Remove White Powdery Mildew from leaves with water
Ensure plants have proper airflow and ventilation

Treat the infected plant to kill spores prevent future growth:
Milk (1 part milk to 3 parts water)
Potassium bicarbonate (Dissolve 1 tbsp of potassium bicarbonate in 1 gallon of water)
Baking soda (1 tbsp baking soda, 1 tsp liquid dish soap, and 1 gallon of water)
Garden Safe Brand Fungicide3
Lost Coast Plant Therapy
GrowSafe