Bordeaux mixture (also called Bordo Mix) is a mixture of copper(II) sulphate (CuSO4) and slaked lime (Ca(OH)2) used as a fungicide. It is used in vineyards, fruit-farms and gardens to prevent infestations of downy mildew, powdery mildew and other fungi. It is sprayed on plants as a preventive treatment; its mode of action is ineffective after a fungus has become established.
It was invented in the Bordeaux region of France in the late 19th century. Hear this story in this video.
If it is applied in large quantities annually for many years, the copper in the mixture eventually becomes a pollutant.