Mildew (?), n. [AS. meledeáw; akin to OHG. militou, G. mehlthau, mehltau; prob. orig. meaning, honeydew; cf. Goth. milip honey. See Mellifluous, and Dew.] (Bot.) A growth of minute powdery or webby fungi, whitish or of different colors, found on various diseased or decaying substances.
[1913 Webster]

powdery mildew a fungal disease of plants caused by an ascomycete of the order Erysiphales, characterized by a powdery white film on the surface of the affected plants. It is damaging to, e.g., roses and lilacs. Also, a fungus that causes such a disease.
[PJC]

Mildew, v. i. To become tainted with mildew.
[1913 Webster]