Here is an organic solution I use on mildew:
- 1 gallon of water
- 1 tablespoon of baking soda
- 1 tablespoon of vegetable oil
- 1 tablespoon of dishwashing liquid
This works by raising the pH of the leaves, which creates an environment which isn't suitable for mildew. It may not get rid of the current mildew, but it will certainly stop it spreading. Also meant to be good is cow's milk believe it or not: mix 1 part milk with 9 parts water and spray the stems and tops of leaves with the solution.
If you remove or collect up any diseased leaves, it's important not to put them in the compost which you are then planning to use in the garden,m as it may spread the mildew to plants in the future. You should also wash any secateurs you use on the mildew too. Good luck with it - it is such a nightmare when a lovely plant gets covered in mildew isnt' it