I have similar problem one tangelos lost all its leaves but the plant still alive. The lime tree is losing leaves slowley. I saw the back of the leaves on the naval orange turning red or light brown. I am not expert but common sence is telling me we are in zone 7B and we are forcing these citrus trees in unnatural enviornment. These trees need sun and 70 degrees weather outside. My theory is as long as the roots and trunk still alive they will recover when they go outside. I have no problem with insects although I spray copper once every two weeks. I think all we have to do is be patient until the winter is over.

I would be concerned if there is TWIG dieback.

How long is it taking for your mix to dry out now that it is indoors?
What is your mix comprised of?
Did you do a give them a time to ajust before they came inside?
How much light are they getting verses what they were use to?
What kind of temps were they exposed to and for how long before you brought them in?
Now, what temps have you exposed them to now?

Have you ever over fertilized?
How about just once let them get to a wilt before watering?

By the way, there is such a thing as over doing it on the fungicides and pest treatments. I would be careful and read the instructions.

I hope it get better soon.

@Foolishpleasure:
"These trees need sun and 70 degrees weather outside"

Well, last winter was the first one where I've owned citrus trees and moved them inside. I didn't have much indoor growth but didn't have these problems last winter.

"I think all we have to do is be patient until the winter is over"

I just hope my plants long enough. I'm pretty confident that if they do, they'll do a lot better outside. The "if" is uncertain, though.

@meyermike
Thanks for the detailed reply. Always enjoy your posts.

"I would be concerned if there is TWIG dieback."

That is what I am experiencing. Sorry if that wasn't clear. Flowers will usually be covered in stick sap, then fall off, then the leaves fall off, and then the twig will turn dark and die.

"How long is it taking for your mix to dry out now that it is indoors? "

The dozen or so trees I have are in a few different mixes. Some are in coconut husk chips and peat. Others are in a peat/compost mix with perlite and fine orchid bark. At least one was in a storebought cactus/citrus mix that I added some more perlite to.

With nearly all except one (which takes ~5 days to dry out), the soil surface will be dry within 3-4 days I'd guess. I water every 5 days approximately. As evidenced by my approximate guess on how long the pots take to dry, I should probably be more certain on the dryness of the plants when I water.

I haven't given properly measuring the dryness of the soil the proper focus, even though I'm aware it's probably the most common indoor plant mistake people make. I've just been eyeballing it, seeing if the surface of the soil is dry. A lot of the time the soil beyond the immediate couple mm surface level of the soil is still a little damp when I water. Sometimes the pots are more dried out, but I don't let this happen consistently. I have had fungus gnats before, have killed them off with Gnatrol and curtailing watering I think 2 times now, and now they are back on some plants. This is probably pretty good evidence of overwatering, isn't it...

"How about just once let them get to a wilt before watering?"

This is a good suggestion. Despite overlooking it in the past, I am going to monitor my water usage more closely, watering less, and see if that clears up any issues. I'm willing to try everything.

Half the plants are in plastic containers, half are in terra cotta containers. This seems to have made no observable difference for the plant health, but I thought I'd mention it because I've read it has some effect.

"Did you do a give them a time to ajust before they came inside? "

"What kind of temps were they exposed to and for how long before you brought them in? "
"Now, what temps have you exposed them to now?"

They were in full, unobstructed, south facing sun. They had sun pretty much the entire daylight hours from April - October. Come October they were moved gradually to a spot that has good morning sun and afternoon shade over about two weeks. Then I moved them to a different yard that is even more shaded for at least two weeks. Then I moved them inside once the nighttime temperatures got consistently into the low 40's. The temperature inside is obviously general room temperatures. The trees are mostly within a few feet of radiators, but right up against windows, which are a little drafty. The humidity in the apartment, given the heat, is pretty low, contrasted with the humid summer weather they got here in the Northeast. I have humdifiers I haven't used and am going to try to see if they help at all.
The lighting inside is generally pretty poor and indirect, but I have a CFL light of variable size on each plant . I don't want to use too much power, but wanted something supplemental, hence the CFL's.

"Have you ever over fertilized?"
Not that I know of.

Outside during the summer, I'd give the trees a scoop of blood meal mixed with a little kelp each time and sometimes I'd mix in a more balanced N-P-K fertilizer whose name escapes me. This would be done every 2-3 weeks. I also once in a while mixed in some old coffee and occasionally urine (both are free sources of N) mixed into the water. The plants seemed perfectly happy with all of this, and everything grew nicely the whole summer with almost no problems. Since moving the plants in, I have done much less fertilization (I had heard it should be scaled back when plants are more dormant and indoors under sub-optimal conditions). All I've done is mix in some old coffee with their water when I have it in my house.

Their water has been NYC tap water since being inside.