In horticulture, mildew is fungi in the order Erysiphales. It is also used more generally to mean mould growth.

In Old English, it meant honeydew (a substance secreted by aphids on leaves, formerly thought to distil from the air like dew), and later came to mean mildew in the modern sense of mould or fungus.

Read more about Mildew:  Plant Pathogens, Household Varieties