The photo is oddly not in focus at critical spots and I can't see the problem.... not really sure if I'm seeing fungal infection.
The blurred areas -- without being at all distinct -- are making me think I'm seeing scale insects. Ha. It would be almost unbelievable if this is actually the case, but I thought I would mention it, just in case.
...since you are using such sandy media, be sure to start giving them Citrus fertilizer -- scratch some along the perimeter of the pot (away from the plants) with a plastic fork(watch out for the roots). Alternatively, put a spoonful in quart of water, dilute --cut in 1/2 at least-- and water with it.
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