When my cukes started to show signs of powdery mildew, I tried 3 different remedies.
1. milk spray
2. kelp spray
3. compost tea
Most immediately on hand was the milk - it's supposed to be 1:10 of skim, but I used 1/2C 2% milk in a 1-1/2qt sprayer filled to the FILL mark with water. Next day or so, I remembered I had powdered kelp, so I sprayed with that according to pkg instructions... then a few days after that, my compost tea was ready, so I used a diluted solution ("add water until it looks like weak tea") in the fine mist sprayer.
I can't tell you which method was most effective, but the powdery mildew was staved off and the cukes are doing well. (I sprayed my pumpkins with the solutions at the same time although they weren't showing any symptoms. They never got the powdery mildew.)
Hope that helps.