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Sclerotinia Stem Rot in Canola

Should You Spray for Sclerotinia? #

Hot, dry conditions across the prairies over the last few years have decreased the number of fields with sclerotinia. But low disease pressure last year doesn’t mean it won’t be an issue this year. This broadleaf plant disease relies heavily on the environment and thrives in saturated soils and wet crop canopies.

A foliar fungicide application is the best option to minimize disease pressure in canola, but the decision to spray is one of the toughest to make. Justine Cornelsen, Agronomic & Regulatory Services Manager, explains that several questions need to be answered before you decide to spray a preventative fungicide:

  • Has the field been wet prior to flowering?
  • Does the forecast have rain or humidity during the flowering window?
  • Is there a dense canola canopy that locks moisture in?

If you answered “yes” to these questions, the field is likely at risk for the disease, and a fungicide application would be recommended. 

Cornelsen explains that the “wet pants test” is one technique to determine if the conditions are right for disease development. “Walk through the canola canopy mid-afternoon on a sunny day. If your pants are wet, this indicates that the crop has enough moisture and humidity within for sclerotinia to grow and spread.” This test may not be scientific but helps determine if the environment for apothecia development and spore release is there. Commercial lab tests are available that determine if spores are moving through the canopy.

The optimal fungicide application timing to manage sclerotinia in canola is at the 20-50% bloom range. “With this fungicide application being a preventative measure, the fungicide must coat the flower petals prior to petal drop,” Cornelsen says. The infection from sclerotinia is caused by petals carrying spores falling into the canopy and landing on branches below. The sclerotinia fungi use the petals as a food source and start to infect further plant tissues.

Sclerotinia Stem Rot in Canola
Graphic Source: Canola Encyclopedia, Canola Council of Canada

With all spray applications, timing is critical. If moisture conditions are favourable for disease development, hitting the 20-50% flowering window with a fungicide is ideal. Follow fungicide label rates and use high water volumes to penetrate the crop canopy. Most fungicide products provide protection that lasts over 2 weeks (dependent on environmental conditions). If humidity or rainfall continues throughout the flowering period, a second application at 50% flowering may be required. If crop staging is variable, split applications might help to coat more flower petals.

Running the economics behind a fungicide application might help make the spray decision easier. The rough yield loss calculation for sclerotinia is the number of plants infected, divided by 2, which equals the potential yield loss. 

Example: If 20% of the plants are infected with sclerotinia, there is a potential of 10% yield loss.

Few canola hybrids offer sufficient resistance to sclerotinia infection to avoid spraying a fungicide when disease conditions are ideal. Getting into your canola fields to scout and examine conditions is a small time commitment with huge yield-saving implications.

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