Mode of Action of Bordeaux Mixture: How Does It Kill Fungi?

Bordeaux mixture (also known as Boóc-đô in Vietnamese) is one of the world’s first fungicides, discovered in the late 19th century and still widely used in agriculture today. Although it is a simple preparation of copper sulfate and lime, it effectively controls many serious fungal diseases in crops.

So, how exactly does Bordeaux mixture work? Why is it effective against fungi while still permitted in organic farming? Let’s explore.

1. What Is Bordeaux Mixture Made Of?

• Copper sulfate (CuSO₄): Provides copper ions (Cu²⁺), the main antifungal component.
• Hydrated lime (Ca(OH)₂): Neutralizes the acidity of copper sulfate solution, reducing plant toxicity.
• Water: Serves as the medium for reaction, forming a stable calcium-copper solution.

When mixed correctly, Bordeaux mixture forms a light blue suspension that adheres well to leaves, stems, and branches.

2. Mode of Action Against Fungi

🧬 Main effect: Copper ions (Cu²⁺) disrupt fungal cells

When Bordeaux mixture is sprayed on plant surfaces:

1. Copper ions (Cu²⁺) are slowly released in humid conditions (dew, light rain).
2. When fungal spores land on treated surfaces, Cu²⁺ ions penetrate fungal cells.
3. Copper ions disrupt intracellular enzymes, interfering with respiration and protein synthesis.
4. As a result, fungal cells are damaged, lose viability, and die.

⚠️ Contact action only:
Bordeaux mixture works on the surface—it does not penetrate plant tissues. Therefore, it is preventive only, not curative once the fungus has invaded inside.

3. Why Is Bordeaux Mixture Effective?

🔹 Long-lasting protection:

• The coating persists for 7–10 days after spraying.
• Copper ions are released gradually under moisture, ensuring prolonged protection.

🔹 Broad-spectrum activity:
Effective against many fungi such as:

• Phytophthora spp. (downy mildew, soft rot)
• Colletotrichum spp. (anthracnose)
• Alternaria spp. (leaf spot, blight)
• Oidium spp. (powdery mildew)

🔹 Low resistance risk:
As an inorganic mixture with a non-specific mechanism, fungi rarely develop resistance—unlike with many modern systemic fungicides.

4. Factors Affecting Effectiveness

✅ For best results: mix properly – spray evenly – apply at the right time.

5. Comparison With Modern Fungicides

6. Conclusion

The mode of action of Bordeaux mixture is through the release of copper ions (Cu²⁺), which disrupt fungal cells—a simple yet effective, sustainable, and low-resistance mechanism. Although it cannot cure established infections like systemic fungicides, it remains a trusted preventive solution, particularly in organic and safe farming systems.

👉 To maximize effectiveness, farmers must mix correctly, spray at the right time, and combine with good field management practices.