I pulled both of my zucchini plants. They were developing a white moldy substance on the leaves and the plants were dying. I did some research and found that my plants had powdery mildew. In my situation, this was caused by lack of air flow. I simply planted too many things together in the corner of the garden and the fence that surrounds the garden was preventing adaquete air flow.
My cantaloupe started to have the same problem. It wasn’t killing the plants yet, but it was obviously a problem. I discovered a cheap, easy solution. Milk! We added 1/2 cup of milk to a gallon of water, and sprayed it directly on the plant. The next day, I could see an improvement. A week later, it looks like nothing ever happened. I wish we would have tried it on the zucchini.