From Lisa Johnson, Dane County UW-Extension horticulture educator:

Apple and crabapple trees: Pruning can typically be done from January through the end of March as long as the trees remain dormant. When pruning fruit trees, be careful not to remove more than approximately one-third of the old growth. Keep in mind that the spurs that bear the flowers and fruit are already on the tree and pruning removes them. Prune for good light penetration and structural strength. Sixty-degree crotch angles between the branch and trunk are the strongest and most fruitful. V-shaped crotch angles are not as strong and don’t produce as much fruit. Remove crossing and rubbing branches, dead branches, diseased branches, suckers that come up around the base of the tree and water sprouts, the vertical fast-growing twigs that usually appear in the middle of the branches. UW-Extension recommends disinfecting pruning shears with 70 percent rubbing alcohol or 10 percent bleach solution. Disinfect shears between cuts when pruning diseased branches or between plants for healthy material. Check for signs of fireblight, a fatal bacterial disease that causes sunken discolored cankers on branches before you start pruning. For more information on pruning apple trees, see UWEX publication A1959 “Training and Pruning Apple Trees” at http://learningstore.uwex.edu for more detailed information.