The grapefruits on my tree have stopped growing.

Question

There is black mold on the leaves. How do I manage the tree?

Answer 1

It sounds like the tree has sooty mold (the black you are seeing), which is actually an indicator of a pest infestation. I would recommend treating the tree with neem oil. This will treat both the pests and the sooty mold fungus. Here is more information that may help: https://www.gardeningknowhow.com/plant-problems/disease/how-to-get-rid-of-sooty-mold.htm
https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm