What is the latest time of year to trim oak trees near Minneapolis Minnesota?

The most efficient and cost effective means of control is prevention. To prevent the introduction of oak wilt, avoid tree wounds during the infection period. Don't prune during April, May or June!! This rule of thumb also holds true for construction activities in and around oak trees. If at all possible, don't work around oak trees during those months. If work cannot be avoided, carry a can of pruning paint and immediate paint each wound created, including those on exposed roots. Even 15 minutes can mean the difference between prevention and disease. http://www.dnr.state.mn.us/treecare/forest_health/oakwilt/whatcanido.html