Question
I have two 50 ft. tall beautiful yellow poplar trees. One looks like it has a disease, looks like it's thinning out and the leaves are half the size as the good tree and appear not as green. There appears to have some bark rot around half the base of the tree up to about 4 inches from the ground. The trunk near the ground appears to move slightly in high winds. How can I save the tree???

Answer
Sounds like the tree is dead already.  Bacteria or fungus gets into the phloem (sap) and cuts off the nutrient flow.  If you see the trenk moving around its probably because the roots are rotting as well and are no longer able to foot hold the soil.  The fungus is at the base of the tree.  

James