1. Variety Description

Sour apple is a variety with large, sweet, crispy fruits, beautiful color, and low susceptibility to pests. It is particularly well-suited to hilly garden soils, drought-resistant, and highly productive under intensive farming. Recently, this variety has become popular and is widely cultivated in the midland and mountainous provinces of Northern Vietnam. The apple trees sold at the center are grafted varieties with large fruits, high yield, and excellent fruit quality. They start bearing fruit 12–14 months after planting, and if pruned early, they may fruit even sooner.

Morphological Characteristics:
The tree grows upright or spreads out widely with drooping branches. The branches are twisted and dense. Leaves are alternate, oval or elongated elliptical, 2–4 cm long, dark green, glossy on the upper surface, with three distinct longitudinal veins and clearly serrated edges. Flowers are small, have five petals, light yellow, and grow in clusters of 2–3 in the leaf axils.

Ecological Characteristics:
This tree is easy to grow and adapts well to various soil types. It has high and stable productivity and is not prone to serious pests. If pruned early, it will bear fruit sooner the following year.

2. Planting and Care Techniques

Planting Time and Density

The best planting season is at the end of the rainy season, ideally from November to December, as the warm weather allows the trees to develop quickly in the following spring.

Planting Method and Density

• Planting distance: 4 x 5 meters

• Digging planting holes: 40 x 40 x 40 cm

• Fertilizer application per hole:

  • 5–20 kg of decomposed manure

  • 0.5 kg of superphosphate

  • 0.3 kg of lime powder

• Mix all fertilizers thoroughly with soil and place them in the hole, forming a mound 20 cm above ground level (do not plant trees directly in the fertilizer).

• If manure is unavailable, 5–7 kg of bio-phosphate fertilizer can be used per hole.

Care and Irrigation

• Watering schedule:

  • First week: Water once daily in the morning or afternoon until the soil is well-moistened.

  • Following weeks: Water every 2–3 days until the end of the first month. Afterward, watering frequency can be reduced but the soil must remain consistently moist.

• Apples require ample water during growth stages, especially before flowering. Drought can lead to smaller fruits, thicker peels, and lower-quality taste.

• Annual fertilization after harvest and pruning:

  • Organic manure: 30–50 kg

  • Superphosphate: 5–8 kg

  • Potassium chloride: 3–5 kg

  • Urea fertilizer: 0.5–1 kg

Pruning Techniques

Apple fruits grow on new branches from early spring, so pruning is essential to encourage strong branch growth for higher productivity. There are two pruning methods:

• Light Pruning: Done annually after harvesting to ensure stable and high yields.

• Heavy Pruning: Applied to young trees (1–3 years old) or older trees that need canopy restructuring. All branches are cut, leaving only three strong main branches from the previous year to encourage better fruiting.

3. Pest and Disease Control

Common Diseases

• Root Rot and Trunk Cracking:

  • Symptoms: Sparse foliage, pale green leaves that fall off, and branches dying from the tip downward. The root system decays, leading to tree death.

  • Prevention: Avoid excessive moisture around the roots; monitor for early signs of cracks and blackened wood tissue.

• Branch Dieback:

  • Caused by the fungus Colletotrichum cloeosporiodes, which invades branches and causes them to dry out and die. The fungus also attacks mature fruits, leading to rot.

  • Prevention: Avoid prolonged direct sunlight exposure and remove infected branches.

• Fruit Disease:

  • Fungal infections during fruit development cause small black spots and cross-like scars on the fruit peel, reducing fruit quality and market value.

  • Prevention: Maintain good air circulation, remove infected debris, and spray fungicides after fruit setting.

• Powdery Mildew on Leaves:

  • This disease is not severe as it mainly occurs when the tree is fruiting, affecting older leaves.

  • Best prevention: Graft trees after September and avoid early pruning, as young shoots exposed to temperatures below 20°C are more susceptible to infection.

• Fruit Rot:

  • This disease commonly appears when fruits are nearing ripeness.

  • Infected fruits rot rapidly—within one week, an entire tree’s fruit may be affected.

Common Pests

• Root-Damaging Insects:

  • Includes termites, grubs, crickets, ants, and mealybugs, usually found in the 0–50 cm soil layer.

  • Symptoms: Yellowing leaves, weak tree growth, and eventual tree death.

• Stink Bugs:

  • These insects suck sap from young shoots and leaves, causing wilting, deformity, and premature fruit drop.

  • Damaged fruits develop black spots, reducing yield and fruit quality.

• Branch-Boring Weevils:

  • These pests create small holes in branches and stems, leading to weakened growth and brittle branches.

  • They are most active from mid-rainy season to early dry season.

Control Methods

• For sap-sucking insects: Use Sherpa (0.1%), Trebon (0.1–0.2%), Depterx (0.2%), Dantinol (0.1–0.2%), Monitor (0.1–0.2%), Bi58, or Báudin.

• For leaf-eating insects: Use Azodrin 50DD (0.2%), Score (0.05%), Aliette (0.3%), or Mancozeb (0.25%).

• For root-damaging pests (termites, ants, beetles): Use Báudin, Lidanfor, or Sevidol. Mix pesticide with sand in a 1:10 ratio and sprinkle around the base of the tree and planting hole.

4. Harvesting

Sour apples are harvested 2–3 months after flowering, when the fruits are large, firm, and brightly colored. The harvest is done manually, ensuring that fruits are not bruised during transportation.