mildew: [OE] Mildew originally meant ‘honeydew’ (which is a sort of sticky substance exuded by aphids and similar insects on to leaves). It is a compound noun formed in the prehistoric Germanic period from *melith ‘honey’ (a relative of Latin mel ‘honey’, source of English mellifluous and molasses) and *daw-waz, ancestor of English dew. The metaphorical transference from ‘honey-dew’ to a less pleasant, fungal growth on plants, etc took place in the 14th century. => dew, mellifluous, molasses
mildew (n.)
mid-13c., mildeu "honeydew, nectar," from Old English meledeaw "honeydew" (sticky stuff exuded by aphids), from Proto-Germanic compound of *melith "honey" (see Melissa) + *dawwaz "dew" (see dew). Similar formation in Old Saxon milidou, Dutch meeldauw, German Meltau "mildew."
First element in many cases assimilated to forms of meal (n.2) "ground grain." As a kind of fungus it is first recorded mid-14c., so called from its being sticky and originally growing in plants. As a verb from 1550s. Related: Mildewed.