The grayish-white growth on leaves of perennial flowers such as phlox, dahlia and bee balm is probably powdery mildew. If severe, the leaves will eventually yellow and die. Powdery mildew fungus thrives during cool, clear nights and sunny warm days. Cut down infected flower stalks and compost them. On plants that have a small amount of infection, spray the following mix; 3 teaspoons of baking soda, 2 tablespoons summer oil and one gallon of water.