Bur oak blight: what the extension services record
What The Morton Arboretum, Iowa State University Extension and University of Minnesota Extension say about bur oak blight: the fungus, the one host, the symptoms by month, the look-alikes Minnesota lists and how the disease cycles each year.
Checked against the sources at the bottom of this page on October 9, 2026. Rules, fees and pay change: the source has the last word.
This page describes what the named sources say. It is not a diagnosis of any tree and does not say what should be done with one. None of the three pages read lists bur oak blight as regulated or notifiable. Each section names its source, all read on October 9, 2026. The full list of diseases is on the tree diseases page. Abbreviation: BOB is bur oak blight.
Pathogen and host
The Morton Arboretum and Iowa State University name the cause as the fungus Tubakia iowensis. The Arboretum calls it a leaf blight and says it is most prevalent on Quercus macrocarpa var. oliviformis, an upland variety of bur oak with relatively small, olive-shaped acorns. University of Minnesota Extension says it infects only bur oaks, the small-acorn variety, and Iowa State says it is known to infect only bur oaks and tells readers to check leaf shape and acorns to confirm the tree is a bur oak. The Arboretum says susceptibility varies a great deal, trees of all sizes are at risk, and it is unknown whether hybrids of bur oak are susceptible. Iowa State describes the fungus as belonging to a group able to live inside plant tissue without causing visible symptoms for a period.
Symptoms as the sources state them
The Arboretum says symptoms become visible in mid-July. The first are purplish spots on the veins on the underside of leaves, then purple major veins on both sides. In August and September the veins die and infection moves to the leaf end, leaving a wedge-shaped dead area. Dying petioles, the leaf stalks, turn brown and carry black pimple-like fruiting bodies visible to the naked eye. The Arboretum says that presence of those fruiting bodies on the petiole is essential to confirming the disease, and that dead leaves or petioles stay attached through winter.
University of Minnesota Extension gives dark brown dots or short lines on lower leaf veins in spring and early summer, random lengths of vein turning brown in mid-summer and fall, some expanding into brown wedge-shaped areas, leaves that may turn completely brown with some dropping early, and small raised black dots on the petiole. Iowa State describes death of tissue along the veins and wedge-shaped browning at leaf tips or sides, and says the fungal structures are microscopic and resemble shields with circular spores. All three place the start low and inside: the Arboretum says the disease starts in the lower, inner canopy and spreads upward and outward, Minnesota says it progresses over several seasons, and Iowa State says that if only part of the crown is affected it usually starts in the lower branches.
Disease cycle
The Arboretum says the fungus overwinters as fruiting bodies on dead petioles that stay on the tree after leaf fall, releases spores in spring that splash in rain onto new foliage, and that the severity in a season depends on spring moisture. Iowa State gives spore production from black pustules in May and says dramatic leaf symptoms do not show until July, worsening in August and September if weather allows. The Arboretum adds that infected leaves look healthy until mid to late July and that secondary infections occur through the season from spores produced under leaf veins in wet periods.
Damage over the years
Iowa State says the disease tends to intensify year to year on an individual tree, and that severely affected trees may die after several years of severe defoliation. The Arboretum says repeated defoliation raises susceptibility to secondary problems, naming two-lined chestnut borer and Armillaria root rot, which it says ultimately cause decline and death. Iowa State says it has observed the disease in Iowa since around 2005, that the fungus has probably been there much longer, and that a shift in climate to more frequent rain events appears to be raising its severity across much of the western two-thirds of the state.
Look-alikes the sources name
University of Minnesota Extension lists bur oak blight among seven causes of oak leaves that are wilted and completely brown. The others are two-lined chestnut borer, Botryosphaeria canker, twig girdler, oak wilt, Ganoderma butt rot and drought. For the borer see twolined chestnut borer, and for oak wilt see oak wilt. Minnesota gives, for oak wilt in the red oak group, leaves that wilt with brown edges and a green centre, symptoms first at the top of the crown, and death in 1 to 3 months. It describes the borer by D-shaped exit holes and S-shaped galleries under the bark. Iowa State says oaks can suffer from other diseases and pests that may be confused with bur oak blight without a detailed examination.
Laboratory confirmation
Iowa State says its Plant and Insect Diagnostic Clinic can investigate and confirm the disease. It says trees infected in earlier years can show pustules on the petioles of dried leaves from the earlier seasons. It points to each state's diagnostic laboratory, and the directory lists the plant disease and insect diagnostic labs in the United States.
Status
None of the pages names a regulation, quarantine or report line.