It sounds to me like you might have an insect problem. If the insects are infesting the willows under their bark, the bark might fall off.

If you could post a photos of the troubled area of the bark that would be very helpful. If you search Garden Web with the words "insert pictures" or "post pictures" you will see a lot of instructions for how to insert the pictures.

One option would be for you to ask an arborist to diagnose your trees problem and to tell you how to treat it. Arborists are listed in the phone book with tree trimmers. There are a lot of scammers who do tree trimming. I would guess that someone who is paying for an add in the yellow pages is established in the community. Before the person comes ask them if they are qualified to diagnose a health problem in a tree.

Another option would be to take pictures to a good nursery and ask them to tell you what they think the problem is and what they can suggest as a treatment. Many nurseries offer this as a free service. Bring along a little sprig from the tree with some leaves so they will be certain of the tree genus.

If you are not certain your trees are willows you might want to find out if they are birches. Many birches have peeling bark and this is considered a decorative feature. The leaf shape is very different on willows and birches so it would not be hard to determine if it is a willow or birch.

Welcome to Garden Web and I hope you find out what is happening with your trees.