The Bordeaux mixture, also known as bordo mix, is a classic fungicide used to prevent the occurrence of certain harmful plant diseases, with downy mildew being one of the most notable. In this article, we’ll provide you with a detailed explanation of what it is and how it works. We’ll also show you how to prepare it manually using the right ingredients, which are readily available to everyone and easily found online.
We’ll also understand how to apply it to our plants, the timing, dosages, and especially for which types of diseases. We want to emphasize that this product is approved for organic agriculture but should be used with proper precautions.
What is Bordeaux Mixture?
The Bordeaux mixture is a surface fungicide, also known as a contact fungicide, based on copper. Copper, with a chemical symbol Cu, is a divalent cation (meaning it has a double positive charge) that replaces calcium and magnesium (Ca and Mg, also divalent) in the polymers that make up fungal cell walls, thus denaturing their structural and enzymatic proteins and damaging membrane lipids.
Copper is also an important catalyst for chlorophyll synthesis. Moreover, it’s among the most effective preventative fungicides we have to combat many of the fungal diseases circulating. Grapes, olive, pome fruits, stone fruits, citrus, solanaceous crops (particularly tomatoes), and more – no crop is exempt from fungal attacks. These attacks can be devastating as they often affect plants comprehensively – leaves, stems, and roots alike. Affected plants exhibit unnatural coloration, their structure deteriorates, and their regular functions are impaired.
Copper can be applied in the form of oxide chlorides, hydroxides, and specifically as the Bordeaux mixture. Homemade Bordeaux mixture is preferred. Commercially available formulations, such as powdered products (which are hydro-sensitive and need to be diluted before use), contain co-formulants that accompany the “active substance.” Many of these co-formulants are adhesive, enhancing the efficacy and applicability. Additionally, some act as stabilizers, aiding, among other things, in preservation. However, everything listed under the term “quanto basta” (as needed) on the label of ready-made Bordeaux mixture remains an uncertainty that the manufacturers do not disclose. Therefore, when using ready-made Bordeaux mixtures, there’s a risk of introducing substances into the soil that we never intended to be there.
Homemade Bordeaux Mixture
What’s Needed for Preparation?
Bordeaux mixture is a simple cold solution that can be easily made even at home. It requires three simple elements:
- Copper sulfate, CuSO4, preferably in powder form (for easier dissolution), but flakes can also be used. It can be easily purchased from any supplier (a good product can be found here).
- Lime, preferably quicklime, which is calcium oxide, CaO. If it’s difficult to find, hydrated lime or slaked lime, calcium hydroxide, Ca(OH)2, derived from the slaking reaction involving quicklime: CaO + H2O → Ca(OH)2, can also be used. The latter product is available online here.
The need to mix these two compounds arises from their chemical nature. Copper sulfate is acidic, with a pH below 7, while lime is basic, meaning it has a pH above 7. Mixing them together results in a neutralization reaction with a pH of 7. By varying the ratios of copper sulfate and lime, acidic mixtures (which act quickly but are highly phytotoxic and caustic due to the higher amount of copper sulfate) or basic mixtures (with a higher amount of lime, less rapid but more persistent due to increased adhesiveness) can be obtained.
What are the Quantities?
To make Bordeaux mixture at home, it’s useful to have a standard quantitative reference. With the appropriate proportions, you can determine the actual amount you need based on your requirements. The dosages of the various components of the mixture are:
- 1 kg of copper sulfate
- 1 kg of lime
Per 100 liters of water
The Right Proportions
Let’s consider a practical example. Suppose we need 10 liters to spray our plants and want to use the mixture as is, without any dilution. How many grams of copper sulfate do I need to mix? The answer is relatively simple:
1000 g (1 kg) of copper sulfate: 100 liters of water = x : 10 liters of water
The x, of course, corresponds to the unknown value we want to find. Therefore, by isolating it, we get:
x = (1000 g of copper sulfate * 10 liters of water)/100 liters of water
x = 100 g (0.1 kg) of copper sulfate
Obviously, the proportion, set with the correct information, will provide the required data each time. We can follow the same analytical procedure for the quantity of lime. For 10 liters, we’ll need 100 g (0.1 kg) of hydrated lime.
