Washing the mildew affected plant with water periodically can help remove the fungus to some extent. There are also other compounds which are effective as Powdery Mildew Treatment.
Some gardeners have found that a solution of baking soda with water can be used to kill the fungi causing mildew to some extent. Neem oil is widely recommended as a fungicide to destroy the fungi causing the mildew on the leaves and stem.
The plants should be planted some distance from each other to improve the air circulation, allowing the leaves and stem to dry quickly. The pruning tools should be disinfected periodically with bleach to kill the fungi.