Question
We have a large Elm tree that has lost at least half of it's leaves already and the remaining look wilted.  Is they anyway to conserve this tree.

Answer
John,
Without laying eyes on the tree it's hard to know if it is insect or disease or too much water or not enough water...  If this is a nice tree and you want to save it you might want to invest in having a certified arborist come and look at it.

In General:
When trees are stressed you want to give them a good long soaking of water once a week.  (Would not apply if the tree is wilting because the area it is growing in is suddenly swamped, of course.  If the problem is a constantly wet soil, some measures would need to be taken to drain it.)  A good long soaking is about 2 hours with a sprinkler or soaker hose, wetting the area under the canopy.

Trees grow best when mulched with chopped up leaves around the base (just like nature does it in the woods!) - if that is not possible, bark mulch is fine.  Fertilizer should be organic where possible and always applied with a light hand... in other words, you don't want to fertilize trees too much if at all.  

There are a host of diseases that attack elms, the most serious being Dutch Elm Disease.  

all the best,
C.L.