We have a grapefruit tree that produced about 100 fruit two years ago, only about 25-40 last year, and this year has no blooms and a lot of limbs with no leaves. There are some places on the trunk with algae-looking spots.
Is Our Grapefruit Tree Dying?
Certified GKH Gardening Expert
The algae looking spots are lichen. They are more a symptom of what is wrong with the tree, rather than the cause. Lichen tends to be attracted to trees that are unhealthy.
As for what could be causing your tree to be unhealthy, there are many, many reasons. It is very hard to diagnose trees without being able to examine them. I would strongly recommend that you bring a tree specialist in to look at it or contact your local extension service and see if they can look at the tree.
If this is not possible, then you can try to treat the tree with a fungicide and a pesticide. These will take care of a significant amount of the problems the tree may have. I recommend neem oil, as it is both a fungucide and a pesticide and is systemic, so will get into the tree itself to fight off what may be attacking it. Reapply it every 1-2 weeks.