Indoor Lemon Plant Losing Leaves
I have two indoor lemon trees, both about 3 feet tall and 3 years old that I germinated from the same lemon (store bought). The one that grew the fastest is thorny and the leaves usually wrinkle, but it usually does fine other than a salt problem early on.
The other lemon tree is practically thornless but has attractive, glossy leaves, some about 4-5 inches long. However, it has begun to lose leaves in the last couple of days. I just shook it a little and 4 more dropped. They're breaking clean off where the stem meets the leaf, and the remaining stems readily pop off the trunk. All the leaves that have dropped are on the lower section of the trunk, not on any upper branches. Some of the leaves are yellowed a little, primarily the central vein.
I can't see any sign of insects or mites, and they've always been indoors. Fungus gnats have been my only issue.
My girlfriend's moving in and we've been doing a lot of rearranging and painting in the adjacent room, so I don't remember when I last watered them or fed them. I know that I've forgotten to use acidic water the last couple of times I watered them (I have rusted stakes in the soil for iron).
The soil is slightly moist. They're planted in a mix of compost soil, shredded sphagnum, sand, and Espoma Plant Tone. Bottoms of the pots are very well draining.
The only thing that's really changed in the last two weeks is that I moved it 3 feet back from the window where it's always been. They're west facing windows so right now it's not getting great light, and it's been overcast nearly every day. Heat hasn't gone on yet, so it's gone from being warm and sunny at that window to chilly and dim.
Do you think it's just time for those to drop or stress or a deficiency? I'm surprised I've gotten these things to grow this long in lower NY with western exposure. I'm probably overdue to add some Plant Tone to it, as these were repotted towards the beginning of the year, and got a slight supplemental feeding a few months ago. I also have a little myers and a kumquat growing next to it that seem fine. If a parasite was to blame, I'm sure they'd have them too.
Thanks for any input, I'm new here.