Potted lemon tree

There are a number of problems that this could be (hard to be sure with out seeing the leaf) but it’s probably leaf or brown spot. The one thing we can be certain of is that it will be a disease, mildew or fungal problem which all have the same cure. Regular spraying of Copper Oxychloride is the best way to keep this sort of thing in check and your tree doing well. There are other Fungal remedies available at your local Palmers Store. You are doing the right thing fertilising the tree and watering through any dry periods, as healthy trees are more resistant to problems.