Posted February 12, 2011
Homegrown: Bur oak threat

Tivon Feeley, of the Iowa Department of Natural Resources forest health program, provided the following information about one threat to a popular Iowa tree:

One of Iowa’s most identifiable trees is under attack from a newly named disease that is currently causing more problems in the state than the high profile emerald ash borer or gypsy moth.

Bur oak acorns (Wikipedia image)

Bur oak blight has been in Iowa since the early 2000s and is already killing trees.   Symptoms of bur oak blight include a v-shaped brown discoloration of leaves and browning veins in July and August.  This disease can cause severe defoliation that can lead to morality of branches or entire trees.  Trees die after the fourth of fifth year of defoliation, usually from a secondary pest.

“Early detection of the disease is the key to managing it,” said Tivon Feeley, with the Iowa Department of Natural Resources forest health program.

Infected bur oaks tend to not drop the leaves in the fall like a healthy tree would. These infected leaves act as a source of inoculums for the healthy leaves that emerge and grow in the spring. The emerging leaves become infected with bur oak blight and the symptoms progress slowly until the leaves start browning in late summer.

Bur oak is native to all 99 Iowa counties and more than half of the counties have bur oak blight.  The disease is killing oaks from southwest Wisconsin to southern Minnesota, most of Iowa and to eastern Nebraska.

In 2008, bur oak ranked second among all tree species in volume of saw timber on forest land and provides substantial value for wood products. It is one of the most beneficial trees for wildlife habitat and provides food through its acorns for many game and non-game species.  It is a significant part of the largest forest type in Iowa.  Wild turkey and deer depend on forests for cover and search out oaks for mast before winter.

The loss of bur oak within the oak-hickory forest type will negatively impact the $1.5 billion contribution that fish and wildlife recreation provides to Iowa’s economy.  Additionally, Iowa’s 3 million acres of forest support an estimated 1.1 billion board feet of merchantable size bur oak, with an economic value to landowners of $330 million.

“Right now we are recommending that woodland owners do salvage cuts of infected trees to get maximum value of the tree before it dies, because we will not get rid of this,” Feeley said. “Bur oak blight is here to stay.”

While the outlook for bur oaks in the countryside is bleak, the outlook for those in town is slightly better.

In urban areas, injecting the fungicide, Alamo™, into the tree appears to have helped trees with their ability to recover.  Trees appeared symptomless for the first two seasons after being injected. However, it is likely that subsequent injections would be required when bur oak blight starts to defoliate the tree again.

“It’s important for tree owners to not re-apply the treatment until the tree starts to defoliate from the pathogen,” Feeley said.

Iowa’s overall urban tree canopy has 12 percent tree cover, of which, 3 percent of the trees are bur oak.  The estimated landscape value and removal cost for urban trees is more than $500 million.  Homeowners can have leaves tested for the disease by sending samples to Iowa State University’s Plant and Insect Diagnostic Clinic to have the fungus confirmed.  Information on the click and how to submit samples is online at

Proper woodland and community tree management have a critical role in creating healthy trees. Maintaining a diversity of trees is the best insurance a landowner can have.  Communities can create tree diversity by not having more than 10 percent of any one species represented.

Here is more information from Tom Harrington, of Iowa State University’s Department of Plant Pathology:

Leaf symptoms of necrosis (death) of the tissue along the veins. (Photo/Tom Harrington)

It’s beginning to look like bur oak blight, or BOB for short, is a long-time resident of Iowa, and he’s here to stay. Bur oak blight has been recognized in Iowa for only the last 6 or 7 years, but the fungus that causes it has probably been here much longer. A shift in climate to more frequent rain events appears to be increasing the severity of BOB throughout much of the western two-thirds of the state. The same phenomenon that contributes to Iowa’s now all-too-common floods may be helping BOB kill our state tree.

A common leafspot fungus, Tubakia dryina, was initially thought to be the cause of the blight on bur oak, but closer examination showed that the cause was a new species of Tubakia. With funding provided by the US Forest Service, we were able to show that there are five species of Tubakia that can infect bur oak, but only one species can cause dramatic leaf symptoms and tree mortality.

The disease tends to intensify year-to-year in individual trees, and if only a portion of the crown is affected, it usually starts in the lower branches and then later progresses up the tree. The fungus overwinters on the petioles of dead leaves that remain attached to branches. Spores are produced in May from black pustules on the petioles of these old leaves, and the spores infect the newly emerging shoots and leaves during rainy weather. Dramatic leaf symptoms do not become evident until July, however, and the severity of symptoms increases in August and September if weather conditions are right. Leaf symptoms include necrosis (death) of the tissue along the veins and wedge-shaped areas of browning at the tips or sides of the leaves. Severely affected trees may die after several years of severe defoliation.

Not all stands of bur oak are seriously affected by the disease, and not all trees are equally susceptible in even the most severely affected stands. Thus far, we have seen severe BOB on only naturally-established bur oak, especially on mature trees on upland sites that appear to be remnants of savannah forests. Bur oak in dense forests and in bottomland sites are less seriously affected. We have confirmed the pathogen in 55 of Iowa’s 99 counties, but most of the severely affected stands have been in the western half of the state. We’ve also confirmed BOB in eastern Nebraska and southern Minnesota.

We are experimenting with fungicide injections for control of BOB in high-value trees. Injections of propiconazole (Alamo) into bur oak in late May or early June, before symptoms appear, have been effective. However, propiconazole at the rate recommended for oak wilt can be phytotoxic to bur oak, and the treatment is costly.

As long as the rains keep coming, BOB will probably continue to intensify on upland sites across much of Iowa, and we could lose a number of stately bur oak. Hopefully our next generation of bur oak should be better adapted to a wetter climate and have the resistance necessary to withstand our long time resident, BOB.

An 18-minute video on the symptoms and other characteristics of BOB is available on the ISU Extension website.

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blight, bur oak, disease, DNR, forestry, fungus, , , ISU, Tivon Feeley, Tom Harrington

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