How to prevent and control citrus yellow leaf disease? A detailed guide to practical techniques

Citrus yellow leaf disease, also known as huanglongbing, yellow shoot disease, yellow blight, green fruit disease, etc., is a devastating disease in citrus production worldwide. It is 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. Let's take a look at citrus yellow leaf disease prevention and control techniques together below!

Symptoms of damage caused by citrus yellow leaf disease

1. Mottled yellowing: After mottled yellowing leaves turn green, local areas fade, forming mottled yellowing. The mottled positions and shapes are very irregular, appearing foggy with no clear boundaries. Most mottling originates from leaf veins, the base or the edges.

2. Uniform yellowing: Uniform yellowing mostly appears on late autumn shoots that sprout 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 of the tree.

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

Conditions for the onset of citrus yellow leaf disease

1. Presence of disease sources in or near the orchard, namely yellow leaf disease-infected trees.

2. Presence of transmission vectors in or near the orchard, namely the Asian citrus psyllid.

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

4. Uneven growth in the orchard with tender shoots emerging at any time can attract Asian citrus psyllids to come and feed and transmit the disease.

Citrus yellow leaf disease prevention and control techniques

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 seedlings.

① Nursery sites should be selected in disease-free areas or locations with good isolation conditions, or use plastic mesh enclosures for closed seedling cultivation.

② Establish a citrus virus-free breeding system. Any selected superior 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 breed disease-free seedlings.

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

④ Strengthen nursery management systems.

3. Control the Asian citrus psyllid

① Strengthen cultivation management. Each new shoot flush should be uniform, and pesticide application should be unified for control. The first pesticide application should be done at the budding stage to prevent adults from laying eggs in the gaps of buds. Pesticides should be mainly organophosphates, and options include isocarbophos, omethoate or mineral oil emulsion.

② Strengthen winter orchard cleanup spraying to eliminate Asian citrus psyllids with poor activity during the overwintering period, which is the key to year-round control.

③ Adjust the fruit tree variety structure. Do not plant Rutaceae plants such as wampee and Murraya in citrus production areas to eliminate alternative hosts for the Asian citrus psyllid.

4. Prevention first, integrated control. Persist in conducting comprehensive inspections for huanglongbing-infected trees after each new shoot flush turns green. Remove each infected tree found, leaving no stump. Currently, the use of physical and biological integrated methods, including low-temperature plasma compound preparations, has shown good results in prevention and control.

5. When rebuilding citrus orchards in diseased areas, diseased and old trees should be removed entirely, the environment cleaned up, and short-term crops planted for one year before replanting citrus. Isolated planting can also be adopted.

6. Develop citrus production through industrialized management methods, 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 in terms of timing (growth stages): The flower bud stage and leaf expansion stage are key periods susceptible to infection. The Chinese herbal medicine Wofengsu has the characteristic of high safety. 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 harboring, and plays a major role in addressing the pathogen base and disease severity in the following year.

② Three-dimensional prevention and control of different parts of the plant (spatially): 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 active ingredients of the solution, and during the conduction process it can kill latent pathogens (in the phloem and overwintering).

③ Three-dimensional prevention and control in terms of application methods: Chinese (herbal) medicines have the characteristic of nutritional revitalization. They prevent and control diseases through two functions: revitalizing to improve immunity and sterilizing. Root irrigation, orchard cleanup (trunk and branch spraying) and foliar spraying are the three-dimensional pathways to achieve these two functions.

8. Combining Chinese and Western approaches: Chinese (herbal) medicine preparations have the advantages of being compounded from multiple ingredients, regulating growth, improving crop immunity, broad-spectrum sterilization, long-lasting efficacy, and preventing and treating diseases from inside out. Conventional single-ingredient pesticides have the characteristic of rapid action (with relatively high resistance). Combining Chinese and Western approaches and using them in compound formulations facilitates complementary advantages and ensures effectiveness in treating severe cases.