I have a few Fairchild mandarins that have developed a problem. The leaves are dying off. It is happening in a large area (about 25% of the tree). Later, the fruit too will turn discolored and then brown and then die.

It happened last winter, as well. Once the heat came on, the problem disappeared. Now that it's cool again it's back. I live in the low desert and have several other citrus trees that were not affected.