Sclerotinia stem rot remains a challenging disease for canola growers because infection levels can shift dramatically from year to year. Even when recent pressure has been low, favourable moisture conditions can quickly make fields susceptible again. Understanding the environmental factors that drive sclerotinia development — and how fungicide timing plays a role — can help growers make informed decisions to protect their crop.
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.
Questions to Ask Before Spraying 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.
Using the Wet Pants Test
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.
Fungicide Timing
The optimal fungicide application timing to manage sclerotinia in canola is at the 20 to 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.
Why Timing Matters
With all spray applications, timing is critical. If moisture conditions are favourable for disease development, hitting the 20 to 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 two 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.
Evaluating Fungicide Economics
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.
Hybrid Resistance Limitations
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.
FAQ
What environmental conditions increase the risk of sclerotinia stem rot in canola?
Sclerotinia stem rot is more likely to develop in saturated soils and wet crop canopies, especially when there is rainfall or high humidity during the flowering period.
How can farmers decide whether to apply a preventative fungicide for sclerotinia?
Farmers should consider whether the field has been wet before flowering, if rain or humidity is forecasted during flowering and if there is a dense canopy that retains moisture. Answering yes to these questions suggests that a fungicide application is recommended.
What is the ‘wet pants test’ and how does it help manage sclerotinia?
The ‘wet pants test’ involves walking through the canola canopy mid-afternoon on a sunny day. If your pants are wet, it indicates there is enough moisture and humidity present for sclerotinia to develop.
When is the optimal time to apply fungicide to control sclerotinia in canola?
The optimal fungicide application timing is at the 20 to 50 percent bloom range as the fungicide needs to coat the flower petals before they drop and potentially carry spores into the canopy.
How long does fungicide protection against sclerotinia typically last?
Most fungicide products provide over two weeks of protection although this is dependent on environmental conditions.
How can farmers estimate potential yield loss due to sclerotinia infection?
Farmers can divide the percentage of plants infected by two to estimate potential yield loss. For example, if 20 percent of plants are infected there could be a 10 percent yield loss.
Do most canola hybrids offer resistance to sclerotinia stem rot?
Few canola hybrids provide enough resistance to sclerotinia to avoid spraying when disease conditions are ideal so scouting and timely management are important.
