Bordeaux mixture is a mixture of copper (II) sulphate (CuSO₄) and quicklime (CaO) 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.

Bordeaux mixture was invented in the Bordeaux region of France in the late 19th century. It was the first fungicide to be developed and still is the most widely used copper fungicide throughout the world. However, it has some drawbacks, such as causing burning of leaves or russeting of fruit when applied in cool, wet weather, and accumulating copper in the soil, which can be harmful to the environment and organisms.

Bordeaux mixture can be prepared using differing proportions of the components. In preparing it, the CuSO₄ and the lime are dissolved separately in water and then mixed. Calcium oxide (burnt lime) and calcium hydroxide (slaked lime) give the same end result, since an excess of water is used in the preparation.

The conventional method of describing the mixture’s composition is to give the weight of CuSO₄, the weight of hydrated lime and the volume of water, in that order. The percentage of the weight of CuSO₄ to the weight of water employed determines the concentration of the mixture. Thus a 1% Bordeaux mixture, which is typical, would have the formula 1:1:100, with the first “1” representing 1 kg CuSO₄ (pentahydrated), the second representing 1 kg hydrated lime, and the 100 representing 100 litres (100 kg) water.

For spraying young, actively growing plants, the amounts of copper sulfate and hydrated lime are reduced, and the formulas used may be 2:2:100, 2:6:100, and so on. For plants known to be sensitive to Bordeaux, a much greater concentration of hydrated lime may be used, as in the formula 8:24:100.

Bordeaux mixture achieves its effect by means of the copper ions (Cu) of the mixture. These ions affect enzymes in the fungal spores in such a way as to prevent germination. This means Bordeaux mixture must be used preventively, before the fungal disease has struck. Thorough coverage of the spray on the plants is necessary. The Bordeaux spray continues to adhere well to the plant during rain, though in the long term it is washed off by rain. Commonly in practice, it is applied just once a year, in the wintertime.

Bordeaux mixture controls many fungal (including oomycete) and bacterial leaf spots, blights, anthracnoses, downy mildews, and cankers. It is widely used to control potato blight, peach leaf curl and apple scab. Some organic agriculture advocates allow its use, so it is often used by organic gardeners in some parts of the world.

However, Bordeaux mixture should be used with caution and moderation, as it can cause phytotoxicity to plants and pollution to soil and water. It should not be applied in hot or humid weather or when rain is expected within 24 hours. It should not be mixed with other pesticides or fertilizers unless compatibility tests are done beforehand. It should not be used on plants that are sensitive to copper or lime, such as blueberries or hydrangeas. It should not be used on edible crops within a week of harvest. It should not be used in areas where copper accumulation or runoff can pose a risk to aquatic life or human health.