Training means pruning young trees for form during the first few years of their existence.
Good nursery stock already has had quite a bit of training by the time it reaches the market. In fact, that is an important trait to look for when you buy. In general, the smaller the wound and the younger the tree, the less infection will result. I. Select a single central leader if the young tree does not have one, and prune out any competitors. One central leader, or stem, is almost always preferable in street trees.