How to treat brown spots on lemon leaves?

[Andrew12]

My lemon tree has been in a big pot for 2 years. I fertilize it quite regularly and it gets a daily drip on a watering system

 

 

Recently it started losing leaves but has now sprouted new leaves with brown spots around a tiny white dot at the center of the brown spot. The brown area is not raised from the leaf service itself. See photos. Help!!