Looks like a bird crap hit. Hits the leaf. Leaf turns yellow as the nutrients are transferred out and then it falls off. Seems to be how citrus trees decide when to shed their evergreen leaves. Just an observation and my opinion. Never read it anywhere. edit: Still, what are the odds 2 similar locations? I seem to remember something diagnostically let me look around a bit. This post was edited by Fascist_Nation on Tue, Mar 18, 14 at 23:41

Hello Ken. Thank you for the detailed response. I have considered that I’m loving it to death and have tried to space out the stress points. This started a few months ago at least when I was definitely not loving it to death. When I first noticed it, I thought it was a light issue, so I moved it to the windows. After about a month of deterioration, I sprayed it with the anti pest/anti mildew then waited about a month, then wiped down the leaves with water to remove the dust and waited a month, then repotted it and pulled some of the dead leaves off. Maybe that’s still too much stress, but it seems to be deteriorating rapidly, so I didn’t want to waste too much time. It has been about three weeks now since i repotted it. The soil seems to be draining/drying fine. I did bare root it to see if there was root rot, but I did not see any, and the old soil smelled fine. I used completely new soil. I think it gets plenty of light by the southerly window. It is near a vent, but we rarely have to use our heat believe it or not, and this all started before it was near the vent. The picture with my hand in it is one of the last shoots that made it past a bud. I included it because of how the leaves are deformed. There are a lot of buds that have sprouted and then just stopped growing entirely by the looks of it. Some of those smaller ones in the picture have been that size for at least a couple months. At this point I’m planning on letting it alone for a while and hoping the repotting and extra light of the spring will revive it. To your point, I don’t want to stress it anymore, but I posted this in case someone could look at the leaves and say, “this is a clear case of this kind of mildew...” or something like that. Thank you again for your response.

The only information you have given us is 'they are the exact same tree' and that may be true but subtle differences in location can change variables in these cases. A boulevard tree, for instance may get some salt from snow plowing. A tree close to other trees will not get as much sun as others in the open. That's why you might see some trees already past peak with fall colors while others that are the exact same species still have green leaves. This year in my country, color change has been scattered with spotty frost pockets and strong winds. Some years everything seems to change at the same time, some years not so much.

I would disagree that you can't fertilize with organic fertilizers in pots, however if you are going to do so you need to be diligent about how you care for your tree, use a medium that is appropriate for and encourages the growth of microorganisms, and avoid plastic pots. You also have to be careful not to use anything that can damage your microorganisms. When you use organic growing methods in containers you absolutely have to treat your soil as, as much of an organism as your tree, and I would not recommend it to someone starting out with citrus. (I grow my own trees indoors in a huge window exclusively under lights and use organic methods. I get fruit every year and my trees grow very well.) Reading your responses it definitely sounds like you have a few problems: water, light, potential overfertilization. You don't "feed" a tree the way you would feed a pet, fertilizer is not "food" in that way, but should instead be applied as necessary to replace nutrients depleted from your soil. Indoors, your trees won't deplete nutrients as quickly due to the low light and slower growth, so you don't need to fertilize as often or at all. As for watering, as mentioned above you should let the tree dry out between deep waterings and watch that your soil doesn't stay wet for too long. (For example: self watering planters are death for many indoor plants). As for lamps, the kind of grow-bulbs or nice looking growing lamps are not going to provide you enough lumens to grow a tree. You could probably use them for little plants that require low light, but a tree needs more than that. A high powered LED lamp is going to be your best bet for a citrus tree. There are options for ones you can install in your house or you could hide them in some way, but if you want your trees to do well you need a lot more light than a standard grow light. I hope this was helpful, and good luck.