Powdery mildew is a plant fungus which effects a diversity of plants. Mildew on plants thrives when it is hot and humid. The first signs are small circular gray or white patches leading to the entire leaf being coated with a powdery mildew. Often the leaves become distorted and may eventually drop.
Generally, unless the pant looses all of its leaves, the plant is not seriously harmed as in some plant diseases. It does however look quite awful. Once the powdery mildew on plants has started there is very little you can do to stop it. Control of powdery mildew is best achieved by catching it at the very first sign of symptoms. You can spray with a baking soda solution or sulfur dust. I prefer to just use the baking soda method.
There are ways to help prevent powdery mildew from starting. Regular pruning will give your plantings good air circulation which is very important. Immediately after you notice diseased plants prune them back right to the ground so they do not infect surrounding plants. Do not put the infected material into your compost. Instead bag it up and throw it out. You can also look for mildew resistant varieties of the plants you choose to grow.
Some plants are more prone to developing powdery milder than others. Here is a list of those plants most likely to cause you trouble:
Baking Soda Solution
1 gallon of water to 1 tablespoon of baking soda
Add 3 drops of Ivory Liquid soap
Add 3 drops of vegetable oil
Mix well and spray on the tops and bottoms of the affected leaves and stems. Clean up the soil around the plant and keep leaf litter away.
Since powdery mildew is related to climatic conditions each year is anew. Just because you had the problem this season does not mean you will necessarily have it next year.