Answer: Insecticides will kill insect pests, but I'm not sure that's what is causing the brown spots on the leaves. Leaf spot and root rot diseases attack ligustrum. Leaf spot fungi attack leaves and can cause premature leaf drop. The disease spots are usually light to dark brown with a yellow border. Control measures include destroying infected leaves and applying fungicide. Both are necessary for control.
Ligustrum root rot can be caused by several disease organisms. Infected plants may become unhealthy, making little growth, and/or part or all the plant will suddenly wilt and die. Examination of the root system reveals few if any remaining healthy, white functional roots. No control measures are effective once the plant is infected.
Hope this information helps you determine just what the problem might be. Good luck with your shrubs.
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