I have a Lisbon lemon tree that is outdoors and is about 35 years old. It has always been a very good producer of fruit but now it appears to be dying. The remaining fruit is quite small and all the leaves have shriveled and turned a light tan color. My local nursery said it was probably root rot and that nothing could be done but wait and see. The soil is clay type but never been a problem in the past. Feeding citrus food has not been done on a regular basis but again, never been a problem. It receives water when the lawn is watered (2-3 times weekly) sinced it is located next to it. I hate to lose this tree. Any suggestions or advise?