How to prevent and control leaf spot disease in Chinese privet (Ligustrum quihoui)?
Ligustrum quihoui, a plant of the genus Ligustrum in the family Oleaceae, is an excellent colored shrub that has been widely planted in recent years. However, as the planting area has expanded and the planting years have increased, some of its shortcomings have gradually emerged. Especially with old seedlings, during the rainy season every year, leaf drop of varying degrees occurs, and it happens year after year, seriously affecting the landscape effect.
According to observation, the cause of this condition is golden privet leaf spot disease. Through preliminary observation and research, the pathogen of Ligustrum quihoui leaf spot disease belongs to the subphylum Deuteromycotina fungi. Round lesions are 4 mm to 5 mm in diameter and are scattered on the leaves.
Under humid weather conditions, a black mold layer (conidia and conidiophores) appears on the lesions. The most suitable temperature range for the growth of this pathogen is 25°C to 30°C, the temperature range for conidial germination is 18°C to 29°C, and the temperature range for pathogen infection is 15°C to 27°C. The pathogen overwinters as mycelium and conidia on diseased plant residues in the soil, with a survival period of about 1 year. Conidia are mainly spread by airflow and rainwater, invading through stomata and wounds on the plant surface, or directly through the epidermis. The incubation period is 10 to 20 days. Under suitable temperature and with the presence of water, spores can germinate within a few hours, and the pathogen can cause repeated infections.
In July, the rainy season begins, and rainfall gradually decreases until September. During this period, high temperature and high humidity accelerate the occurrence of the disease. The disease begins to occur in late July and reaches its peak in mid-August, causing large-scale leaf drop. The plants gradually grow new leaves again from mid-to-late September. In plots where the disease occurs year after year, whether the newly grown leaves or the old leaves that have not fallen, the leaf color appears darkened. The older the seedling area, the more severe this phenomenon. Based on years of practice, the following prevention and control measures have been summarized.
1. The planting site should be conducive to drainage, and water accumulation must be avoided.
2. Planting density should be reasonable. The greater the plant density, the more severe the disease. As Ligustrum quihoui grows, combine pruning with reasonable branch training to enhance internal ventilation and light penetration and reduce humidity and temperature.
3. Reduce sources of disease infection. Remove diseased plant residues from the planting site and remove weeds at any time. This not only reduces infection sources but also increases ventilation and light penetration, helping to create a healthy growing environment for the plants.
4. Strengthen fertilizer and water management to increase the plant's own resistance. In road greening, the soil conditions are generally rather poor, and combined with insufficient fertilization, it is easy to cause plant malnutrition. Applying fertilizer once in spring and broadcasting compound fertilizer during the rainy season along with rain are both beneficial for improving the resistance of golden privet and promoting early recovery after disease.
5. Prune at the appropriate time and treat wounds promptly. Heavy pruning can be done in winter, but pruning should be reduced during the rainy season to reduce wounds and lower the chance of pathogens invading through wounds. Spray fungicide immediately after pruning. Practice has proven that the more frequent the pruning during the rainy season, the more severe the disease. In addition, pruning tools should be disinfected.
6. Chemical control. Starting from late June, spray once about every 10 days until the end of the rainy season in September. Optional agents include 50% mancozeb wettable powder at 500 times dilution, 40% Dafudan at 400 times dilution, 70% thiophanate-methyl wettable powder at 1000 times dilution, and 50% carbendazim wettable powder at 1000 times dilution. To prevent the development of pathogen resistance, these agents must be used alternately. Mixing the above agents with 400 mL of streptomycin gives better results.
7. Nursery land should practice crop rotation for more than 2 years, and diseased soil can be treated with formaldehyde solution.