פירוש על עבודה זרה 100:6

The Talmud now suggests that the difficulty is why placing manure is permitted while pruning is prohibited. But this too can be easily explained. It is permitted to preserve a tree during hol hamoed and the sabbatical year. One need not let the tree die and evidently placing manure on cuts in the tree is meant to prevent them from dying. But pruning strengthens the tree and is therefore prohibited.