Powdery mildew is very difficult to eliminate, so keep things clean to eliminate its spread to other plants and trees.
Powdery mildew creates a white powdery substance on the leaves of plants and trees, which affects the plants' ability to produce energy through sunlight (leading to stunted growth and reduced yield and making the plant/tree look unwell and not very aesthetically pleasing).
Cleaning every leaf is usually recommended to treat the problem, but not very feasible on most plants and trees. One of the best ways to deal with powdery mildew is to remove all traces of leaves after they've dropped in the fall/winter. Practicing good 'ground hygiene' now will help the overall health and vigor of the plant or tree next Spring.
Use a leaf blower or rake and even your hands to gather together all affected leaves under and around your plants/trees that had leaves with powdery mildew. Your goal is to get rid of every affected leaf, bud, fruit or vegetables the best you are able. Put them in the trash - don't compost.
Next Spring, make sure your plants have good air circulation around them, and avoid any kind of overhead watering to help contain the spread of this disease.