Always based on the “standard” quantities we’re using, let’s get two containers. They must be non-metallic and clean. Their capacity will vary based on the amounts of copper sulfate and lime we need to use. Let’s assume we need containers with capacities of 20 and 150 liters.
Finally, we need a wooden stick to mix the mixture (avoid mixing with your hands).
How to Prepare Bordeaux Mixture
Once you have the necessary materials, preparing the Bordeaux mixture involves three simple steps:
- Dissolve 1 kg of copper sulfate in 10 liters of water in our 20-liter container. Ensure that the mixture dissolves completely, leaving no residues. To do this, stir to speed up the process. A useful method, when using copper sulfate flakes, is to soak them in warm water the night before. This will facilitate complete dissolution.
- Dissolve 1 kg of lime in the remaining 90 liters of water, in the 150-liter container.
- After dissolving the two compounds separately, mix them together, adding the copper sulfate to the lime mixture, in that order. Then stir to thoroughly mix everything.
When to Apply
Bordeaux mixture is certainly a product to use within 24 hours of preparation. This is because it might lose effectiveness over a longer period. It’s a preventative treatment that should be applied before rain. For instance, if rain is expected in the morning or there’s high humidity, you can consider using it the evening before. This is because fungi like downy mildew need water to enter the plant. Additionally, they overwinter in the soil and require heavy rain to project their spores onto the green parts. Once the spores land on the lower side of the leaf, they enter through the stomata, forming mycelium and initiating secondary infection. This then spreads throughout the plant due to water mobility. Of course, if it doesn’t rain frequently or the humidity level isn’t high, the treatment might not be necessary.
It’s advisable not to spray very small or recently germinated plants. Although copper does not penetrate through the cuticle of plants (which is a waxy layer protecting plant organs), when this cuticle is thick, copper can be harmful to very young or highly hydrated organs. This is because its action is phytotoxic, resulting in an abnormal curvature of leaves that tends to close toward the central vein from both sides of the leaf margin.
Using copper is also not recommended during the flowering phase. This is because it could hinder flower attachment, i.e., fruit formation.
How and How Much Bordeaux Mixture to Apply Based on the Type of Disease
Bordeaux mixture, based on the severity of the attack, can be used just after preparation, as is. It’s important to ensure that the plant is completely covered with the mixture, particularly applying it to the lower side of the leaves where stomata are present. Appropriate equipment for spraying is necessary, usually a backpack sprayer.
Furthermore, it’s essential to use the appropriate personal protective equipment, such as gloves and a mask.
For each crop species and type of fungal attack, it’s recommended to apply the following doses:
- Grapes: against downy mildew, rot, and botrytis, use around 400 – 600 g/hl
- Pome fruits: against fire blight, apple scab, cankers, sooty blotch, and rust, use about 600 – 1200 g/hl
- Stone fruits: against peach leaf curl, brown rot, sooty blotch, cankers, and fire blight, use approximately 100 – 1200 g/hl. At least two winter treatments are recommended: after leaf drop and immediately after pruning.
- Citrus fruits: against gummosis, citrus canker, sooty blotch, and citrus scab, use around 700 – 800 g/hl
- Olive trees: against peacock spot and sooty blotch, apply about 700 – 800 g/hl
- Tomatoes: against alternaria, anthracnose, downy mildew, and bacterial spot, use around 600 – 800 g/hl
- Other vegetables: against alternaria, anthracnose, downy mildew, bacterial spot, cercospora, and rust, use approximately 300 – 500 g/hl
Bordeaux mixture can also be effective against powdery mildew, although not through the direct action of copper on the fungus. In this case, protection works by thickening the outer film of plants and their fruits.
