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. Occasionally, BOB can infect swamp white oak (Q. bicolor); 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 of BOB become visible during mid-July. The first symptoms are purplish spots on the veins on the lower side of the leaves. Next, the major leaf veins display a purplish color on both the upper and lower sides of the leaves (Fig. 1).
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 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.
Fruiting bodies on dead petioles from the previous season release spores during the spring and this is considered 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.