Bur Oak Blight

Need To Know

  • Bur oak blight symptoms begin in the bottom of the canopy and move upward.
  • Leaves will die along the vein tissue and brown at the tips in wedge shapes. 
  • Severely affected trees can die after years of severe defoliation as the disease intensifies year after years. 
  • Raking or pruning is not a practical management method, but fungicide applications in the late spring has shown promise. 

Overview of bur oak blight

Leaf symptoms of necrosis (death) of the tissue along the veins. 

Bur oak blight (BOB) has been observed in Iowa since around 2005, 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.

Symptoms of bur oak blight

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. 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.

Signs of bur oak blight

Fungal structures of the fungal pathogen Tubakia iowensis are microscopic and resemble shields with circular spores.

Fungal structures of the fungal pathogen Tubakia iowensis
Fungal structures of the fungal pathogen Tubakia iowensis

Disease cycle of bur oak blight

Bur oak blight is caused by the pathogen Tubakia iowensis and belongs to a group of fungal organisms that are capable of living as an endophyte (inside the plant tissue) without causing apparent symptoms for a period. 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.

Type of Sample Needed for Diagnosis and Confirmation

The Iowa State University Plant & Insect Diagnostic Clinic can help you investigate and confirm if your plant has this disease. Please see our website for current forms, fees, and instructions on collecting and packing samples. Contact information for each state's diagnostic laboratory for U.S. residents can be located on the NPDN website.  If you have a sample from outside of Iowa, please DO NOT submit it to the Plant & Insect Diagnostic Clinic without contacting us.

Want to submit a sample? Follow the instructions at Submitting Trees and Shrubs

Management of bur oak blight

Bur oak blight is known to only infect Bur Oaks. Therefore it is important to observe the characteristics of the tree (leaves shape and acorns, see Bur Oak characteristics in this ISU Forestry Extension website at (https://www.extension.iastate.edu/forestry/iowa_trees/trees/bur_oak.html) and determine if the problematic tree is, in fact, suffering from bur oak blight.

Oaks can suffer from other diseases and pests that may be confused with some of the symptoms if not a detailed examination and monitoring of the trees is conducted.

Petioles of brown leaves attached from last fall

Consider submitting a sample to our clinic for confirmation. When sending us a sample, collect a couple of branches with symptomatic and healthy leaves.  Include a couple of acorns from the tree if possible. Look for branches with leaves attached that have purple to red veins or large chlorotic-necrotic wedged areas. Trees infected in prior years can have pustules located on the petiole of dried-out leaves from the previous years. The diameter of the branches is not relevant.

Sanitation is not a practical measure to manage bur oak blight. The source of the pathogen, pustules on last year’s petioles, remain attached to the tree and pruning could reduce the pathogen numbers, but is not practical in larger trees. Raking the leaves can do little to reduce the pathogen numbers as the pathogen overwinter on petiole pustules that remain attached to the tree.

Fungicide treatments have shown promise in preliminary studies. See Dr. Harrington's video "potential management with fungicides"Application (injection) is recommended after full leaf expansion in the spring (late May, early June) to slow down the transition of the pathogen from dormant infection (endophytic) to actively causing symptoms.  Fall treatment will not cure an infected tree or eliminate the pustules (where the fungus overwinter) and therefore is not recommended.

For more information visit the Pest Alert - Bur Oak Blight downloadable for free at https://store.extension.iastate.edu/Product/Pest-Alert-Bur-Oak-Blight

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 oaks should be better adapted to a wetter climate and have the resistance necessary to withstand our longtime resident, BOB.

Adapted from the article "It looks like bur oak blight (BOB) really isn't that new" published originally on 2/9/2011

Please keep in mind the considerations on the resource below when choosing an arborist on the publication “Choosing and arborist” available for free download at https://store.extension.iastate.edu/Product/Choosing-an-Arborist


Fungicide applications may be avoided by following good Integrated Pest Management practices like those listed in this encyclopedia article. Often, the only preventative application is effective to manage plant diseases. If the problem requires a fungicide, state law requires the user to read and follow all labels accordingly. For more information, read Proper fungicide use.  

Last reviewed:
January 2022

Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Yard and Garden, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on September 1, 2016. The information contained within may not be the most current and accurate depending on when it is accessed.