Latest Prevention and Control Techniques and Methods Tutorial for Citrus Yellow Leaf Disease

Citrus yellow leaf disease is also known as huanglongbing, yellow shoot disease, yellow blight, green fruit disease, etc. It is a devastating disease in citrus production worldwide, caused by a gram-negative bacterium limited to parasitism within the phloem, and can infect a variety of Rutaceae plants including Citrus, Poncirus, Fortunella, and Murraya paniculata. Let us take a look at the prevention and control techniques for citrus yellow leaf disease below!

Damage Symptoms of Citrus Yellow Leaf Disease

1. Mottled yellowing: After the leaves with mottled yellowing turn green, parts of them fade again, forming mottled yellowing. The positions and shapes of the mottling are very irregular, appear foggy, and have no clear boundaries. Most mottling originates from the veins, base, or edges.

2. Uniform yellowing: Uniform yellowing mostly appears on autumn shoots and late autumn shoots that emerge after local temperature drops in autumn. The new shoot leaves do not turn green and gradually form uniform yellowing, mostly appearing on the outer canopy, sunny sides, and top.

3. Nutrient-deficiency-like yellowing: Nutrient-deficiency-like yellowing is not true nutrient deficiency. It is caused by local root rot due to huanglongbing, which reduces nutrient absorption capacity and causes leaf nutrient deficiency. It mainly manifests as symptoms similar to zinc deficiency and manganese deficiency, and is an auxiliary symptom for identifying huanglongbing.

Conditions for the Onset of Citrus Yellow Leaf Disease

1. There is a source of disease in or near the orchard, namely yellow leaf disease-infected trees.

2. There is a virus-transmitting vector in or near the orchard, namely the Asian citrus psyllid.

3. The Asian citrus psyllid can carry the virus after feeding on diseased trees for a cumulative total of 5 hours.

4. The orchard is uneven in growth, with tender shoots emerging at any time, which can attract Asian citrus psyllids to come and feed and transmit the virus.

Prevention and Control Techniques for Citrus Yellow Leaf Disease

1. Strict quarantine system to prevent diseased seedlings and diseased scions from being introduced into disease-free areas and new planting areas.

2. Cultivate disease-free nursery stock.

① Nursery land should be selected in disease-free areas or places with good isolation conditions, or use plastic net sheds for closed seedling cultivation.

② Establish a citrus virus-free propagation system. All selected fine variety lines must be tested through indicator plants or polymerase chain reaction (PCR). Obtain shoot-tip seedlings through shoot-tip grafting virus elimination technology, and operate according to virus-free procedures to propagate disease-free nursery stock.

③ When the “system” has not been established, rootstock seeds should be collected from the fruit of disease-free trees. Soak the seeds in hot water at 50-52°C for 5 minutes, then after preheating, soak them in hot water at 55-56°C at constant temperature for 50 minutes. Scions should be collected from identified disease-free mother trees, soaked in 1000-fold tetracycline hydrochloride solution for 2 hours, then rinsed clean with water before grafting.

④ Strengthen the management system of the nursery.

3. Control the Asian citrus psyllid

① Strengthen cultivation management so that each new shoot flush emerges uniformly, and apply pesticide uniformly for control. The first pesticide application should be made at the budding stage to prevent adults from laying eggs in the bud gaps. Pesticides are mainly organophosphorus compounds, and options include isocarbophos, omethoate, or mineral oil emulsion, etc.

② Strengthen winter orchard cleaning and pesticide application to eliminate Asian citrus psyllids with poor activity during overwintering, which is the key to year-round control.

③ Adjust the structure of fruit tree varieties. Do not plant Rutaceae plants such as wampee and Murraya paniculata in citrus production areas, to prevent Asian citrus psyllids from transferring to alternative hosts.

4. Prevention first, integrated control. Persist in conducting a comprehensive inspection of huanglongbing-infected plants after each new shoot turns green. When one is found, dig it out, leaving no residual stump. At present, the use of integrated physical and biological methods, including low-temperature plasma compound preparations, has relatively good results in prevention and control.

5. When rebuilding citrus orchards in diseased areas, the diseased and old trees should be dug out across the whole area, the environment cleaned up, and a one-year short-term crop planted first before planting citrus again. Isolated planting can also be adopted.

6. Develop citrus production through industrialized management, achieving unified planning of development scale, unified varieties within the park, unified measures in production, unified technical guidance, and unified procedures in management. This can effectively control the disease incidence rate.

7. Three-dimensional prevention and control

① Three-dimensional prevention and control during the growth period (in terms of time). The flower bud stage and leaf expansion stage are key periods when infection readily occurs. The Chinese herbal medicine Wofengsu has the characteristic of high safety, and appropriate high-concentration use at this time plays a major role in disease prevention and control throughout the entire growth period. After fruit harvest in autumn is a key period for pathogen reproduction and latent survival, and plays a major role in addressing the pathogen base and degree of disease damage in the coming year.

② Three-dimensional prevention and control at different parts of the plant (in terms of space). The Chinese (herbal) medicine preparation Kuifuling has the characteristic of good conductivity. The roots, bark, leaves, and fruit of the tree can all absorb the effective ingredients of the medicinal liquid, and during the conduction process it can kill latent pathogens (in the phloem and overwintering).

③ Three-dimensional prevention and control in terms of usage. Chinese (herbal) medicines have the characteristic of nutritional rejuvenation. They prevent and control diseases through two functions: improving immunity through rejuvenation and sterilization. Root irrigation, orchard cleaning (branch and trunk spraying), and spraying are three-dimensional pathways for achieving these two functions.

8. Combination of Chinese and Western: Chinese (herbal) medicine preparations have the advantages of being compounded from multiple ingredients, regulating growth, improving crop immunity, broad-spectrum sterilization, long lasting effect, and preventing and treating diseases from the inside out, while conventional single-ingredient pesticides have the characteristic of quick effect (with relatively greater resistance). The combination and compound use of Chinese and Western medicines is conducive to complementary advantages and ensures effectiveness in treating severe cases.