Erin, a few things right off the bat. Check the roots to see if you're dealing with rot rot. What potting soil do you have this little tree in? Secondly, 40 degrees is nothing for citrus, especially most mandarins (especially if it's a Satsuma variety, extremely cold tolerant). BUT, we did have temps below that recently, depending on where you are in relation to the coast. Where are you, exactly? I'm in Vista, about 6 miles inland, but up in the hills, so it's milder on my property, save for some cold pockets (my lot slopes downhill, north to south, so the south side of my lot is literally a hole that the cold air rolls down into and sinks - only 1 citrus tree down there and it's a variegated lemon that is holding its own, amazingly, right next to an avocado tree, also hanging in there!) I think you've probably got either a bad case of root rot, or some sort of extensive root damage. Can you not plant this poor little guy in the ground?? Here are the 3 main things that can cause catastrophic leaf drop (these come from the Univ. of Az Coop Extension, very helpful info and quite applicable to us in the San Diego area):

1. Over fertilization. Prevent by following fertilizer application instructions on the product. Treat by watering heavily to flush the fertilizer below the root zone.

2. Inappropriate watering strategy, too much, or not enough water: As a general rule citrus need a deep watering once a month in the winter and once a week in the summer (in ground suggestion, NOT for potted plants, it may be less frequently in the winter, unless we get Santa Anas, more frequently in the summer, especially if very hot. Use the "index finger" test to tell.) Apply water to the entire root zone, well out beyond the drip line. Apply enough water to wet the soil at least a foot below the surface. Check the depth of the irrigation with a long handled screw driver, you will only be able to push the screw driver as deep as the water has gone.

3. Citrus Mesophyll Collapse: Grapefruits are particularly susceptible but any citrus may be affected. It is not uncommon for grapefruit trees to be affected and other nearby citrus to be fine. Leaves and fruit of citrus trees dry up and drop. Stems and branches remain green and living. Rarely is the whole tree affected. Mesophyll collapse occurs when there are unseasonable weather changes. For example if it gets abnormally cold in October for several days and then heats back up above average temperature in November. This fluctuation stresses the trees as they kick into and out of and then back into their winter slow down. It is especially stressful after a dry summer. MANAGEMENT: Remove fallen fruit to prevent attracting fungal and bacterial diseases. Continue to water the tree on its normal cycle and it will leaf-out again in the spring. Do not prune out the defoliated branches until spring, and then only after the tree has completely leafed out. Adding this: Start fertilizing as SOON as you see any sign of leaf push. I had this happen to two of my in-ground grapefruit and was horrified. Was glad to know this was the cause. Happened last winter when we had all the cold, wet weather. Totally freaked out my lovely Rio Red grapefruit. And the owner.

Patty S.

Hi Patty & anyone else that reads....
Thanks for your response, it was very informative. I live in Mission Beach and unfortunately have concrete for back and front yard; no place to plant. I could give him back to the family member that has the space and gardens but want to try to keep him:) I will research root rot and root damage (I'm a first time tree person; only have had houseplants and potted herbs). I am feeding it E.B. Stone organic citrus and fruit tree food. I don't have the bag anymore of potting soil but I know it was for a citrus because I took the plant to the garden store and was recommended given where I live. The tree is now hanging out in my kitchen and I haven't done anything except put him by the window for direct light. It has lost all his leaves and I don't see any leaf push. Yikes! I will do some homework and respond.

EB Stone is a great product (I use their products quite a bit, in fact), and their fertilizers are very goo. BUT you'll want to use much faster draining soil. I would recommend EB Stone's Cactus potting soil. Citrus really can't tolerate wet feet, and their roots will rot. And, put your tree outside :-) It needs the warmth and full sun, which it will not get being inside. Heck, we live in "citrus country", so keep you tree outside. Also, I would do the scratch test on a couple of branches, to see if the cambium layer is still green. Is not, you may have a dead tree. I would re-pot this tree right away, using a better draining potting soil, and make sure not to water it unless you can feel that the soil is getting dry (use your index finger and stick it all the way down to feel). You may not get any new leaves for a bit, since our citrus are rather dormant this time of year. You may not see any new leaves for a bit, maybe around March or so, when our weather warms back up. We're due for some rain this weekend, so make sure your pot drains well (has a large hole in the bottom), and make sure it's not getting runoff water from your roof, so the soil doesn't get water logged.

Patty S.