prevent rain to splash on the ground it will send spores on leaf, add ground cover.
put plant in well aerated place.
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for powdery mildew you can try
Mix 5 ml (1 teaspoon) of baking soda (found in the pantry) in 1 liter of water and spray this solution on the plants prone to disease. For bicarbonate to adhere better to the leaf, you can add a few drops of insecticidal soap or dish soap to the solution. Note that this treatment only prevents the disease, not cure it. If you start treating after the leaves already show the typical white powder of the disease, you may be able to stop the progression of the disease, but the white already present will remain for the rest of the season.
Milk also has some efficacy as a preventative fungicide against white. Recipe? A milk part for nine parts of water that we spray until saturation. Milk powder would be as effective as whole milk.