Iowa is experiencing an unusually warm February with temperatures high enough to signal the early start of spring biological activity, according to forestry officials. The state has already accumulated early-season heat, often tracked using growing degree days, which are used to predict when insects become active.
“We typically suggest to stop pruning oaks around March 15,” said Tivon Feeley, forest health program leader with the Iowa Department of Natural Resources. “The unseasonably warm February has moved that date up. Pruning oak trees should stop soon out of an abundance of caution to prevent the spread of oak wilt.”
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.
A healthy tree can be infected through open wounds during the growing season where the fungus is carried from a diseased tree to a healthy tree by a small beetle or through root grafts between oak trees of the same species.
Feeley says symptoms of oak wilt on infected trees are usually visible in June or July. Look for 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 latex or 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 www.iowadnr.gov/oakwilt.