What does Bordeaux mixture do? A detailed guide to usage precautions
Bordeaux mixture is an inorganic copper fungicide. The chemical composition of its active ingredients is CuSO4·xCu(OH)2·yCa(OH)2·zH2O. In 1882, the Frenchman A. Millardet discovered its fungicidal effect in the city of Bordeaux, hence the name Bordeaux mixture. It is a sky-blue gelatinous suspension prepared from about 500 g of copper sulfate, 500 g of quicklime and 50 kg of water. The ingredient ratio can be appropriately increased or decreased as needed. It is generally alkaline and has good adhesion properties, but its physical properties break down after long storage, so it is best prepared and used fresh or made into dehydrated Bordeaux powder. Mix with water when using.
The fungicidal principle of Bordeaux mixture
Bordeaux mixture itself has no fungicidal effect. When it is sprayed on the plant surface, it is adsorbed onto the crop surface due to its stickiness. During metabolism, plants secrete acidic liquids, and together with the acidic substances secreted by bacteria when invading plant cells, a small amount of basic copper sulfate in the Bordeaux mixture is converted into soluble copper sulfate, thereby producing a small amount of copper ions (Cu2+). After Cu2+ enters the pathogen cells, it coagulates the proteins in the cells. At the same time, Cu2+ can also destroy a certain enzyme in the cells, so that the metabolic processes in the bacterial body cannot proceed normally. Under the influence of these two effects, the bacteria are poisoned and die.
Targets of Bordeaux mixture
To control apple early defoliation disease, anthracnose and ring rot, spraying can begin after apple petal fall with a lime double-strength Bordeaux mixture at 200-240 times dilution, once every half month, alternating with other fungicides, for a total of 3-4 sprays. To control apple fruit rot (ring rot, anthracnose), spray a lime double-strength or high-strength Bordeaux mixture at 200 times dilution 10-15 days before diseased fruit appeared in previous years, once every 15-20 days, for 3-4 consecutive sprays, and stop use 25 days before harvest. To control apple moldy core, begin spraying a lime double-strength Bordeaux mixture at 200 times dilution at the apple bud stage. To control apple and pear rust, spray a lime equal-strength Bordeaux mixture at 160 times dilution on the Chinese junipers around the apple orchard. To control grape black rot, anthracnose and downy mildew, spray a lime low-strength Bordeaux mixture at 160 times dilution, once every 12-15 days, for a total of 2-4 sprays. After the rainy season begins, using Bordeaux mixture to control grape diseases gives good results.
Usage precautions for Bordeaux mixture
1. Pay attention to raw material selection: use pure, high-quality, white quicklime lumps and pure blue copper sulfate. Note that copper sulfate should not contain green or yellow impurities.
2. Master the preparation method: first take 1/3 of the water to prepare the lime milk, fully dissolve and filter for later use; then take 2/3 of the water to prepare the copper sulfate solution, fully dissolve for later use. Slowly pour the copper sulfate solution into the lime milk, stirring while pouring, or pour the copper sulfate solution and lime milk slowly and simultaneously into the same container, stirring while pouring. Never pour the lime milk into the copper sulfate solution, otherwise the prepared liquid will settle quickly and easily cause phytotoxicity. Note that Bordeaux mixture must be used as soon as it is prepared; the liquid prepared on the same day should preferably be used up on the same day, and should not be stored for long, still less overnight, nor diluted. Metal containers should not be used to prepare Bordeaux mixture, especially not iron utensils, to prevent chemical reactions that reduce efficacy.
3. Pay attention to the timing of application: after the rainy season arrives, generally during the fruit bunch closure and stone hardening stage (late July to early August) it is used to control grape anthracnose, white rot, black pox, downy mildew, etc. Before use, spray once generally with 80% mancozeb; 10-15 days after spraying, spray once with a half-strength Bordeaux mixture. Spraying must be uniform and thorough, and the berries and rachis must all be covered. After coloring, use pollution-free pesticides and no longer spray Bordeaux mixture. After harvest, a lime equal-strength Bordeaux mixture (1:1:200) can be used for control.
4. Use the agent reasonably: Bordeaux mixture is alkaline, and its active ingredients include calcium and copper, so it cannot be mixed with fungicides or insecticides such as lime sulfur mixture, carbendazim, thiophanate-methyl and mancozeb. When Bordeaux mixture and fungicides or insecticides are used separately, there must be an interval of 10-15 days.
5. Spray safely at the right time: when using Bordeaux mixture, avoid high-temperature and high-humidity weather. Spraying Bordeaux mixture at hot noon or on dewy mornings easily causes lime and copper ions to rapidly accumulate, burning leaves and berries. Generally, spraying after 15:00 is relatively safe.
6. Different crops react differently to Bordeaux mixture: when using it, pay attention to the safety of copper sulfate and lime for the crop. Crops sensitive to lime include potato, grape, cucurbits, tomato, pepper, etc. After these crops are treated with Bordeaux mixture, phytotoxicity easily occurs under high temperature and dry conditions, so lime equal-strength, low-strength or half-strength Bordeaux mixture should be used, and it is generally not used on young seedlings. Crops very sensitive to copper include peach, plum, apricot, Chinese cabbage, lettuce, soybean, common bean, etc.; use with caution, and test before use.
Preparation precautions for Bordeaux mixture
1. The preparation container must not be a metal vessel (mainly metals chemically more active than copper); after spraying, the equipment must be washed in time to prevent corrosion (mainly for metals chemically more active than copper). Metals chemically more active than copper mainly include: magnesium, aluminum, zinc, iron, tin, etc. The reason vessels made of these metals cannot be used to hold it: Bordeaux mixture contains copper sulfate, and metals chemically more active than copper will displace it from the solution, causing it to deteriorate. For example, iron: Fe+CuSO4===Cu+FeSO4; aluminum: 2Al+3CuSO4===3Cu+Al2(SO4)3
2. It must not be used on cloudy and rainy days, foggy days, or in the morning when the dew has not dried, to avoid phytotoxicity. It cannot be mixed with alkaline pesticides such as lime sulfur mixture; the interval between the two agents is 15-20 days. Stop use 20 days before fruit harvest. Some apple varieties (such as Golden Delicious, etc.) tend to develop fruit russeting on young fruit after spraying Bordeaux mixture, so other pesticides can be used instead.
3. Do not first prepare a concentrated Bordeaux mixture and then add water to dilute it. The Bordeaux mixture prepared in one go is a colloidal suspension and is relatively stable; adding water again will form precipitates or crystals, affecting quality and easily causing phytotoxicity.
4. Do not pour lime milk into dilute copper sulfate, as the Bordeaux mixture prepared this way is extremely unstable and prone to precipitation.
5. Do not pour concentrated copper sulfate into lime water, as the Bordeaux mixture prepared this way is unstable and of poor quality.
6. The copper sulfate in Bordeaux mixture is highly toxic. If Bordeaux mixture is accidentally swallowed, a large amount of egg white should be consumed.