The severity of the attack isn’t the only factor influencing Bordeaux mixture dosages. Weather conditions also matter, for example. Additionally, as mentioned before, the phenological stage of our crops and the aggressiveness of the diseases to be fought must also be taken into account.
As you’ve seen, Bordeaux mixture is a compound that contains a heavy metal, copper. If this accumulates in the soil, it inhibits and suppresses useful microbial activities for humus synthesis. Therefore, it’s crucial to avoid excessive use. Stick to the legal limit of about 4 kg/ha/year (28 kg/ha/year over 7 years).
Use it only when there’s a real risk of fungal attack.
As previously mentioned, Bordeaux mixture is a preventative treatment. But before resorting to its use, another prevention step should be taken, paying special attention to irrigation. Proper water management is essential. If our crops are prone to frequent rain and humidity rises significantly, an initial action could involve aerating, perhaps by reducing leaf volume whenever possible to lower relative humidity.
- MDPI (International Journal of Molecular Sciences): “Double- or Triple-Tiered Protection: Prospects for the Sustainable Application of Copper-Based Antimicrobial Compounds for Another Fourteen Decades” – The article reviews the long-standing use of copper-based antimicrobial compounds in controlling phytopathogens, tracing their evolution from the introduction of the Bordeaux mixture to the present.
- MDPI (Horticulturae): “Effects of Exogenously Applied Copper in Tomato Plants’ Oxidative and Nitrogen Metabolisms under Organic Farming Conditions” – The paper discusses the role of copper, an active substance in plant protection products, in combating diseases in both conventional and organic tomato cultivation.
- MDPI (International Journal of Molecular Sciences): “Photosynthetic Characteristics and Chloroplast Ultrastructure Responses of Citrus Leaves to Copper Toxicity Induced by Bordeaux Mixture in Greenhouse” – This study investigates the effects of Bordeaux mixture on citrus leaves, focusing on photosynthetic characteristics and chloroplast ultrastructure responses to copper toxicity.
- MDPI (Horticulturae): “Organic Production of Snap Bean in Bulgaria: Pests and Diseases Incidence and Control, Soil Fertility and Yield” – The research aims to determine the impact of organic production practices on snap bean pests and diseases infestation, soil fertility, and yield, with a mention of Bordeaux mixture.
- MDPI (Fungi): “Control Efficiency and Yield Response of Chemical and Biological Treatments against Fruit Rot of Arecanut: A Network Meta-Analysis” – This comprehensive study evaluates various treatment combinations for controlling fruit rot disease in arecanut, including the use of Bordeaux mixture.
- MDPI (Polymers): “Grafted Pullulan Derivatives for Reducing the Content of Some Pesticides from Simulated Wastewater” – The article presents data on the use of grafted pullulan derivatives as flocculating agents for removing certain pesticide formulations from model wastewater, with a mention of Bordeaux mixture.
- MDPI (Journal of fungi): “Control Efficiency and Yield Response of Chemical and Biological Treatments against Fruit Rot of Arecanut: A Network Meta-Analysis” – The study evaluates various treatment combinations for controlling fruit rot disease in arecanut, including the use of Bordeaux mixture.
- MDPI (Chemosensors): “Potentiometric Sensor Based on Layered Pillararene—Copper Composite” – This research develops a solid-contact potentiometric sensor using layered pillararene and Cu2+ ions films, with a mention of Bordeaux mixture.
- MDPI (Polymers): “Electrochromic Self-Electrostabilized Polypyrrole Films Doped with Surfactant and Azo Dye” – The study focuses on the electropolymerization of a pyrrole monomer using various dopants, including Bordeaux mixture.
- Journal of Xenobiotics: “Effects of Copper on the Early Development of Xenopus laevis: The case of CuSO4 and Bordeaux Mixture Solutions” – The research investigates the effects of copper, specifically from Bordeaux mixture, on the early development stages of Xenopus laevis.