Question
we have about 50 pecan trees and most all of them have this "mossy" looking growth on them.  the limbs with the growth don't appear to be healthy and the limbs break off.  how do i treat this and help the trees grow strong again?

Answer
Hi Becky,

I did not get a picture of the "mossy" growth, but based on your description, this sounds like lichens.  Unfortunately, the lichens are not the cause of the problem.  They are normal on trees and indicate that the trees are receiving more light than normal.  Lichens grow on the surface of the bark, but do not actually penetrate into the tree.  They are often found on tree limbs where light penetration is great.  Sometimes, people confuse lichens with a pathogen because they may appear on dead or dying limbs.  They are present on these limbs because the thin or absent canopy means that there is an abundance of light.

This is a link to a fact sheet from Ohio State that will give you more information about lichens.
http://ohioline.osu.edu/hyg-fact/3000/pdf/HYG_3312_09.pdf

As for the problem with the limbs dying back and breaking off, I cannot tell you the exact problem based on the description.  I think that you will need to take a close look at the leaves and exact pattern of the problem when the trees leaf out.  I suggest that you review this fact sheet that has information about the appearance of different types of problems on pecan.  You can see that there are many, many possibilities for the cause of the dieback.  
http://aggie-horticulture.tamu.edu/fruit-nut/fact-sheets/evaluating-pecan-proble

In general, when trees have dying limbs, it is best to remove these limbs by pruning.  If there is a pathogen, it can help prevent spread to other parts of the trees by removing the dead wood.  Dead and dying wood is attractive to insects such as borers which will attack and make the problem much more severe.

Many of the problems with pecan are related to nutritional problems.  If you have not had a soil test, you might consider getting one done.  Your local county extension office (look up in the phone book) can coordinate the testing, usually for a small fee ($10-20).  

I hope that this information helps. If you have additional information to provide about the appearance of the leaves in the spring, I will try to help you narrow it down.  Often, cleaning up the trees by pruning and providing the proper fertility will improve the appearance and vigor of the trees.

Good Luck!