Bur oaks are native to all 99 counties in Iowa and are arguably one of Iowa’s toughest native trees. However, this year, many of these trees are showing signs of bur oak blight.
Bur oak blight is a leaf fungus that causes severe defoliation. While the effects of the blight have been recognized more recently, the fungus that causes the disease has been around for a long time. Shifts in the amount of precipitation, changes in temperatures and increased humidity levels seem to have elevated the effects of the fungi from harmless to fatal.
“Although the fungal infection occurs in May, the browning leaves do not show up until late July,” says Tivon Feeley, DNR forest health program leader. “The leaves on the lower branches are the first to turn brown and after several years of repeated infection the whole canopy.”
Symptoms of bur oak blight include browning leaves during late summer, black fungal structures grown along the leaf’s veins in August, and sickly looking leaves that remain on the tree during winter months.
According to Feeley, the DNR is beginning to receive calls about bur oak blight damage, and he expects the calls will increase as the damage will become more apparent through September.
There is no way to prevent bur oak blight. Some bur oaks are more tolerant of the blight and do not show any symptoms at all.
“We have had some success controlling bur oak blight with a chemical called propiconazole ,” says Feeley.
Feeley suggests contacting the Iowa State University Plant Diagnostic Clinic to make sure the tree has bur oak blight. For information on submitting a sample visit http://www.ent.iastate.edu/pidc/
“If it has bur oak blight,” says Feeley, be sure to contact a qualified arborist to inject the chemical into the tree in the spring.”
For more information about bur oak blight, contact a DNR district forester in your area. A list of DNR staff by county is available at www.iowadnr.gov/contact. More information is available under “forest health” at www.iowadnr.gov/Conservation/Forestry/