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Bordeaux mixture is a broad-spectrum, protective fungicide that has been widely used in fruit tree cultivation for many years. It is known for its long application history, broad usage range, strong efficacy, and the fact that resistance has not yet developed. The active ingredient in Bordeaux mixture is basic copper sulfate. However, improper preparation or application can lead to phytotoxicity, which may damage the fruit and reduce its market value. Therefore, it's important to understand the causes and how to prevent them.
Phytotoxicity from Bordeaux mixture can occur due to several reasons. First, if copper sulfate isn't fully dissolved during preparation, undissolved particles can remain in the solution. When sprayed on fruit, these particles may cause reddish-brown spots or freckles, lowering the fruit’s commercial value. Second, applying Bordeaux mixture during the early stages of fruit development or when fruits are ripening can result in rust spots, rough skin, and reduced smoothness, affecting the fruit’s appearance.
Third, certain fruit trees such as peaches, apricots, and plums are highly sensitive to copper ions. Using Bordeaux mixture during their growing season can lead to copper toxicity, causing leaf drop and fruit loss. If the lime content in the mixture is too low, apple varieties like Golden Crown, hawthorn, and persimmon trees are more likely to suffer from phytotoxicity. Conversely, if the lime content is too high, grapes, longan, non-nuclear purple, and other varieties may also be affected.
Fourth, mixing or alternating Bordeaux mixture with sulfur-based products or antibacterial agents without proper intervals can increase the risk of phytotoxicity. Lastly, applying Bordeaux mixture under high temperatures can lead to lime-related phytotoxicity. If the spray remains wet after application, especially before rain or when leaves are still damp, copper may dissolve more easily, increasing the concentration of soluble copper on the leaves and causing burns. After some time, wind and rain can further increase the soluble copper content on the leaves, leading to drying and damage.
To prevent phytotoxicity, the following measures should be taken:
1. Use high-quality materials: Choose pure, white, light-weight lump lime and blue copper sulfate crystals. Avoid using yellow or green powders.
2. Follow the correct preparation procedure: Dissolve copper sulfate in hot water first, filter the solution, then add 90% of the total water. Prepare the lime solution separately and slowly pour the copper sulfate into it while stirring continuously. Never pour lime into copper sulfate, as this can cause rapid precipitation and phytotoxicity.
3. Choose the right timing: Avoid using Bordeaux mixture one month after flowering or near harvest time to prevent surface rust and color issues. During this period, use alternatives like carbendazim or thiophanate.
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 according to their sensitivity. For example, use lime-type or multi-type mixtures for copper-sensitive trees, and lime-equivalent for less sensitive ones like pears. Grapes, which are sensitive to lime but tolerant of copper, can use a half-strength lime ratio.
5. Mix carefully: Do not combine Bordeaux mixture 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 only during clear, dry periods, avoiding midday heat in summer. In rainy seasons, try to avoid spraying when rain is expected and increase the lime proportion slightly to reduce the risk of damage.
By following these guidelines, farmers can safely and effectively use Bordeaux mixture while minimizing the risk of phytotoxicity.