First, yellow dragon disease, caused by the entire plant or partial shoot yellowing, manifests as young trees showing overall yellowing or mottled branches with a yellowish appearance. This condition is often due to viral infection and can significantly affect tree health.
To manage this issue, it's crucial to use disease-free seedlings and isolate planting areas. Timely removal of infected shoots is essential, and any diseased plants should be cut and burned immediately to prevent further spread. This helps in controlling the source of infection effectively.
Second, fertilizer damage can also lead to yellowing of the entire plant or parts of the shoots. Symptoms include yellowing in both young and mature trees, often due to improper fertilization practices.
For this problem, it's important to excavate and prune rotting roots to healthy sections. Apply fungicides at the right time to restore root vitality. Organic fertilizer should be applied appropriately based on the tree’s age, vigor, fruit load, and soil conditions. Avoid over-fertilizing, and ensure that animal manure is properly composted before application. Foliar fertilizers can also be used to enhance nutrient uptake. Fertilization should be done in designated ditches, followed by soil coverage and proper irrigation to avoid root damage and water stress.
Third, waterlogging can cause yellowing of shoots, similar to symptoms seen in fertilizer damage. The identification process is comparable, but the focus here is on drainage management.
The approach to treating root rot is similar to that for fertilizer damage. However, special attention should be given to the orchard's drainage system—opening ring ditches and central ditches to manage water accumulation after rain and ensure adequate watering during dry periods. This helps maintain strong root vitality.
Fourth, poor soil quality with less than 2% organic matter can lead to yellowing due to nutrient deficiencies such as magnesium and zinc. Symptoms include yellow pigmentation and leaf discoloration in both young and mature trees.
To address this, expand planting holes, turn over the soil, and apply organic fertilizer to increase organic matter content to at least 2.5%. Foliar sprays containing nitrogen, phosphorus, potassium, calcium, magnesium, and zinc can help improve tree growth and overall health.
Fifth, other external factors can also cause shoot yellowing:
1. Excessive use of herbicides can harm beneficial bacteria, contaminate soil, and lead to yellow shoots. This results in soil degradation, compaction, reduced oxygen levels, and poor root development, which ultimately causes yellowing.
2. Nematode infestation damages root vitality, causing root nodules and poor nutrient absorption. To control nematodes, ensure all animal manure is well-composted, and apply appropriate nematicides like Miller or Kedan according to recommended rates.
3. Overly heavy ring cutting or damage from pests like longhorn beetles can cause yellowing. Adjust ring cutting based on tree age, vigor, and fruit load, and monitor pest activity regularly to prevent damage.
4. On certain rootstocks, excessive fruiting can weaken the tree. Manage fruit load appropriately, apply balanced fertilization, and ensure proper irrigation to maintain tree health and prevent premature aging.
Sixth, comprehensive measures are essential for preventing yellowing in trees. The main principle involves using disease-free seedlings, avoiding materials from infected areas, and following standardized citrus cultivation practices. Focus on implementing three key measures: balanced nutrition, improved orchard management, and effective irrigation and drainage systems.
1. Implement a full nutritional formula for citrus. Balanced fertilization is critical, including "cold fertilizer," "strong bud fertilizer," "strong fruit fertilizer," and "autumn shoot fertilizer." The recommended ratio of nitrogen, phosphorus, potassium, calcium, and magnesium is 1: (0.3–0.4): (1–1.2): (0.4–0.5): (0.14–0.15). Use both top-dressing and foliar fertilizers as needed.
2. Reform orchard cultivation and management. Expand soil, improve soil structure, and incorporate organic matter. For young trees, promote root development through deep trench fertilization. For mature trees, prune roots after harvest and update the root system. During cold seasons, improve soil pH with lime, and apply root-strengthening agents and foliar sprays to enhance tree resilience.
3. Prune trees to improve light and air circulation, promoting better growth and reducing disease risk. Ensure efficient irrigation and drainage systems to maintain optimal moisture levels.
4. Perform thorough winter cleaning of the orchard and implement frost protection measures. Apply organic-based cold fertilizers in deep trenches before winter, trim diseased branches, and burn or bury fallen fruits. Use high-quality pesticides and apply trunk whitening treatments to protect trees from cold and pests. A typical whitening agent formula includes 10 kg of water, 4 kg of quicklime, 1 kg of sulfur, and 0.1 kg of salt.
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