Bur Oak Blight
Bur oak blight 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.
Signs and Symptoms
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.
Type of Sample Needed for Diagnosis and Confirmation
In spring and early summer, when symptoms on new leaves may be on the way but not yet evident, we rely on inspecting the petioles of brown leaves attached from last fall.
Bur oak blight (BOB) is caused by the pathogen Tubakia iowensis and belong to a group of fungal organisms that are capable of live 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.
Bur oak blight is known to only infect Bur Oaks. Therefore 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.
Oaks can suffer from other diseases and pests that may be confused with some of the symptoms if not a detail examination and monitoring of the trees is conducted.
Consider submitting a sample to our clinic for confirmation. When sending us a sample, please fill out a form (https://store.extension.iastate.edu/Product/Plant-Problem-Diagnosis-Form), and collect a couple branches with symptomatic and healthy leaves. Include a couple acorns from the tree if possible. When collecting a sample, 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. 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 eliminates the pustules (where the fungus overwinter) and therefore 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 oak 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 at http://www.ipm.iastate.edu/ipm/hortnews/2011/2-9/buroakblight.html
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