A black, green, or whitish area caused by a fungus that grows on things such as plants, paper, cloth, or buildings, usually if the conditions are warm and wet. (Cambridge Dictionary)
A superficial usually whitish growth produced especially on organic matter or living plants by fungi. (Merriam – Webster)
A thin whitish coating consisting of minute fungal hyphae, growing on plants or damp organic material such as paper. (Oxford Dictionaries)
Mildew is a soft white fungus that grows in damp places. (Collins Dictionary)
Getting Rid of Powdery Mildew on Plants.
Mildew vs. Mold