The degree of fruit coloration is closely linked to sugar content. Any factor that enhances the sugar levels in fruits can significantly improve their coloration. To achieve a more vibrant red color in apples, several key strategies are commonly employed:
First, selecting high-quality varieties and rootstocks plays a crucial role. Popular choices include 'Marshal', 'Red Fuji', 'Red Jonagold', and other new red-bud cultivars, which are known for their superior coloring potential.
Second, improving light exposure is essential. Pruning is a fundamental practice to ensure adequate sunlight reaches the fruit-bearing areas. This involves minimal cutting, controlling excessive branches, and conducting seasonal pruning. In April and May, bud rubbing helps reduce overcrowding, while June brings tip pinching and September features thinning to maintain an open canopy. Proper pruning ensures leaves don’t block sunlight, promoting anthocyanin development.
Additionally, leaf and fruit thinning during the coloration period can enhance fruit appearance. Removing shaded or unhealthy leaves close to the fruit improves direct light exposure. Reflective films, such as silver ones laid on the ground under the tree canopy after mid-September, can further boost coloration by reflecting light onto the lower parts of the tree. Studies show that this method increases redness by 45% compared to bagging alone, with a 12.6% improvement in the coloring index.
Third, applying foliar fertilizers and growth regulators can support fruit quality. Spraying products like 2116 fruit tree fertilizer twice between May and July helps supply trace elements and regulate tree growth. CA2000 is also used to prevent scab and improve internal fruit quality. However, these should not be mixed, as it may reduce effectiveness. Spraying 2116 two weeks before harvest can advance coloration by 12–17 days and increase the coloring index by 24.2%. Ethephon, when combined with sodium naphthalene acetate, can promote color and reduce fruit drop, but it's best used on vigorous trees.
Finally, managing fertilization and irrigation is vital. Overuse of nitrogen can lead to poor coloration and reduced sugar content, especially in red apple varieties. Therefore, nitrogen application should be carefully timed. Research from Laiyang Agricultural College shows that applying nitrogen in late May results in better coloration than later applications. After September, water should be controlled, with small, frequent irrigation to avoid cracking and enhance color development.
By combining these practices—varietal selection, light management, foliar treatments, and proper nutrition and irrigation—growers can significantly improve the color and quality of their apple crops.
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