SUMMER vs. DORMANT PRUNING
Use summer pruning to train young trees and shorten the time to full fruit production. On mature trees, summer pruning involves mainly: 1) removing vigorous, upright shoots that are not needed to create permanent branches and 2) heading shoots to control tree height. Summer pruning is done in both spring and summer. When useful, bend and stake any shoots of young trees that you want to grow in a different direction during the spring and summer. Bending branches hastens branch development compared to removing or heading those in undesirable locations and waiting for a new branch to form.
If trees received appropriate summer training and pruning, far less dormant pruning is necessary. However, the absence of leaves provides a clear view of the framework of the tree, so thin or head any branches that were not adequately summer pruned.
When you dormant prune, it is preferable to prune stone fruits in late February to early March rather than in the fall or early winter. Pruning wounds made late in the dormant season heal faster than those made earlier, allowing less time for disease organisms to infect the wound; also, there is less rain after February. Furthermore, spores of many organisms causing branch diseases are more prevalent with early season rains than later. This is especially true with Eutypa disease, which infects apricots (and grapes) and causes severe gumming and branch die back, so it is especially important to prune apricots late in the season. Apples and pears can be safely pruned at any time.
From TRAINING AND PRUNING FRUIT TREES written by Cooperative Extension, University of California - Sacramento County.