Actually there were at one time commercial fungicides which controlled powdery mildew, but the powers-that-be decided that we would be better off without them, and they have never been replaced. However, there are several organic remedies which the Brussels Mafia are unable to effectively ban, and these do provide some control over the disease - at least in its earlier stages.
It can, in fact, completely ravage some crops, and I`ve had late peas so badly crippled that the stems died before the pods were mature enough to pick, so if you`ve got crops that are not yet mature it`s a good idea to consider preventative or controlling organic sprays