Lime Tree in Container

Question

We have a lime tree in a container. The leaves have black on them. What is causing this?

Answer 1

That sounds like sooty mold. It is a type of fungus and will coat branches and leaves in black powder. This article might help:
https://www.gardeningknowhow.com/plant-problems/disease/how-to-get-rid-of-sooty-mold.htm