News Releases

What’s Wrong with My Oaks?

May 1, 2009

Bur Oak is the most common tree around the Iowa Great Lakes. Since the early 2000s, residents have noticed that the leaves of some trees turn brown in late summer and then fall prematurely. New research at Iowa State University (ISU) has identified the cause, although much remains to be learned about the problem and its long-term effect on the trees.

Dr. Tom Harrington, Professor of Plant Pathology at ISU, has named the condition “Bur Oak Blight” and identified its cause—a new strain of the Tubakia fungus. Tubakia in its usual form is a fairly harmless fungus that causes leaf spots on red oaks and other trees. The origin of the new strain is unknown.

Symptoms first appear in late July when brown blotches appear on the leaves, often along the veins. As the summer continues the blotches increase in size until they eventually cover all or most of the leaf. The leaves then begin to fall, although some may remain on the tree over winter. Most or all of the leaves on a tree may be infected, causing the tree to stand out among other, neighboring trees.

According to Dr. Harrington, once a tree in infected, the fungus over-winters inside the tree and invades the new growth each spring, traveling through the twigs into the veins of the leaves. It is unknown how the disease spreads from tree to tree, or why some trees become diseased while adjacent trees remain unaffected.

The disease’s long-term effect on infected trees is also unknown. It does not kill the oaks outright, but may weaken them by decreasing the food produced by their leaves. Stressed trees are more susceptible to other diseases and insect pests, including the two-lined chestnut borer. Some infected trees are declining, but it is not known if the disease caused the decline or simply attacked an already weakened tree.

How should one manage infected oaks? Since the disease stresses trees, other stresses should be prevented or minimized. According to Peter van der Linden at Iowa Lakeside Lab, homeowners should water trees during severe drought, avoid digging, filling, or compacting the soil around trees, and consider mulching the ground under the trees with wood chips. The roots of the trees spread as far or farther than the branches, so it is important to protect as much of the ground under the trees as possible. Dr. Harrington suggests that dead branches be removed to reduce populations of the two-lined chestnut borer.

There is at present no proven cure or treatment for the disease. Lakeside Lab will be conducting experimental treatments this summer to determine if two widely used fungicides are effective in control.

For more information, residents may call van der Linden at 337-3669 ext. 5 or e-mail him at peter-vanderlinden@uiowa.edu.