ISpindle Gall is unsightly but Harmless.  It is caused by a small  mite called  Eriophyids.  They live under the bark over winter and crawl under the folds of emerging spring  leaves and cause an irritation that causes the leaf to make cells that grow around the mite - so that is the gall that you see.  Once the tree fully leafs out it doesn't get any worse because the mite needs a growing leaf to make the  gall.  The mite produces eggs within the gall and they emerge to repeat the cycle next year.   Removing  affected leaves and keeping your tree healthy is the best way to control the mites but again, the mites really don't hurt the tree overall.