Moisture boosts risk of sclerotinia stem rot: Canola Council

Moisture in the canola canopy — before, during and after flowering — promotes the sclerotinia cycle that can produce severe sclerotinia stem rot infection in crops.

Sclerotinia stem rot is the most common and costly disease of canola on the Canadian Prairies, and its severity is directly related to moisture. If a field has regular rains or high humidity or both, from two weeks before flowering and through flowering, then infection will occur.

“If rain or humidity continues after flowering, disease severity will increase and yield loss can be significant,” says Clint Jurke, agronomy specialist with the Canola Council of Canada at Lloydminster, Sask. “The farm’s location and a field’s crop rotation history don’t matter so much anymore. Sclerotinia stem rot can strike anywhere.”

With early fields coming into flower across the Prairies, the management season for sclerotinia has begun. To scout, growers and agronomists could look for tiny mushroom-like apothecia in fields that were seeded to canola last year, and have petals tested for sclerotinia ascospores.

“But the more important scouting steps are to look at the yield potential of the crop and check the current moisture situation for each field,” Jurke says. “A crop with a dense canopy and good yield potential, good soil moisture and lots of morning dew will likely provide a return from a fungicide application — as long as conditions stay moist.”

If conditions turn hot and dry after flowering, sclerotinia infection may not be as severe as it would be if conditions stay humid and moist inside the canopy. Unfortunately, rainfall over the two weeks after flowering is hard to predict, so the decision has to be based on current conditions.

Fungicide application timing for sclerotinia stem rot management is between 20 and 50 per cent bloom. This timing is important. For fungicide to be effective, it has to land on petals before they fall into the canopy.

Sclerotinia ascospores cannot infect plants directly. They need dead tissue, such as fallen petals adhering to leaves and stems, to continue their cycle. Decaying petals give ascospores the energy to infect the plant. Once inside the plant, the fungus grows up and down the stem, eventually cutting off moisture and nutrient flow and killing the plant.

“Remember that fungicides will reduce the severity of infection but will not eliminate sclerotinia completely, especially if conditions are favorable all through flowering,” Jurke says.

“Current sclerotinia tolerant varieties may reduce infection level, but with extended moisture through the sclerotinia risk period, fungicide will often provide a return on investment even when these varieties are grown,” he adds.

Canola can reach 20 per cent flower four to five days after first flower — so prepare to assess the sclerotinia stem rot risk as soon as flowering starts. — Canola Council of Canada

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