It looks like scale insect infestation. The insects suck sap from leaves, branches, and fruits resulting in yellowing of leaves and branches. Severe infestation leads to falling of leaves, fruits, and reduction in plant vigor.
The honeydew secreted by insects attracts ants and will also lead to the development of sooty mold (black fungal covering the leaves, twigs, and fruits).
Scale insects can be controlled by spraying fish oil rosin soap or neem oil or basic soap spray (add 2 tablespoons of liquid dish detergent or baby shampoo to 1 gallon of water). Also, it will be useful if you have an ant barrier of a 3-4 inch band with tangle-foot/tape around the trunk of the citrus to avoid ants (since ants move the scale insect from one plant to another).
Posted by: Dr. Ravishankar Narayana
Posted: January 2, 2018