You might notice that your commercial fungicide gives very specific instructions for use relating to temperature. Most require you to apply the fungicide spray when the weather is neither too hot nor too cold. This will help prevent the plant from getting harmed in the spraying process. Always check with your specific manufacturer to confirm its safe temperatures.

Heat Limitations
Most fungicidal sprays will limit your spraying to the morning or the afternoon when there is less heat from the sun. The instructions will also note not to spray the fungicide at temperatures above 85 degrees Fahrenheit. If you are not sure what the temperature is, place a thermometer next to the area you intend to spray and check the temperature. If you don't have a thermometer, wait until you know that the temperature is below 85 degrees Fahrenheit.

Cold Limitations
Fungicidal spray instructions also note that you cannot use the spray in colder conditions such as those below 55 degrees Fahrenheit. Thus, don't spray your fungicide early in the morning before the sun comes up or late at night after it goes down unless you can confirm the temperature is above 55 degrees Fahrenheit.

Leaf Burn
Applying fungicide to a plant in hot conditions could cause its leaves to actually burn. The chemicals in the fungicide heat up a lot in the sunlight and they can burn black spots or holes in the plant's leaves if the leaves are wet with fungicidal spray over 85 F.

Fungicide Effectiveness
Fungicidal spray is not as effective at very high or very low temperatures when it is still wet. If you spray the fungicide when it is very hot, it will evaporate off the plant rather than soak into it. This will leave little residue to prevent fungi such as powdery mildew from infecting the plant.

From Network Resources