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 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 daybefore 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.
