Bordeaux mixture

Bordeaux mixture (also called Bordo Mix) is a mixture of copper(II) sulfate (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 preventative; 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. If it is applied in large quantities annually for many years, the copper in the mixture eventually becomes a pollutant.

In addition to its use to control fungal infection on grape vines, the mixture is also widely used to control potato blight, peach leaf curl and apple scab. It is approved for organic use, so is often used by organic gardeners where nonorganic gardeners would prefer other controls.

Bordeaux mixture achieves its effect by means of the copper ions (Cu²⁺) 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 pre-emptively, 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 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.

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