The Iowa Department of Natural Resources is advising residents to wait just a little longer before pruning their oak treas. “It’s best to wait until after Oct. 15 to start pruning oak trees,” says Tivon Feeley with the forest health program. “Then you can prune your oaks all winter long with the goal to stop pruning by March 1.” This strategy is aimed at prevention of “oak wilt,” which is caused by a fungus that has been present throughout the state for many years. Iowa’s red, black and pin oaks are more susceptible, but it can also infect white and bur oak. Black, pin or red oaks usually die within the same summer they are infected while with white and bur oaks it can often take a number of years to succumb to the disease. Oak wilt can spread from infected trees to healthy ones in two ways. The first is by a small beetle that carries spores of the fungus from one tree to another and any tree with an open wound during the growing season is susceptible. The other is through root grafts from nearby oaks. For example if a red oak is infected and there are other oak trees within 50 to 100 feet, their roots are likely grafted together and the fungus can move from one to the other. Symptoms usually include leaves turning a bronzed brown along the outer margins and they can still have some green on them when they fall from the tree. The defoliation tends to start at the top. For trees that have been pruned or damaged during the growing season, treat with a wound dressing such as acrylic paint. Do not use pruning paints/sealants as these slow the tree’s ability to seal over the wound.
