Milk, water, baking soda mixture for powdery mildew
Hi- research studies indicate that commercial bicarbonate (sodium and potassium) fungicides are not very effective against powdery mildew in cucumber family crops.
We would not recommend mixing and spraying the materials you reference.
Elemental sulfur may be the most effective organic fungicide for controlling powdery mildew. Horticultural oil may also be somewhat effective. Do not spray either product during hot weather.