🌿 Bordeaux mixture

Bordeaux mixture is a fungicide made from copper sulfate and slaked lime in water. It is sprayed on grapevine leaves to protect them from downy mildew (Plasmopara viticola).

Downy mildew was first observed in France near Coutras in 1878 and caused significant losses by 1882. That year E. David, manager of Château Beaucaillou, noticed that plot edges whitewashed with lime and copper sulfate to deter grape thieves also had less mildew. Alexis Millardet ran trials in 1883 and 1884. An IVES Technical Reviews paper calls the discovery entirely fortuitous.

David's 1884 recipe mixed 8 kg of copper sulfate in 100 L of water with a lime slurry of 15 kg of lime in 30 L of water. Gayon's 1887 formula with 2% copper sulfate became the standard. A present-day preparation uses 2 kg of copper sulfate and 1.3 kg of slaked lime per hectolitre of water. Burgundy mixture replaces the lime with sodium carbonate.

The copper salts on the leaves prevent the spores of downy mildew from germinating, so the infection does not start.

Copper accumulates in vineyard soils, at 1 to 400 mg per kg of dry soil, most often in older vineyards, and it is toxic to aquatic and terrestrial organisms. In the EU copper is a candidate for substitution under Regulation (EU) 2018/1981, and the limit is 4 kg per hectare per year, averaged over 7 years (28 kg/ha). In organic farming copper remains the only effective product against downy mildew.