Willows are susceptible to a number of canker diseases. These are fungi that get into the plant and can kill sections of the tree. Once infected, there's not much that you can do for that particular branch. The main way to prevent it is to avoid wounds or injuries to the tree or branches, because cankers usually get a start in a branch wound. Also, a healthy tree is more likely to fend off a canker than one under stress. So make certain that the tree has adequate water and nutrition.