Powdery Mildew is a Fungal Disease that spreads rapidly. It looks like white powder and starts in small patches. The spores of this fungus is airborne and can be transferred from plant to plant with insects.
Powdery Mildew-New Zealand Institute of Horticulture, A serious powdery mildew infestation (right)-AgricoCanada
If not treated quickly it can result in the loss or stunted growth of your plants. Roses are most susceptible to this fungus but almost any plant, tree, grass or ornamentals can be affected.
1) Increase air circulation between plants.
2) When watering only water the leaves if there is time for the plant to dry off.
3) A tablespoon of baking soda in a spray bottle will help kill this fungus.
4) A weak solution of skim milk and water sprayed on the fungus also works. (about 1 part milk to 10 parts water)
5) Sulfer or copper sprays are effective controls for any fungus type disease.
6) Organic fungicides can be purchased in most garden shops and home improvement centers.