Questions of the Week

How does hot weather affect fungicide performance? 

Fungicide labels do not have temperature restrictions. Fungicide companies will say to avoid spraying in temperatures over 27°C or 28°C. On hot days, the best time to spray is in the morning or evening, before or after the temperature peaks. However, it often comes down to when the aerial applicator can squeeze in the job. When risk assessment indicates a need to spray, timely application with reduced efficacy is better than late-window (or no) application.

Heat presents three risk factors for fungicide performance:

• Hot weather can result in rapid plant growth and a thin cuticle. This can lead to potential crop burning from the spray.
• Oil-based adjuvants can increase heat damage to petals, causing them to abort.
• Rapid vaporization of spray droplets can reduce coverage.

Delta T. The temperature recommendation is related to Delta T, or “wet bulb depression,” which accounts for the evaporative potential of the air. On a hot summer day, Delta T can be high enough to evaporate spray droplets before they can spread over the target area and provide maximum protection. This can be an issue for fungicides. “We want the droplets to stay liquid as long as possible,” says sprayer specialist Tom Wolf, who wrote this article about Delta T.

Tank contamination. Fungicide surfactant can be a tank cleaner, lifting herbicide residue from sprayer plumbing. A sprayer spa day before fungicide spraying can remove herbicide contamination before it causes damage. 

Timing. Canola may look quite yellow but has it reached 20 per cent flower on the main stem? Once petals start to fall, crop is at about 30 per cent flower.

What happens to sclerotinia stem rot risk in hot weather?

Infection may slow when day time highs reach 30°C, but it will quickly resume in the evening when canopy temperature drops and humidity increases. Canopy humidity is also more important than temperature, so use readings from inside the canopy, if possible, instead of from the weather station in the yard. Sclerotinia risk assessment tool

What will protect canola flowers from heat blast?

Hot days (28-30°C and up) from bud to mid-flowering stages can reduce flower reproductive function. This can reduce yield. Cooler nights, and adequate soil moisture should reduce heat blast and allow the crop to recover more quickly. 

We don’t have research to show reliable results from any product. If trying something, consider strip trials to see how they work.

What insects threaten canola this week?

Nothing major. 

• Saskatchewan Ministry of Agriculture staff have heard of diamondback moth larvae near threshold in southwest Saskatchewan. 
• In some isolated cases in northern Alberta, lygus are at 25-30 per 10 sweeps and maybe stopping canola from coming into flower. That is not the usual window for lygus spraying, but control could be required if lygus prevent canola from flowering. 
• This just in… Three Saskatchewan locations have surpassed 300 adult moth counts, based on this week’s brand new map. This moves the risk in those spots from low to “uncertain”.
• Provincial pest updates: Alberta Saskatchewan Manitoba