Answer: This type of symptom can be an indication of a variety of problems. One simple explanation is that there is physical damage to the branch either breakage or perhaps exposure to extra cold against a window pane or perhaps a hot draft from a heating vent.
Another explanation is a root problem corresponding to that branch, for example if a container does not have a functioning drainage hole and the soil has become waterlogged, or if the plant has too small a root system to support all its top growth. With citrus, you may find that you need to prune occasionally to keep the top and roots in balance.
Another possibility is that there is an insect such as spider mite or scale on that branch, or a fungal infection spreading slowly across the plant.
All of these cases would require some careful inspection and detective work to figure out the cause of the problem but I hope this gives you some ideas of what to look for.
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