The Bordeaux mixture
In agriculture one of the most used remedies for the prevention against pathogenic fungi is the Bordeaux mixture. Used already in Roman times, it is the most widespread and easy to prepare pesticide. The Bordeaux mixture is a compound given by the chemical reaction of copper sulphate with the calcium hydroxide present in the lime. The active ingredient that serves to inhibit fungal cells is copper in the form of cation. The copper to be transported and therefore widespread must be in a watery environment and therefore the Bordeaux mixture it must always first be dissolved in water to be used. The source of copper is the sulfate itself while the lime serves to neutralize the sulphate which, being acidic, could give phytotoxic effects if used directly as such.
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Different types of Bordeaux mixture can be prepared, acting on the quantities used for the preparation of the mixture, based on the effect we want to obtain. The acid mixtures are prepared by increasing the dose of copper sulphate and are indicated if we want to obtain an immediate effect, which we are looking for when there is an aggressive mycotic attack already in place. These mixtures are generally phytotoxic, causing damage to the leaf apparatus and for this reason used only in case of need. Alkaline mixtures are indicated if we want to have a long-term action as the higher content of lime present confers greater adhesiveness to the slurry. Finally, neutral mixtures have intermediate properties, with good fungicidal activity and medium duration action.
Preparation of Bordeaux mixture
For the preparation of the Bordeaux mixture, 2 kg of copper sulphate pentahydrate and 1.4 kg of lime per 100 liters of water are generally used. Using these doses, the so-called neutral mixture is obtained while, increasing the dose of lime, the alkaline dose is obtained and the acid dose is reduced. To carry out the correct preparation of the Bordeaux mixture, it is necessary to dissolve the copper sulphate in a non-metallic container containing 80 liters of water, gradually adding it and mixing so as to obtain a homogeneous blue solution. In another container containing the remaining 20 liters of water the lime must be dissolved and mixed until complete dissolution. Finally add the first solution to the second, continuing to mix in an energetic and constant manner.
Bordeaux mixture: Usage and doses of Bordeaux mixture
The Bordeaux mixture acts by direct contact with the aerial apparatus of the plants, so it is necessary to nebulize the solution on the leaves and never use it for irrigation. It is important to keep in mind the fact that once prepared the Bordeaux mixture cannot be stored for a long time and is difficult to dispose of as potentially toxic to the environment. The doses of use vary according to the season and the variety of plant. In general, winter is the best time to perform the treatment, taking advantage of the plant's vegetative rest period, using 1.8 kg of Bordeaux mixture per 100 liters of water. Subsequent treatments should be done weekly using smaller doses, generally 400 grams per 100 liters of water and in case of rain 600 grams per 100 liters of water. The last two treatments, which must be carried out before flowering, are carried out at a distance of two weeks and with doses of about 800 grams per 100 liters of water. The treatment must never be carried out during flowering or during the production of fruits and vegetables.