Certified GKH Gardening Expert
There are many fungal/bacterial infections that cause damage to citrus. The first thing will be to change the care of the tree.
You will need to make the environment more suitable for the tree, and less suitable for the infection, since they prefer quite different environments. This will mean letting the soil dry out, very thoroughly and completely, down to about 3 or 4 inches between watering. It may also mean improving air flow to the area around the tree, while preserving the humidity levels. This can be achieved with a fan and humidifier, to keep the humidity around 40-50 percent.
Next will be a treatment. A fungicide or bactericide may be necessary. These articles will help:
Two things that will mitigate many of these issues, and are good to have around for citrus- DOLOMITIC LIME and IRON SULFATE. Applying once per year, according to packaging instructions, and whenever there is an infection will help to resolve the issue.
Here is a collection of articles that will help you to care for citrus: https://www.gardeningknowhow.com/category/edible/fruits/citrus