I have three Sapote seedings and all have this problem to some extent, tho the one pictured below is the worst. They are inside, in a south window. Putting them outside is not an option. The condition starts as tiny dots, and the green layer is removed on both the top and underside of the leaf for each spot. We have looked at them through a 20x loupe and also a 60x field microscope and can see no evidence of insects, nor of fungal spores on the leaf. I really want to save the plants. (In my climate they have to be indoor plants1) Can anyone tell me what can be causing this, and what treatment, if any, should be. Or do I have to get rid of them? Thank you, I will appreciate any help!