Oak trees are most easily infected by overland spread in the springtime, from bud swelling until two to three weeks past full leaf development. The Wisconsin Department of Natural Resources recommends that you avoid pruning, cutting, or wounding oak April through July (April, May, June, and July) in urban areas. Observations and unpublished research have shown that overland infection can occur after July, yet these mid-summer through early fall infections are not common. To take a very cautious approach, do not prune or otherwise wound oaks from April to October.
In some years, spring comes much earlier. If daytime temperatures begin to reach the 50-degree mark, stop pruning oak at that time, even if it is still the middle of March.
Apply tree wound dressing to the last 3 growth rings.
If you must prune when temperatures are above 50 degrees or between April and October, cover the wound surface with tree wound paint immediately. Tree wound paint can actually slow the natural closing of the wound, so limit the use of wound paint to the situation described above.