This fall I had an outbreak of white powdery mildew on a bunch of my Euphorbias. It came in on a medusoid Euphorbia, and probably sporulated there. A couple of weeks later, it popped up on a few nearby plants, concentrated at the growth tips. Through a combination of soap spray and alcohol swabs, I think I have eliminated the active infection. But from what I've read, it's for life. So I'm wondering if anyone else has experienced this pest and has figured out a good chemical control method.
While I was looking around, I came across this article, which is pretty informative.