Question
I have a huge globe willow tree that has a branch with something seeping out of
it that looks like spit. When it drips on the lawn it kills the grass. Also has a
rotten smell. What is it?

Answer
It is probably sap coming from a wound in the tree.  The wound could have been caused by an insect, woodpecker, or wind damage.  Bacteria will colonize this area and cause the sap to ooze from the tree.  It would also have a rotten smell.  Unfortunately, there is nothing that you can do for this situation.  Keep the tree watered during periods of dry weather and fertilize lightly.  This is about all that can be done.  Good Luck.