Powdery Mildew is a fungal disease that can affect many common plants in both wet and dry conditions, but is most prevalent in cool, dry weather with high humidity. Powdery Mildew spores require dry conditions to spread and are suppressed by excess moisture.
It is identifiable by the white or gray "powder" of spores and fungal strands that form on the outside of leaves (especially new growth), unopened flower buds and sometimes fruit. Left untreated, Powdery Mildew can deform and even kill plants. It is easily spread by the wind and especially common on plants grown in too much shade.