Answer: The sucker is coming from the rootstock and will not be a lisbon lemon but another type of citrus. If the top of your tree died, the roots are trying to send up more leaves so it can survive and that's why it sent up the sucker. You should cut the sucker off from where it emerges on the trunk or soil. Without leaves you lisbon lemon tree won't survive, so do what you can to encourage new growth. Water it well, fertilize it lightly and prune it back a little. With any luck that will shock it into developing new leaves and flowers. Hope this information helps.
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