Mildew attacked the leaves of cucumber plants and zucchini (and other summer squash) plants this summer. Despite this, we harvested abundant quantities of each, so one could just call this part of our ecological tithe, especially since we weren't eating the leaves. Yet there was enough concern that we did try to address the problem.
We altered our watering, so as not to water from above near these plants (as well as the tomatoes, given the threat of tomato blight). Water on leaves helps spread disease, and it's water into roots which counts. We added a soaker hose in the area of these plants as well.
We also tried a couple pesticides. The first was a dilute mixture of hydrogen peroxide. The effectiveness was questionable, requiring multiple regular applications to seem to work. The second was baking soda and soybean oil (vegetable oil) diluted in water: about a tablespoon of baking soda with a half gallon of water; the oil helps the baking soda stick to the leaves. A few days later, many of the leaves of the squash plants appeared free of mildew. However, it is unclear how much of this is new growth unyet touched by mildew, since we cleared away the dead leaves.