Disease name: Bur oak blight (BOB)
Pathogen name: Tubakia iowensis (fungus)
Hosts: Bur oak (Quercus macrocarpa)
Bur oak blight (BOB) is a leaf blight caused by the fungus Tubakia iowensis. It is a disease of concern to anyone who has bur oak trees on their property. BOB is most prevalent on Quercus macrocarpa var. oliviformis, an upland variety of bur oak characterized by its relatively small, olive-shaped acorns.
Susceptibility of bur oaks to BOB is highly variable and trees of all sizes are at risk. It is unknown whether hybrids of bur oaks are susceptible. Repetitive defoliation of trees by BOB increases susceptibility to secondary problems such as the two-lined chestnut borer (Agrilus bilineatus) and Armillaria root rot, which ultimately cause tree decline and death.
Symptoms
Symptoms of bur oak blight (BOB) become visible during mid-July. The first symptoms are purplish spots on the veins on the lower side of the leaves (fig. 1). Next, the major leaf veins display a purplish color on both the upper and lower sides of the leaves.
In August and September, symptoms will worsen, with veins dying and the infection moving to the end of the leaf, leading to a wedge-shaped dead area. The dying petioles (leaf stalks) turn entirely brown, and develop black pimple-like fruiting bodies on the surface that are visible with the naked eye (fig. 2). The presence of these fruiting bodies on the petiole is essential to the confirmation of BOB. Dead leaves and/or dead petioles will remain attached to the stems throughout the winter. The disease starts in the lower, inner sections of the canopy and spreads upward and outward as it progresses.
Disease Cycle
Bur oak blight overwinters as fruiting bodies on dead petioles from the previous season. In spring, these fruiting bodies release spores and this is the primary infection. The spores are spread by splashing rain and infect the newly-emerged foliage. The intensity of the disease within a season is dependent on spring moisture.
Although infection occurs in spring, leaves will appear healthy until mid-to-late July. The foliar symptoms progress throughout the month of August and early September. Secondary infections will occur throughout the season from spores developed under leaf veins during wet periods.
Fruiting bodies are produced on dying petioles in late summer. Entire leaves, or just petioles, will remain attached to the stems until the following spring and repeat the cycle.
Management
Cultural Management
Currently there are no cultural methods to manage bur oak blight, other than reducing stress on the tree through proper care. This may help reduce the severity of the disease. Removing leaf litter under the tree will not reduce the infection rate the following year, since the petioles, which are the source of new infection, remain attached to the tree. Plucking off petioles is likely not an effective method as it is not practical on large trees.
Chemical Management
A fungicide treatment in spring after full leaf expansion is the most effective method for managing bur oak blight. Results from studies conducted at Iowa State University showed injections of propiconazole reduced symptoms in the fall and in the following year. Yearly applications are not recommended. Fall applications of fungicides are not recommended for bur oak blight.
The pesticide information presented in this publication is current with federal and state regulations. The user is responsible for determining that the intended use is consistent with the label of the product being used. The information given here is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement made by The Morton Arboretum.
For current pesticide recommendations, contact The Morton Arboretum’s Plant Clinic (630-719-2424 or plantclinic@mortonarb.org).