Powdery mildew thrives in temperatures between 60 and 80 degrees. Dry, shady conditions are ideal, as are areas with poor air circulation. Planting disease resistant cultivars and making sure you allow for good air flow are two ways to guard against powdery mildew. Inspect plants regularly during warm, dry conditions, and remove any leaves that show signs of infection. Destroy (do not compost!) infected plant parts. A spray made with baking soda, if applied weekly at the first signs of infection, can protect plants against further damage. Plants that are badly infected should be ripped out and destroyed to prevent the disease from spreading further.