Bordeaux Mixture | In the Oxford Companion to Wine (4th edition) Dr Richard Smart describes Bordeaux Mixture or la bouillie Bordelaise as it is called in French as ‘a copper-based spray made from a mixture of lime, copper sulfate, and water first recorded in 1885 by Alexis Millardet, Professor of Botany at Bordeaux University, as an effective control of downy mildew (peronospera) and which halted the devastation caused by the latter which had begun in 1883.’ The treatment is said to have been discovered because downy mildew was not present on those rows sprayed with the blue-staining copper sulfate to deter thieves. 

Bibliography

Oxford Companion to Wine 4th edition ed. Jancis Robinson MW and Julia Harding MW (Oxford University Press, 2015, p.95-6)