Destroying fallen infected leaves in autumn will reduce the amount of infectious spores next spring. Mulching and watering reduces water stress and helps make plants less prone to infection. Promptly pruning out infected shoots will reduce subsequent infection.
Most powdery mildew fungi have a host range restricted to a relatively few, related plants, but these can include wild relatives which can be sources of infection, e.g. wild crab apples may be sources of infection for apple orchards.
Seed producers sometimes offer powdery mildew-resistant cultivars of both vegetables and ornamental plants, check catalogues for details.
Because most of the growth of powdery mildews is found on the plant surface they are easily targeted with fungicides.
Edibles and ornamentals: Myclobutanil (Bayer Garden Systhane Fungus Fighter concentrate) can be used on ornamentals, apples, pears, gooseberries and blackcurrants.
Ornamentals only: Myclobutanil (Westland Rose Rescue and various other products, all as ready-to-use sprays), tebuconazole (Bayer Garden Multirose Concentrate 2) and triticonazole (Scotts Fungus Clear Ultra) can be used on ornamentals. Some formulations of myclobutanil, tebuconazole and triticonazole contain insecticides to control pests. Avoid these unless an insect pest problem is specifically identified.
Plant and fish oil blends (Vitax Organic 2 in 1) may be used on all plants.
Fungicides for gardeners (Adobe Acrobat pdf document outlining fungicides available to gardeners)