Prevent powdery mildew on plants

Powdery mildew affects many vegetable species including tomato, beans and cucurbits. (Scot Nelson/UH CTAHR)
Powdery mildew appears on many different plants but is host specific and will not transfer from one species to another. (Scot Nelson/UH CTAHR)

Cheetah asks: I lost most of my last bean crop to powdery mildew. I want to try growing beans again. How can I prevent and treat this disease?