North Iowa Outdoors: Time to Stop Pruning Oaks

Spring is on its way.

The Iowa Department of Natural Resources (DNR) suggests finishing pruning oak trees by the end of the second week in March to prevent the spread of oak wilt.

“The best way to prevent the spread of oak wilt is to not prune any oak tree between the middle of March and the start of November,” says Tivon Feeley, with the Iowa DNR’s forest health program.

Oak wilt, caused by a fungus, has been present in Iowa for many years. Iowa’s red, black, and pin oak are more susceptible to oak wilt, but it can also infect white and bur oak. Black, pin, or red oak usually die within the same summer they are infected. White oak and bur oak can often take a number of years before they succumb to this disease.

Oak wilt can spread from infected trees to healthy trees in two ways: a small beetle that carries spores of the fungus from a diseased tree to a healthy tree with an open wound during the growing season, and through root grafts connecting nearby oaks. For example, if a red oak is infected and there is another red oak within 50 to 100 feet, there is a good chance that the roots of these trees are grafted and the fungus can move from the diseased tree to the healthy tree.

Feeley says symptoms to look for on infected trees usually include leaves turning a bronzed brown along the outer margins of the leaves. These leaves can often still have some green on them as they fall from the tree. The defoliation tends to start at the top of the tree.

The best way to prevent the spread of oak wilt is to prevent any wounding to oak trees during the growing season. If a tree is wounded from storm damage or pruning is required during the growing season, treat the wounds immediately with a wound dressing such as acrylic paint.  Do not purchase pruning paints/sealants.  Those products slow the tree’s ability to seal over the wound.

Learn more about oak wilt prevention and control at


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