What Is Bur Oak Blight?
Bur Oak Blight (BOB) is a fungal disease caused by Tubakia iowensis, a fungus first identified in Iowa in the early 2000s and named for our state. It infects bur oak trees (Quercus macrocarpa) and is now recognized as a serious threat to bur oaks throughout the Midwest and Great Plains.
Bur oaks are iconic trees in Iowa's landscape, tough, long-lived, and drought-tolerant under natural conditions. But BOB has emerged as a significant stressor, particularly in urban and suburban settings where trees are already dealing with compacted soils, pollution, and reduced root zones.
Iowa-specific concern: Bur oak blight is particularly prevalent in Iowa. The fungus thrives in our humid summers, and repeated annual infections weaken trees significantly over time. Don't confuse it with normal summer stress, if your bur oak looks burned by midsummer, call us.
What Causes Bur Oak Blight?
The Tubakia iowensis fungus spreads through:
- Wind-blown spores: carried from infected leaves and branches to healthy tissue during the growing season
- Insect activity: insects can transport spores from infected to healthy trees
- Wet, humid conditions: the fungus thrives in warm, moist conditions, particularly with cool nights and warm, wet days in late spring and early summer
Stressed trees are far more vulnerable. Drought, root damage, soil compaction, and construction damage all make a bur oak much more susceptible to BOB infection.
Symptoms to Watch For
Bur oak blight symptoms typically appear starting in late spring or early summer and progressively worsen through the season:
- Leaf browning: starts at the tips and edges, moving inward; leaves take on a scorched appearance
- Dark streaks along the veins or edges of leaves
- Premature leaf drop: infected leaves fall early, starting from the lower crown and moving upward
- Branch dieback: in advanced infections, entire branches die, especially in the upper portions of the crown
- Canker formation: in severe cases, cankers form at the base of infected branches where bark darkens and sinks
- Repeated year-over-year decline: BOB returns annually, with each infection weakening the tree further
How We Treat Bur Oak Blight
BOB is challenging to eliminate completely, but a combination of fungicide applications, proper pruning, and cultural management significantly reduces its impact and keeps your tree healthier year over year.
Fungicide Applications
Preventative fungicide treatments, applied when buds swell in spring, before infection begins, can substantially reduce the severity of BOB. We use fungicides such as chlorothalonil, thiophanate-methyl, or propiconazole depending on the situation. Timing is critical: applications must coincide with bud break and be reapplied as needed through the growing season, particularly during wet periods.
Pruning Infected Branches
Removing dead and heavily infected branches reduces the fungal load in the tree and improves air circulation in the canopy. All infected material is disposed of properly, never composted, as this spreads spores.
Supporting Tree Health
A vigorous, healthy bur oak handles BOB much better than a stressed one. Our recommendations typically include:
- Proper mulching around the base (no mulch volcanoes - the root flare must stay exposed)
- Soil aeration to relieve compaction and improve root oxygen and water access
- Fertilization with a slow-release product to help the tree regain vigor
- Adequate watering during dry periods without over-irrigation
- Shortstop growth regulator to redirect energy from top growth to root development, improving stress tolerance
Service Area
We treat bur oak blight throughout Central Iowa, including Roland, Story City, Ames, Boone, Nevada, Huxley, Ankeny, Gilbert, and surrounding communities. If you're unsure whether you're in our service area, give us a call.