Not sure what the causative organism is, but it is very common, and fairly harmless - not worth treating.

Resin

I know that this was posted quite a while back but I was just wondering if you figured out what the black spots are and what you did about it. My willow has the same problem and I am not sure what to do.

I also have about 5 weeping willows with this issue and I would like to know what to do.. if anyone can help I would really appreciate it.. thanks

The damage is more cosmetic than anything else. Like Resin says, "Not worth treating'.
Willows have their problems. Correct pruning is very important.
Mike

I know this is old thread, but what do you mean correct pruning? What needs pruned on a newly planted weeping willow tree?

Very little-on a willow or any other new transplant. Best to let the tree have all the leaves it can in the first year or two, then use good pruning techniques to achieve your goals. Large subject, well outside the scope of a thread about insignificant blemishes on some willow leaves.