There is black mold on the leaves. How do I manage the tree?
The Grapefruits on My Tree Have Stopped Growing.
Certified GKH Gardening Expert
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