” says Jurke.

In Western Canada, a large number of acres are devoted to canola production each year, as well as other host crops, including pulse crops, sunflower and soybeans. Furthermore, rotations are often tighter than recommendations, giving the pathogen more opportunity to develop.

Sclerotinia stem rot stem lesion. photo: Clint Jurke

New tools

There are new tools available to help growers quantify how much pathogen is present at the time of flowering, including DNA petal tests and spore traps. 20/20 Seedlabs, for instance, developed the Spornado, a simple spore catcher that traps airborne spores on a specialized filter cassette in the field. Currently, Spornado focuses on sclerotina stem rot and Fusarium graminearum.

In reality, it’s the weather that determines if the disease will become a big problem; in particular, how much rain falls prior to flowering, and how much rain falls during flowering.

“If we could have a really good weather prediction system that would tell us how much rainfall is going to happen, then we would know even more specifically how much risk is there,” says Jurke.

Since weather prediction isn’t perfect, farmers are left to make their best guesses. It really comes down to moisture and how much moisture was in the crop in the two to three weeks leading up to the flowering period, and how wet and humid the canopy is during the flowering period, he says.

What makes managing sclerotinia stem rot even more challenging is the window to respond is fairly narrow. Most fungicides need to be applied at the 20 to 50 per cent bloom stage, and the majority target 30 per cent bloom.

Sclerotinia stem rot-infected stem with newly formed sclerotia inside. photo: Clint Jurke

“It’s only a matter of days for the crop to be in that stage,” says Jurke. “A grower needs to be scouting the fields right at bloom stage to make their estimation as to how wet it has been prior to that period, and what the forecast is going to be for the next couple of weeks.”

The other factor that’s important to consider is the actual yield potential of the crop. If the crop is not good, the cost of application is not justified. Crops with higher yield potential in the 50 to 60 bushel per acre range warrant protection.

New varieties offer tolerance

Currently, there are a couple of varieties (45CS40 and D3154S) that are considered sclerotinia stem rot tolerant. Jurke refers to them as “partially resistant” because they don’t provide complete control of the disease.

“The pathogen still infects the plants but what happens is the infection just moves slowly through these varieties compared to other ones.”

Using a tolerant variety can reduce disease impact by an estimated 60 per cent. “But that level of control is not as good as a fungicide,” he adds.

A close-up of a sclerotinia stem rot leaf lesion. photo: Clint Jurke

Jurke says there’s much excitement about new germplasm that’s currently under development. In Asia, new resistance genes have been identified, and are currently being backcrossed and bred into current resistant varieties. However, it will be five to seven years before those varieties come to market.

“But these look like truly resistant varieties, like they could potentially be replacing fungicide applications,” says Jurke. “When those hit the market, that will be pretty awesome.”

About the author

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Melanie Epp

Melanie Epp is a freelance farm writer.

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