Copper(II) sulfate (CuSO4), is a common salt of copper. Copper sulfate exists as a series of compounds that differ in their degree of hydration. The anhydrous form is a pale green or gray-white powder, while the hydrated form is bright blue. The archaic name for copper(II) sulfate is “blue vitriol” or “bluestone”. The most common form of copper sulphate is in the pentahydrate form and there are over 100 manufacturers producing around 200,000 tonnes per annum. Approximately three-quarters of copper sulphate pentahydrate is used in agriculture, principally as a fungicide, but also for treating copper-deficient soils. Two commonly used fungicide solutions containing copper sulphate are the Bordeaux Mixture and the Burgundy Mixture.
What is a Bordeaux Mixture ?
The Bordeaux mixture (also known as the Bordo Mix) was developed in the 19th century in France to control a downy mildew that caused problems on vines in the Bordeaux region of France. It contains a blend of copper sulphate and hydrated lime in water and is used to control different diseases on a wide range of plants and crops. It is used mainly to control garden, vineyard, nursery and farm infestations of fungi. It is the copper ions in the bordeaux mixture which kills the fungal spores by preventing the spores germinating. As such, the product is only effective as a prevention and not as a treatment for the fungus after it has formed.
Among Bordeaux’s many uses are applications in autumn and winter to manage:
- Fire blight on pears and apples;
- Leaf curl and shot hole on peaches and nectarines;
- Downy mildew and powdery mildew on grapes;
- Peacock spot on olives;
- Walnut blight on walnut; and
- Black spot on roses.
How to prepare the Bordeaux Mixture:
Ideally solutions should be prepared fresh. 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 (pentahydrate), the second representing 1 kg hydrated lime (calcium hydroxide), and the 100 representing 100 litres (100 kg) water. As CuSO4 contains 25% copper, the copper content of a 1% Bordeaux mixture would be 0.25%.
For small-scale preparation: add 3 1/3 tablespoons of copper sulfate and 10 tablespoons of hydrated lime in 5 Litres of water.
To make – dissolve the copper sulphate in half the water and dissolve the hydrated lime in the other half of the water. When both are fully dissolved, mix the two solutions together and stir well.
Please Note: it is important that metal containers in particular, iron, are not used to make up the Bordeaux solution. Use wooden or plastic containers.
Bordeaux mixture has been found to be harmful to fish, livestock and—due to potential build up of copper in the soil—earthworms.
Because Bordeaux can leave a blue-green discoloration on plants or painted surfaces, use it on dormant, deciduous plants that are away from buildings and fences.
How to use Bordeaux Mixture.
Spraying with Bordeaux mixture should be carried out before the season of spores starts. Typical usage solutions are 0.5% to 1.0% Bordeaux mix. Do not use solutions stronger than 1% as this may harm plants. Apply at 2 to 3 week intervals to give complete protection.
When applying Bordeaux, be sure to wear protective clothing, including goggles, because the spray deposit is corrosive, can permanently stain clothing, and is difficult to wash off.