mildew, a conspicuous mass of white threadlike hyphae and fruiting structures produced by various fungi. Mildew is commonly associated with damp cloth, fibres, leather goods, and several plant diseases (downy mildew and powdery mildew). Mildew-causing fungi use these substances as sources of food for growth and reproduction. Mildew often produces a distinctive smell.
The terms mold and mildew are sometimes used interchangeably. Although mildew can cause respiratory irritation in sensitive individuals, household molds, such as those from indoor water damage, are generally associated with more serious health effects.