Mildew is defined as a thin, superficial, usually whitish growth consisting of minute hyphae (fungal filaments) produced especially on living plants or organic matter such as wood, paper or leather. Molds are similar superficial, often "woolly", downy, or furry growths of unspecified color, typically on food or suggesting decay. In horticulture, mildew is either species of fungus in the order Erysiphales, or fungus-like organisms in the family Peronosporaceae. It is also used more generally to mean mold growth. In Old English, mildew meant honeydew (a substance secreted by aphids on leaves, formerly thought to distill from the air like dew), and later came to mean mold or fungus.