Learn why open wounds on trees can lead to disease in the spring, summer, and fall…
Tree trimming produces open wounds on trees where branches have been removed. Trees will then produce substances that aid in sealing wounds. Many insects have adapted over time to be able to detect these substances and are attracted to wounded (i.e. newly pruned) trees; it is easier to burrow under a trees bark at the site of a wound. Adult flying insects that cause or spread tree disease do not emerge until spring, making winter an excellent time of year to prune most trees.
In some cases, like emerald ash borer http://www.mda.state.mn.us/emeraldashborer, the insects themselves are a problem. In other cases, insects spread diseases from one tree to another: oak wilt http://www.extension.umn.edu/environment/trees-woodlands/oak-wilt-in-minnesota/ and dutch elm disease http://www.extension.umn.edu/garden/yard-garden/trees-shrubs/dutch-elm-disease/.
Think before you prune. Wait until winter if you can. If the tree must be pruned during the growing season, make sure you seal all cuts immediately.