I'm trying to grow many bonsais (Lemon, Pine, Cypress, Chestnuts, Peach). I was doing fine and all of the bonsais are in the same place. However, recently my lemons started to show some kind of an illness. As in the photos above, their leafs are drying and they have never been anhydrous. I took very good care of them. They are in pots outside of my window. The window is looking to north side and there is another apartment on the cross side. So they do not expose to burning sun more than one hour in summers. I live in Istanbul, Turkey.
Can you tell me, why this happens? And how to take care of it?