Powdery Mildew

Powdery and downy mildews cause a white material to form on foliage.  These mildews are a common problem in damp, cool, or overly hot and dry weather, infecting many plants.  You can prevent mildews by watering around plant stems instead of sprinkling foliage.  Be sure to destroy all infected plants.  Thin the remaining plants for better air circulation.  In severe cases, apply a fungicide.