Leaf drop can be perfectly normal. The average healthy orange tree has about 140,000 leaves. Washington Navels can loose over 3,000 leaves a day, during the peak leaf drop period in the spring. Valencia's will lose about 1,500 leaves a day. During the three month period of March - July, Navel Orange trees can discard a total of 28,000 leaves, and Valencia approximately 19,000. - Millet

www.mastergardeners.org/tips/april.html

Does Yuzu normally drop a lot of leaves during the winter, even in warm climates?

I recently got a small Yuzu in a container; it's indoors and started dropping many of its leaves in December. Its soil is kept moist but not soggy; it doesn't get much sunlight but has a fluorescent bulb next to it. A Nippon Orangequat kept in the same conditions has only minimal leaf drop and has actually started sprouting many new leaves!

Yesterday (January 26) I was at the Green Arrow nursery in Southern California, where it rarely gets freezing temperatures, yet I noticed that all the Yuzu plants had dropped the majority of their leaves, compared to the other citrus which had normal looking healthy green leaves.

I have two Yuzu planted in the ground here. We had one night that dropped down to 24 F. Neither tree dropped any leaves. Yuzu is one of the most hardy of citrus.

I agree with Laaz, in you case the problem is not the tree, it is either in the container or the environment surrounding the tree. - Millet