Looks like maybe greasy spot; it is caused by a fungus that usually originates in dead leaves on the soil under the tree, hence orchard sanitation is very important. Greasy spot, like most fungal infections requires high humidity conditions.

You can google citrus greasy spot and find lots of ways to treat it. I prefer to prevent it; and for that I use a systemic fungicide; and only stronger measures when there is a significant outbreak that has not been controlled by the systemic. I am a big fan of Bayer Silvacur; but there are other good products. Ask your local nurseryman.