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Bordeaux mixture is a broad-spectrum protective fungicide that has been widely used in fruit tree cultivation for a long time. It is known for its broad application range, strong efficacy, and the fact that resistance has not yet developed. The active ingredient is basic copper sulfate. However, improper preparation or application can lead to phytotoxicity, which can harm the quality of fruits and even cause damage to the plants. Therefore, it's important to understand the causes and how to prevent them.
First, let's look at the common causes of phytotoxicity from Bordeaux mixture:
1. Incomplete dissolution of copper sulfate during preparation can result in residue being sprayed onto the fruit. This may cause red-brown spots and freckles, reducing the commercial value of the fruit.
2. Applying Bordeaux mixture during the early stages of fruit development or when the fruit is ripening can lead to rust spots, rough skin, and reduced smoothness, negatively affecting the appearance of the fruit.
3. Trees such as peaches, apricots, and plums are particularly sensitive to copper. Using Bordeaux mixture during their growing season can lead to copper ion poisoning, causing leaf drop and fruit loss. Additionally, if the lime content is too low, certain varieties like apples (especially golden crown), hawthorns, and persimmons may suffer from phytotoxicity. On the other hand, if the lime content is too high, grapes, longans, and non-nuclear purple varieties are more likely to be affected.
4. Mixing or alternating Bordeaux mixture with sulfur-based products or antibacterial agents too frequently can also lead to phytotoxicity.
5. High-temperature conditions increase the risk of phytotoxicity due to lime. If the spray solution is not dry before rain or if leaves are still wet, copper can separate and penetrate the leaves, increasing soluble copper levels and potentially burning the leaves. After some use, wind and rain can further increase the concentration of soluble copper on the leaves, leading to damage commonly referred to as "wind and rain injury."
To prevent these issues, here are some key recommendations:
1. Use high-quality materials. When preparing the mixture, choose pure, white, lightweight lump lime and blue copper sulfate crystals. Avoid using yellow or green powders.
2. Follow the correct preparation method. Dissolve copper sulfate in hot water in a non-metallic container, filter it, and then add 90% of the total water. Filter out any residue. Then, mix the lime solution (with 10% water) into a large tank or cement tank, and slowly pour the copper sulfate solution into it while stirring continuously for 2–3 minutes. Never pour the lime into the copper sulfate solution, as this can cause rapid precipitation and increase the risk of phytotoxicity.
3. Choose the right time for spraying. Avoid using Bordeaux mixture within one month after flowering or when the fruit is nearly mature, to prevent surface discoloration or damage. During this period, alternatives like carbendazim or thiophanate can be used instead.
4. Adjust concentrations based on tree species. Sensitive trees like peaches, apricots, and plums should avoid Bordeaux mixture during the growing season. For other trees, adjust the formulation and ratio accordingly. For example, lime-type or multi-type mixtures are suitable for copper-sensitive species, while lime-equivalent types work well for pears. Grapes, which are sensitive to lime but tolerant of copper, can use a half-strength lime ratio.
5. Be cautious when mixing. Bordeaux mixture should not be mixed with sulfur-based products or antibacterial agents. When mixing with organophosphorus pesticides, ensure sufficient time between applications—wait 10 days after sulfur and 20–30 days after Bordeaux before applying sulfur again.
6. Monitor weather conditions. Spray in clear, dry conditions, avoiding high temperatures during midday in summer to prevent lime-related toxicity. During rainy seasons, try to avoid spraying during bad weather, and consider increasing the lime proportion slightly when preparing the mixture.
By following these guidelines, farmers can safely and effectively use Bordeaux mixture without causing harm to their crops.