Sclerotinia stem rot and hot weather

July 12, 2017 - Issue 16

Hot, dry weather may have changed the sclerotinia stem rot risk scenario for many canola growers.

Hot weather during flowering can increase flower and pod abortion and reduce overall yield potential. Return on investment from a fungicide applications is more likely if yield potential is at least 30 bu./ac., with ROI going up as yields increase beyond that threshold.

Hot, dry weather during pod fill and ripening also reduces the progression of sclerotinia within the plant.

Heat and drought can lead to missing pods, and also greatly reduce the level of sclerotinia stem rot infection. Credit: Nicole Philp

Sclerotinia stem rot needs a moist environment. Higher yields usually coincide with rainfall and dense humid canopies that lead to sclerotinia infection. This combination greatly improves the fungicide ROI.

If a field has regular rains or high humidity or both from two to three weeks before flowering and through flowering, this will get spore-releasing apothecia going in time to be active during flowering. If moist conditions continue after flowering, severity of the disease could be high and yield loss significant. Morning dew is a good indicator of ample humidity. Walk through the field in the morning. If your pants get wet, there’s enough moisture for sclerotinia.

Even on hot days, with high humidity in canola canopy, it may be worthwhile spraying. The canopy, if thick, can have its own more-moist micro-climate. Also keep in mind that even though the sclerotinia fungus does not like to grow over temperatures of 30°C, lower night temperatures will allow for fungal growth.

With these conditions and average or better yield potential, a fungicide application at 20-30% flower is warranted. When considering fungicide cost ($14 to $25 per acre, depending on the product) and application ($7 per acre for custom ground, $11 for aerial), a fungicide would have to preserve around 3 bu./ac. of yield to break even. This should be easily achieved in a high-risk, high-yield field.

Spray performance in hot weather. Note that hot conditions can reduce fungicide performance by increasing evaporation of the spray droplets. Some companies say to avoid spraying in temperatures above 25-27°C, but that may not be necessary for all fungicides. Check with the supplier. A tip is to keep the water volumes at a minimum of 10 gallons per acre and droplets in the coarse range. This will help to reduce rapid droplet evaporation while still maintaining very good target coverage.

Late-window scenario. If conditions were hot and dry before flowering, then turn moist during flowering, the economic return from fungicide could be higher with a late-window application (around 50% flower). This would be especially true if moisture also promoted a longer flowering window due to later compensatory growth. With lower seeding rates resulting in more branching, canola fields can be at 50% flower for a week or more. Keep in mind however that infection to side branches tends to have a lower overall effect on yield than infection on main stems, and that late-window applications for some products may be close to their pre-harvest intervals.

Base the spray decision on this year’s conditions, not last year’s experience. A lot of growers felt burned by sclerotinia last year, realizing too late that they probably should have sprayed. The 2016 experience may give some a more sensitive trigger finger this year. Use last year’s experience as a guide, but make this year’s decision based on this year’s conditions. Last year at this time conditions in many areas were wet during flowering and yield potential was high — prime conditions for sclerotinia stem rot and good return on investment from fungicide.

Further reading:

Use 50-mesh screens for Proline
Sclerotinia risk assessment tool
NDSU sclerotinia risk map
Canola Encyclopedia sclerotinia stem rot
Sclerotinia management: Common questions

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