How to prevent and control brown spot and defoliation disease in golden privet (Ligustrum × vicaryi)

Some years ago, because golden privet (Ligustrum × vicaryi) has advantages such as easy propagation, drought resistance, few pests and diseases, tolerance to pruning, a long golden-leaf period, and good effects in large color-block landscape grouping, it was widely planted, especially for patterned flower-bed shapes and large color blocks, where the landscape effect was particularly good. However, due to excessively high planting density and inadequate maintenance and management, brown spot and defoliation disease occurred every year during the rainy season (August to September in the Luoyang area), causing leaves to fall one after another, new leaves and tender buds to fail to emerge in time, and even entire plants to die in patches, severely damaging the landscape effect. In response to this situation, in 2003 we observed and recorded the growth and disease occurrence of golden privet, invited experts to identify the pathogen, and adopted comprehensive preventive measures, effectively reducing the harm of this disease during the peak period of the rainy season.

Disease symptoms

In the early stage of leaf disease, water-soaked, chlorotic small round spots appear, which then turn purple or brown and gradually expand into round, oval, semicircular or irregular lesions, 5 to 10 mm × 3 to 5 mm in size; in the later stage, the center of the lesion on the upper leaf surface is light yellowish brown or grayish white, slightly raised, with a brown edge that is darker than the center, and relatively obvious concentric rings appear on the lesion, with irregular water-soaked light brown rot around the lesion; the center of the lesion on the lower leaf surface is sunken and lighter in color than the upper surface; under humid weather conditions, many small black mold spots are scattered on the lower leaf surface, and in severe cases a small number of black mold spots also appear on the upper surface of the lesion. The disease first appears on old leaves, leaf tips, leaf margins and leaf bases, then gradually develops toward the middle; in severe cases the lesions join together and reach 1/2 to 2/3 of the entire leaf, at which point the leaves fall one after another with only a gentle touch to the plant.

Pathogen

After identification, the pathogen of this disease is Alternaria, belonging to Deuteromycotina, Hyphomycetes, Moniliales. Genus Alternaria.

Disease occurrence pattern

The pathogen overwinters as mycelium and conidia on diseased fallen leaves and dead branches; conidia are spread by airflow and rainwater and invade through wounds and stomata, with an incubation period of 10 to 20 days; the optimum growth temperature for the pathogen is 25°C, and the optimum temperature for conidial germination is 20°C to 29°C.

Excessive application of nitrogen fertilizer, deficiency of phosphorus and potassium fertilizer, soil compaction, poor drainage, overly dense planting, poor ventilation, insufficient fertilizer and water in summer, poor tree vigor, and abundant rain in autumn all aggravate the occurrence of this disease.

Comprehensive control

1. Ensure proper planting: the planting site should be chosen with slightly higher ground in the middle and lower ground around it to prevent water accumulation; the planting density should preferably be 36 to 49 plants per sq m. It should not be too dense. As the seedlings grow, they should be thinned in time, pruned, and have leaves removed.

2. Reduce sources of infection: in autumn, clear away diseased fallen leaves, dead branches and dead plants; before sprouting in spring, spray 5 to 7 degrees Baumé lime sulfur mixture to kill overwintering sources of inoculum; combined with pruning, remove diseased branches, diseased leaves, densely planted seedlings that affect ventilation and light penetration, and random-growing branches.

3. Strengthen maintenance and management: apply an appropriate amount of compound fertilizer in spring, topdress with phosphorus and potassium fertilizers such as potassium dihydrogen phosphate and potassium sulfate in early summer, and during summer irrigation adhere to the principle of fewer times but sufficient amount, watering thoroughly each time, avoiding spray irrigation from the top of the plants; pay attention to drainage during the rainy season in autumn.

4. Chemical control: from mid-May, spray once every 10 days for 2 consecutive times; from mid-June to mid-August, once every 7 days; from early September to late September, once every 10 days for 2 consecutive times.

Through experiments, we selected several pesticides with remarkable control effects: before disease onset, spray systemic fungicides such as carbendazim at 300 to 500 times, and thiophanate-methyl, Likejun, and Jiameling at 700 to 1000 times; at disease onset, use protective fungicides such as 1% lime equivalent Bordeaux mixture, 50% mancozeb WP at 300 to 500 times, 50% procymidone WP at 700 to 1000 times, and chlorothalonil at 500 to 700 times; to avoid the pathogen developing resistance, protective agents and systemic agents should be used in rotation.

5. Biological agent control: such as Telike and X8, although their current scope of application is not yet very wide, they have already shown obvious advantages, and practice has proven that the effect is relatively good.