I planted a willow tree four years ago and it appeared to be very healthy. This summer the bark on the main trunk peeled off, not off the branches as of yet. An arborist company first thought it may be sun scald, but after reviewing the pictures they said it isn't. They referred me to you to see if there is an insect or something else in my area that would cause this.
The picture does not show the upper portion of the tree. If it isn't dead, it will be soon. A tree cannot survive without its protective bark. The damage may have been caused by bark beetles or even an animal. There appears to be new growth coming from the base. As quickly as willow grows, you may want to train it into a single stem and see what happens.