Recently has been getting more sun because of the days are getting longer, but it was positioned at the same window for over a year. Haven't sprayed it with anything except with some water on the hotter days.

What are your care habits? I don't think I have seen discoloring like that. I really hope that someone can give you more information. I'm very curious as to what is wrong with your tree and if it can be fixed.

Are all leaves affected or only older, lower leaves?

Amanda: I have to say that I'm not likely using the proper soil for these trees in the first place because haven't had a chance to switch over to a proper mix. I recently moved them over to a 3 gallon pot several months ago. The Miracle Gro might be too compact at this point. I water them as needed, but I haven't done much. I have 4 other lemon trees that are in good health beside it (not included in the photos above). This one is the only one out of 5 of them that are showing these symptoms.

Vladimir: It's the newer ones near the top. I've observed dark brown spots that first appear. Then the condition of the leaves deteriorate very quickly. Within a day or two they become brown and discoloured like the top photo. It's rather a rapid deterioration, which is of concern. I've cut off the affected leaves. The affected leaves are mature ones that have been on the tree for couple months now...

I am stumped. I hope someone can come up with an answer for you. It does not seem to be fungal or bacterial.

they haven't been exposed to pesticides, have they?

subtropix: No pesticides.

I'm sorry I can't help...no idea. Any chance you can take your tree outdoors for summer though? They do so much better. Also, I would change the potting mix to 5-1-1 ideally.

looks like it could be sunburn to me

All I see is fertilizer damage

freedomfries, please describe your fertilization regimen, i.e., the exact fertilizer you use, the concentration, if in solution, or the volume, if a solid, and how often you fertilize.

Just looking at the tree one can see that the tree is not getting anywhere close to enough sunlight. Everything is hanging downward and the internodes are much to extended in length. .

pip313: These haven't been fertilized. Fertilizer damage would have been more apparent when they were repotted in the new soil (if there was higher level of fertilizer with Miracle Gro) and wouldn't appear several months after being repotted.

Vladimir: I haven't applied any fertilizer. I've only repotted them into the 3 gallon containers with miracle gro.

OJ: Well, I am pretty further north so we get short days during the winter. This specific tree is genetically predisposed to produce giant leaves. The nodes are extended on the right branch that started growing less than a year ago. The main branch on the left is fine in terms of node spacing.

Yes it actually would. Miracle grow is notorious for dumping nutrients after some time.

Pip, how does Miracle Grow "dump" nutrients months after it was applied?

It's miracle grow potting soil he used. Which contains time release balls that like to all "pop" at the same time. Usually 3-6 months after being potted.

Thanks for the answer, pip. I thought you were referring to the MG fertilizer.

No problem, I've had this problem before with this poopy soil