Dense, deeply ridged bark has allowed bur oaks (Quercus macrocarpa) to survive the lightning-started wildfires that keep thin-barked trees off America’s prairies. Now an icon of the Midwestern grasslands, the 60- to 80-foot bur oak thrives in averagely moist, sunny conditions across U.S. Department of Agriculture plant hardiness zones 3 through 8. Two things distinguish this majestic tree from other members of its species -- exceptionally large, mossy-capped acorns and susceptibility to fatal bur oak blight.
The Tubakia iowensis fungus responsible for bur oak blight began attacking trees across the Midwest in the 1990s. One of several Tubakia species fungi that cause leaf blight in bur oaks, it's by far the most serious. While this blight threatens all varieties of bur oaks, it's especially serious on the subspecies Quercus macrocarpa var. oliviformis concentrated in Minnesota, Iowa and eastern Nebraska. Isolated mature trees growing on upland sites have a greater risk of infection than bottomland forest trees.
Blight symptoms surface in early summer on the lower half of an oak's canopy. Purplish-brown spots discolor and kill the leaf-backs' veins as they expand. V-shaped dead areas then cover the interveinal tissue, giving the leaves a scorched look. Many of the infected leaves die. Over time, the leaf spots spread to upper-canopy branches. Unlike healthy bur oaks that drop their leaves in fall, blight-affected ones retain much of their infected foliage. Even when wind clears the diseased branches, the leafstalks stay attached.
The blight fungus overwinters in raised black structures on the attached, diseased leaf stalks. During wet spring weather, these structures release infectious spores onto new leaves. Once a bur oak contracts the disease, it remains infected, developing symptoms each spring and summer as the infection consumes the new foliage crop. After several years of leaf loss, the affected branches -- and eventually the entire tree -- die.
Because bur oak blight spreads each spring from previously infected leaf stalks remaining on a tree, keeping the soil free of contaminated leaf litter is useless. Adequate water and fertilizer may improve the diseased tree's vigor enough to extend its life span. Watering in dry spells whenever the top 3 to 4 inches of soil are dry reduces drought stress. Scatter a 15-10-10 granular fertilizer at the rate of 1 cup for each 100 square feet of the tree's root area, extending outward from the trunk in a radius equal to the tree's height, and water it into the soil. Make the first yearly application when the leaves begin emerging in spring and repeat every six weeks until new growth stops.
Injecting an infected bur oak's trunk with propiconazole fungicide after its spring leaves completely expand lessens the severity of its symptoms over the next year. Only certified professionals, however, have the equipment and qualifications to perform the injection.
- Missouri Botanical Garden: Quercus Macrocarpa
- United States Department of Agriculture Forest Service Northeastern Area: Pest Alert -- Bur Oak Blight
- University of Illinois Extension Home, Yard & Garden Pest Newsletter: Bur Oak Blight
- University of Minnesota Plant Disease Clinic: Bur Oak Blight (BOB) -- There’s a New Kid in Town
- 1979 Sunset New Western Garden Book; David E. Clark
- Texas Cooperative Extension: Tree Planting and Care
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