What to Do About Leaf Drop in Golden Privet During the Growing Season? Causes and Control Measures
In 1999, golden privet began to be trial-planted in the Hengshui area of Hebei, and the planted area then increased rapidly. However, starting in 2002, a small number of local golden privets began to drop leaves during the growing season, showing patchy bare spots, and the problem kept spreading. By mid-June 2003, spots appeared successively on the leaves of golden privet over a large area; the spots appeared on old leaves later than on young leaves, then progressed to drying up, leaf drop, and the branches gradually dried up from top to bottom, with a small number dying. The dead patchy areas were mostly in the middle of the planting site, while the plants around the edges grew better. After making inquiries in many directions, the author found that there were mainly two causes of the leaf drop.
Caused by disease. According to expert research, this spot disease is caused by the pathogenic fungus Alternaria (Afternaria) in the order Hyphomycetales, subdivision Deuteromycotina. In the early stage, small brown spots appear on the leaves, surrounded by purple-red halos, and black mold-like substances are visible on the spots, namely the conidiophores and conidia of the pathogen. As the temperature rises, several lesions sometimes join together, and finally the leaves scorch and fall off. Alternaria fungi mainly overwinter as mycelium on diseased residues on the soil surface; conidia spread through airflow or contact between branches and leaves, and invade the host through wounds, stomata, or directly. The incubation period is 10 to 20 days. The optimum temperature for pathogen growth is 26°C, and the optimum temperature for spore germination is 18°C to 27°C. Under suitable temperature and high humidity, spores can germinate within a few hours. Last year, the Hengshui area entered the rainy season early in June, and it lasted until September. Because the plants were planted densely, with poor ventilation and light transmission, a relatively stable environment of high humidity and suitable temperature formed among the plants, which was very favorable for the germination and invasion of pathogen spores, causing the disease to break out severely.
Scale insect damage. Scale insects are relatively small, about 2 mm, and crawl quickly on branches before they become fixed. Where branches and leaves are too dense and ventilation and light transmission are poor, the damage from pests and disease is severe. This often causes leaf drop, and in serious cases it can also cause the plants to wither and die.
To control leaf drop in golden privet during the growing season, the author adopted the following control measures, which gave good results.
1. Choose the terrain well and get planting right. Keep the planting site on relatively high ground; it must never accumulate water. Plant at a reasonable density, and as the plants grow, thin out branches rationally to enhance internal ventilation and light transmission and reduce humidity.
2. Strengthen cultivation management and apply more organic fertilizer. Use nitrogen and phosphorus fertilizers together so that the seedlings grow vigorously and their stress resistance is enhanced. Avoid over-applying quick-acting nitrogen, which causes excessive vegetative growth.
3. Clean up the planting site and block the source of the pathogen. After each pruning, promptly remove the pruned leaves and branches from the planting site, and remove weeds at all times, always keeping the planting site clean and free of debris. This reduces overwintering mycelium and blocks the source of the pathogen.
4. Prune at the right time and treat wounds promptly. Once the rainy season begins, prune little or not at all to reduce wounds and lower the possibility of pathogen invasion. No matter when pruning is done, a fungicide should be sprayed immediately afterward for protection.
5. Spray regularly for prevention and control. Disease control must be based on prevention first, combining prevention and control. Starting in late June, spray a fungicide every 7 to 10 days, and spray an additional fungicide after each rain, until the rainy season ends in September. In the early stage of the disease, systemic fungicides such as carbendazim, thiophanate-methyl, and Likjun can be used. To control scale insects, imidacloprid can be used together with the fungicide before the insects become fixed. In the later stage, protective fungicides such as Bordeaux mixture and mancozeb can be used. To prevent the pathogen from developing resistance, systemic fungicides and protective fungicides can be used alternately.