Bordeaux mixture has been used for nearly 200 years to treat crops against cryptogamic diseases. These are capable of wreaking havoc in the vegetable garden, among other things, and rather than treating relentlessly with chemical antifungals, we resort to this famous cupric mixture. This is where it makes sense. So let’s find out what exactly is the composition of this blue powder. Let’s also see how to properly use Bordeaux mixture to preserve the good health of plants, whether preventive or curative, without however abusing it since Bordeaux mixture is sometimes criticized because of its proven toxicity.
What is Bordeaux mixture?
It’s about a micronized powder of turquoise blue color active against algae and pathogenic fungi. It is an algaecide and a fungicide. It is obtained by neutralizing with slaked lime a solution of copper sulfate. It is therefore the copper ions that constitute its active ingredient. This cupric mixture, that is to say which contains copper, having been discovered by chance in the Médoc, was baptized by logic Bordeaux mixture.
What to treat with Bordeaux mixture?
All areas of the garden can benefit from a preventive or curative treatment with Bordeaux mixture, whether it is the vegetable patch, the orchard, the vineyard or the ornamental garden. It is therefore of great help against:
- La tavelure,
- peach leaf curl,
- the criblure,
- The gummy,
- The jewelry,
- Bacterial canker, etc.
If it has been used for two centuries, it is because it is effective. It can be used in organic farming but in small doses.
This copper compound offers very good results since the germination of mushroom spores is stopped after the treatment. It is also useful for inhibiting the proliferation of bacteria in the garden. In addition, this copper compound is toxic to algae. It is also for this reason that it makes it possible to overcome mildew, a disease responsible for an oomycete, kind ofalgae-mushroom (Plasmopara viticola).
On the other hand this blue powder absolutely must not be used to treat pool water and it is of no use against an infestation of mites or insect pests whether on legumes, fruit trees, hedges or flowering plants. You must know it.
How to use Bordeaux mixture?
We should’nt never use pure Bordeaux mixture. We absolutely dilute it in water in order to spray it where necessary, i.e. on plants sensitive to attacks by microscopic pathogenic fungi or which are already affected by a cryptogamic disease.
The dosage to be strictly observed must not exceed 20 mg of Bordeaux mixture per liter of water. But this is a maximum. In many situations, it is recommended to treat with only 10 or 12 g/L. The reason is simple: the plants treated with this highly dosed cupric mixture are doomed to certain death and the treatment will not be any more effective on the cryptogamic disease which is targeted. There is therefore really no point in overdosing this treatment, whether one is looking for a preventive or curative action.
Experienced gardeners are able to visually quickly realize that a plant has been treated with an excessive dosage: blue drops are visible on the tips of the leaves. If this phenomenon is observed, the only solution is to water the plants abundantly in fine rain in order to save them, but let’s face it: the Bordeaux mixture will infiltrate the soil.
Rather than having a heavy hand, it is sometimes necessary to resort to a natural wetting product pour increase the adhesion of Bordeaux mixture on the plant parts to be treated. This prevents this contact fungicide from being washed away too quickly by the slightest downpour. In 1 liter of water added with 10 to 20 g of Bordeaux mixture, one can therefore add one or other of the following wetting agents:
- 5 tablespoons of black soap,
- 20 ml dormant oil or horticultural oil which is a mineral oil.
Some people opt for adding 10 cl of skimmed milk. However, it should be noted that this is not the best solution because bacteria and certain pathogenic fungi feed on milk proteins.
Is Bordeaux mixture toxic?
The toxicity of Bordeaux mixture is real for man as soon as the dosage exceeds 0.3 g per kg of body weight as detailed in many expert reports. It is also toxic to animals and the environment. It is therefore essential to strictly respect the doses. Let’s not forget that the copper what does Bordeaux mixture contain is washed away by the waters rain and watering, then accumulates in the ground or in groundwater. Copper toxicity is even greater in acidic soil than in neutral soil. It is well understood that every gardener must use Bordeaux mixture with great moderation.
In order not to fall into excess, it is quite possible to replace the cupric mixture from time to time with horsetail or nettle manure, but here again the dosages must be respected so as not to cause any unfortunate impact on the health and environmental level. Manure should not be dosed at more than 8%.
And to avoid cryptogamic diseases, as much adopt the right gardening methods such as space your plantations sufficiently, make the right sizes to ventilate the heart of trees and shrubs, or even avoid wetting the leaves of vegetable garden plants too much when watering.