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Identification of apple diseases caused by lack of factors
Apple orchards can suffer from a variety of physiological disorders caused by nutrient deficiencies in the soil or imbalances in tree physiology. These conditions often lead to stunted growth, abnormal leaf and fruit development, and ultimately impact yield, appearance, and fruit quality. Common issues include apple leaf spot disease, yellow leaf disease, fruit shrinkage, and bitter pit disease.
(1) Apple Leaf Spot Disease: This condition is primarily caused by a deficiency of zinc in the soil, commonly found in sandy or alkaline soils. It affects new shoots and leaves, causing them to become narrow, elongated, and curled at the edges. Leaves turn pale green or develop uneven coloration, becoming brittle and prone to early fall. The internodes are shortened, and overall growth is weakened. Affected trees have fewer flowering buds, resulting in small, pale flowers that struggle to set fruit. The fruits produced are small and misshapen, while young saplings show poor root development and older trees may experience root rot and sparse crowns, leading to low yields.
(2) Yellow Leaf Disease: This is a physiological disorder caused by a lack of iron, typically occurring in saline or calcareous soils. Seedlings and young trees are most affected. Symptoms start with the youngest leaves, which turn yellow while the veins remain green, forming a net-like pattern. As the disease progresses, the leaves lose their color, drop prematurely, and the tips of new shoots die back. This condition is more severe in the upper canopy than in the lower parts. In seedlings, symptoms appear first on the leaves at the tip of the shoots, showing similar patterns.
(3) Apple Fruit Shrinkage: This is a physiological issue caused by boron deficiency, commonly seen in mountainous or sandy soils, especially during dry years. It affects the fruit from flowering through harvest, leading to fruit drop, reduced yield, and poor quality. Some varieties also show symptoms on shoots, buds, and leaves. There are three main types of fruit shrinkage:
- Dry-Spot Type: Appears about two weeks after flowering, with brown spots forming on the shaded side of young fruit. The flesh becomes translucent, then turns brown and shrinks, creating dry, sunken areas. The fruit becomes hard, rough, and unpalatable, often falling prematurely.
- Cork Type: Begins after fruit set and becomes more common in later growth stages. Watery spots form in the flesh, turning brown and soft. Corky tissue develops along the fruit’s core, making it bitter. Early infections result in small, deformed fruit, while late cases may cause only slight irregularities. Red varieties may show premature color change and easy fruit drop.
- Rust Type: Seen mainly in susceptible varieties, this type features brown, horizontal stripes around the stem area. The skin cracks but the flesh remains soft and tasteless.
(4) Bitter Pit Disease: Caused by calcium deficiency in the tree, this condition is worsened by excessive pruning, overuse of nitrogen fertilizer, and poor water and nutrient management. Varieties like Guoguang, Green Banana, Gold Crown, and Red Star are particularly vulnerable. Symptoms begin when fruit nears maturity and continue during storage. Initially, dark-colored lesions appear on red fruits, or dark green on green or yellow-green fruits, surrounded by a halo. Lesions become sunken and brown, with spongy, bitter necrotic tissue beneath the skin. In later storage stages, the affected tissue is prone to bacterial infection and rot.