Plants infected with fungi also may have insects creating the problem. For example, fungi such as sooty mold usually occur when insects such as aphids or scale are present secreting honeydew. The gardener needs to treat both problems by using a fungicide and an insecticide because each product on its own will not treat both problems. Using a separate sprayer for each product is the most effective and safest way of application, but sometimes mixing the two is possible. Always test your fungicide and insecticide for mixing compatibility before making a large batch, recommend the experts at Pennsylvania State University.
Step 1
Read the product labels before mixing. Note the mixing directions, safety precautions and if there are special requirements and instructions involved.
Step 2
Fill a 1-pint glass jar with a cap with water. You will use this to mix the fungicide and insecticide together to check for mixing compatibility. Mix the products in an area that has no contact with food, children or pets such as the garage.
Step 3
Add the appropriate amount of insecticide, per the label instructions, to the jar of water. Add the appropriate amount of fungicide, per the label instructions, to the jar of water and insecticide.
Step 4
Seal the jar with the cap and shake thoroughly to mix the fungicide and insecticide together. Allow the jar to sit undisturbed for approximately one-half hour, according to Pennsylvania State University.
Step 5
Check the jar after the allotted time to see if the mixtures have uniformly mixed together with only a slight separation of the two products. If the mixture does not clump and contain sludge, the products are safe for mixing. If clumping occurs, do not mix the two ingredients together but apply separately.
Step 6
Fill a pump-up sprayer with the specified amount of water, per the fungicide's and insecticide's labels.
Step 7
Add the specified amount of insecticide and fungicide to the sprayer. Shake the sprayer, thoroughly mixing the two products together.

From：www.gardenguides.com