/ˈmilˌd(y)o͞o/
Noun
1. A growth of minute powdery or webby fungi, whitish or of different colors, found on various diseased or decaying substances. (wiktionary.org)
2. A fungi in the order Erysiphales. It is also used more generally to mean mold growth. (wikipedia.org)
3. A whitish coating of minute fungal hyphae, growing on plants or damp organic material. (Google Dictionary)
4. Any of various fungi that form a superficial, usually whitish growth on plants and various organic materials. (The American Heritage® Dictionary of the English Language)
5. A superficial coating or discoloration of organic materials, such as cloth, paper, or leather, caused by fungi, especially under damp conditions. (The American Heritage® Dictionary of the English Language)
6. A plant disease caused by such fungi. (The American Heritage® Dictionary of the English Language)
Verb
1. Affect or be affected with mildew. (Google Dictionary)
Word origin: From Old English meledeaw “honeydew” (sticky stuff exuded by aphids), from Proto-Germanic compound of *melith “honey” + *dawwaz “dew.”