Apple trees, once they enter the full fruiting stage, naturally experience variations in yield from year to year—often referred to as "big and small years." In years with heavy fruit production, the tree is burdened with too many fruits, which may lead to high yields but poor fruit quality. This not only reduces economic returns but also weakens the tree’s health, potentially leading to premature aging or even death. To prevent such issues, it's crucial for apple growers to manage tree load carefully by thinning fruits, ensuring stable, high-quality, and high-yield production.
The balance of fruit load directly affects tree vigor and overall orchard management. If a tree has low vitality, insufficient water and nutrients, or reduced photosynthetic capacity, its fruit load should be smaller. Conversely, if the tree is strong and well-nourished, it can handle a higher fruit load.
According to international research, each 100 grams of apple requires about 200 square centimeters of leaf area. For every additional 25 grams of fruit, an extra 75 square centimeters of leaf area is needed. To ensure consistent production, the leaf area must increase by 200 square centimeters per 100 grams of fruit. A typical apple weighing around 175 grams would need a total of 625 square centimeters of leaf area to support that year's yield without compromising next year’s.
To achieve a balanced fruit load and avoid the cycle of big and small years, proper fruit thinning is essential. Spacing out flowers and fruits helps save tree energy, increase fruit size, and improve fruit set rates. This process is typically done during the flowering season when conditions are favorable and there is no risk of late frost. The timing usually starts early in the bloom period and continues until the end.
When thinning flowers, it's common to remove excess inflorescences using methods like "single-to-one" or "several-to-several." Additionally, some outer flowers may be removed to promote better distribution. These practices have been proven effective in improving fruit quality and tree health.
Fruit thinning generally begins seven days after flowering and should be completed within 30 days. There are several ways to determine how many fruits to keep:
1. **Fruit-to-fruit ratio**: A common guideline is 25–30 fruits per one. For varieties like Qiaohua Anvil (e.g., Marshal system), the ratio is often 35–40:1.
2. **Top bud method**: For certain varieties, such as the Marshal Department, one fruit is left per four top buds, resulting in about 15,000 fruits per 1,000 square meters. Other types, like land Austria, leave one fruit per five top buds, averaging around 12,000 fruits per 1,000 square meters.
3. **Dry week method**: Based on research from the Chinese Academy of Agricultural Sciences, this method helps determine the optimal number of fruits based on environmental factors.
4. **Spacing method**: Larger fruits and weaker trees should have more space between them. For example, large fruits like Marshal or Fuji should be spaced 20–25 cm apart. When thinning, remove small, deformed, or diseased fruits, keeping only the largest and most upright ones. Inner and lower parts of the canopy should be sparsely pruned, while the upper and outer parts can be more densely kept.
The key to successful fruit thinning is timing—early intervention is always better. In stable climates, thinning can begin immediately after flowering. However, in areas with unpredictable weather or frost risks, it's safer to preserve more flowers first and thin later.
**Chemical thinning methods** are also widely used:
1. **Chemicals and concentrations**:
- Carbaryl at 1000–2000 ppm
- Naphthaleneacetic acid at 10–20 ppm
- Ethephon at 150–200 ppm + naphthalene acetic acid at 7–10 ppm
- Trichlorfon (90%) at 1000 times, sprayed two weeks after full bloom
- Guoguang: Cevaine at 2000 ppm 10 days after blooming
- Ethylene at 300 ppm + naphthalene acetic acid at 20 ppm
- Trichlorfon (90%) at 800 times + naphthalene acetic acid at 15–20 ppm
- Lime sulfur applied two days after blooming
2. **Preparation of chemicals**: Accurate mixing is critical. The concentration is measured in ppm (parts per million), where 1 ppm equals 1 gram of chemical per 1 million grams of water. For example, 25% carbaryl WP contains 0.25 kg of pure carbaryl per 1 kg of product.
Proper chemical application ensures effective fruit thinning without damaging the tree or reducing yield. By combining manual and chemical methods, apple growers can maintain a healthy balance, leading to better long-term orchard performance and higher profitability.
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