Question
We have two holly trees in our S. New England back yard.  Each is 15-20 feet tall and are planted within 10 feet of each other.  One has recently exhibited a dramatic change in leaf color to a milky white.  The leaves have rapidly fallen off this one tree with other seemingly OK.  The leaves appear much more supple/flexible than normal.  Other than this recent development, the trees have been relatively healthy and we guess their age to be 50+ years.  We keep the area under them cleared of leaves, but do not prune regularly.  This one tree does have older damage in a low crotch, possibly a previously removed limb.  Both are against a 6' fence to buffer wind.

We hope this has provided enough information to assess the situation, but if you have specific questions, please let us know.  Your advise on a solution/remedy is gratefully appreciated.

Answer
It sounds like you have Powdery mildew, a disease of leaves of many plants. To control this disease you will need to spray the leaves (both sides) with a fungicide. There are several that will work and since I am not sure if one can be found in your location I am attaching a web site that discribes the disease and controls.
http://www.ext.vt.edu/pubs/plantdiseasefs/450-603/450-603.html