leaves and shoots may become distorted and discolored.
Powdery mildew is a fungi that requires living tissue to grow. The fungi grow as thin layers of mycelium on the surface. Mycelium is the vegetative part of a fungus made of threadlike tubes.
Powdery mildew spreads as wind-borne spores. Spores do not need water to germinate; they actually die in water. Powdery mildew survives only on plant tissue and dormant buds and does not survive in the soil.

The first thing to do is ask your nursery person if there is a hybrid type of your plant which is resistant to powdery mildew--most of the time there is. Dig your infected plant out and transplant the new one in. For small plants this is usually the best solution.

Next, make sure you are not planting next to a wall or fence that will block circulation, as this is critical to powdery mildew. Keep the inside of plants trimmed out to help circulation. Also, water in the early morning so the soil will dry out before nightfall. Humidity around the plant is a big factor in fungus disease. Remember fungus spores will multiply fastest in warm, dark, damp places.