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Tree removal services by Accurate Tree Services + H&H Arborists in the Greater Madison, WI

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Bur Oak Blight: Why Wet Springs Are Hurting Your Bur Oaks

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You certainly aren’t the first Madison-area homeowner or property manager to walk outside and see that your bur oak seems to have developed splotchy brown leaves by mid-August. You may have even looked around the tree to see if you could spot the cause. 

But unfortunately, this particular issue doesn’t leave many obvious calling cards, and the problem may have actually begun months ago, during the spring. 

Called bur oak blight (BOB), this disease starts infecting Wisconsin’s bur oaks early in the year, but the symptoms only appear during mid- to late summer.  Adding further confusion to the issue, it doesn’t necessarily cause serious tree illness every year — it’s most severe during years with wet springs. 

We’ll explain everything you need to know about this disease, including the treatments available, below. 

Bur Oak Blight: A Fungal Disease of Bur Oaks

Bur oak blight is a fungal disease caused by Tubakia iowensis. It primarily affects bur oaks (Quercus macrocarpa), and it has become an increasingly important concern in Wisconsin and other portions of the Midwest.

Fungal diseases are fairly common in trees, but many are relatively minor. They cause cosmetic damage ranging from leaf spots to defoliation, but the damage is often limited to that year’s foliage. The next spring, the tree produces new foliage and often continues to thrive. 

However, BOB is different. It infects the leaf petioles — the leaf “stems” — in addition to the actual leaf blades. 

It then alters the manner in which the trees shed their leaves, causing the infected petioles to remain on the tree while the blades drift away. These infected petioles can then persist through winter and serve as a source of infection again in spring. 

However, not all springs are created equal. Some — especially cool, rainy springs — cause much worse BOB outbreaks than others. That’s primarily because frequent rainfall and prolonged periods of leaf wetness create favorable conditions for infection.

Over time, this recurring cycle can cause affected bur oaks to lose significant amounts of healthy foliage year after year. Although bur oak blight does not typically kill otherwise healthy trees quickly, repeated infections can cause trees to decline over time.

What Trees Are Susceptible to Bur Oak Blight?

Though it has occasionally been found to infect swamp white oaks (Quercus bicolor), BOB primarily affects bur oaks.  However, not all bur oaks appear to be equally susceptible, and some varieties appear to be at higher risk than others. 

For example, Quercus macrocarpa var. oliviformis — an upland variety of bur oak that produces relatively small, olive-shaped acorns — seems to be the most frequent target of the disease. By contrast, a large-acorned variety of the bur oak (Quercus macrocarpa var. macrocarpa) appears much less susceptible. 

It’s not entirely clear whether the difference is due to genetic variation between the two varieties, differences in the sites where they typically grow, or a combination of the two.

Variation among individual trees may also play a role, as it is possible to find severely affected bur oaks growing right beside apparently healthy ones. 

So, while all bur oaks are worth monitoring, trees that grow on upland sites and produce relatively small acorns may warrant particularly close attention.

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Bur Oak Blight: Symptoms of the Disease

One of the tricky things about bur oak blight is that symptoms don’t usually become obvious until well after the initial infection occurs. Trees are typically infected during spring, but noticeable leaf discoloration often doesn’t develop until July or August.

Some of the most common signs and symptoms of BOB include:

  • Brown or Purple Lesions Along the Leaf Veins: Foliar discoloration often begins along the veins on the lower portions of affected leaves and gradually spreads outward.
  • Wedge-Shaped Areas of Dead Leaf Tissue: As the lesions expand, they can form distinctive wedge-shaped areas of browning bordered by leaf veins.
  • Large Portions of Leaves Turning Brown: Individual lesions may eventually merge, leaving much of the leaf brown and giving the canopy an overall scorched appearance.
  • Premature Leaf Loss: Severely affected trees may begin shedding damaged foliage before normal fall leaf drop.
  • Dead Leaves Remaining on the Tree: Some affected leaves may remain attached well into the fall rather than dropping normally.
  • Retained Petioles: Infected leaf blades may eventually fall while their petioles remain attached to the twigs.  
Symptom Location Can Be a Clue

BOB symptoms often begin in the lower portions of the canopy and become more widespread as the disease intensifies over successive years.

Bur Oak Blight: Support, Prevention, & Treatment

Unfortunately, BOB can’t be controlled in the same ways many other fungal diseases can. Because the fungus overwinters in infected petioles that remain attached to the tree, several common management strategies that work for things like anthracnose simply aren’t effective.

Things that generally won’t help control BOB include:

  • Raking and removing fallen leaves. The fungus primarily survives in infected petioles that remain attached to the tree, rather than the leaves scattered across the ground.
  • Pruning out infected foliage or petioles. Removing individual infected petioles from the canopy of a mature bur oak isn’t practical and won’t reliably eliminate the disease.
  • Initiating treatment for this year’s problems. By the time leaves begin developing obvious brown lesions in summer, there’s no way to repair the damage they’ve suffered. 

Fortunately, there are still several things you can do to protect affected bur oaks.

A few strategies that may help address BOB-susceptible trees include:

  • Keep trees as healthy as possible. Provide supplemental water during droughts, protect the root zone from compaction and construction damage, and avoid unnecessary injuries.  
  • Monitor the tree from year to year. Mild or occasional BOB usually doesn’t warrant aggressive treatment, but monitor trees for frequent, repeated infections.  
  • Consider fungicide treatment for previously affected trees. Trunk injections can reduce BOB symptoms and may provide protection for more than one growing season.  
  • Have the diagnosis confirmed. Other diseases and environmental problems can cause similar symptoms, so consider having an ISA-certified arborist submit samples for laboratory testing.

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Bur Oak Blight: FAQ

Bur oak blight can certainly cause confusion among homeowners and property managers, given the somewhat unusual infectious cycle it presents. To help you better understand this important tree disease, we’ll answer some of the most common questions our clients have below. 

Will Bur Oak Blight Kill My Tree?

Bur oak blight does not typically kill otherwise healthy trees — at least, not very quickly. However, severe infections that occur repeatedly can reduce the amount of healthy foliage a tree produces and cause the tree to decline over time.  

Does Bur Oak Blight Affect Other Types of Oak Trees?

BOB primarily affects bur oaks, although the pathogen has occasionally been found on swamp white oaks. Even among bur oaks, some varieties and individual trees appear considerably more susceptible than others.

Why Is Bur Oak Blight Worse After a Wet Spring?

The fungus responsible for BOB infects developing foliage during spring, and prolonged periods of moisture create favorable conditions for infection. That’s why cool, rainy springs can lead to particularly severe symptoms later in the summer.

Can Bur Oak Blight Spread from One Tree to Another?

Yes. The fungus produces spores that can infect susceptible foliage, so BOB can occur on multiple bur oaks in the same area. However, susceptibility varies considerably among individual trees, which is why a severely affected bur oak may sometimes grow beside one showing no symptoms.

Should I Rake Up Leaves from a Tree with Bur Oak Blight?

Raking leaves is generally not an effective way to control BOB. Unlike many fungal leaf diseases, the BOB fungus primarily survives the winter in infected petioles that remain attached to the tree. Removing fallen leaves therefore does little to eliminate the source of infection.

Can Bur Oak Blight Be Treated with Fungicides?

In many cases, yes. Fungicide trunk injections can reduce symptoms in repeatedly affected trees and may provide protection for more than one growing season.  

When Should I Call an Arborist About Bur Oak Blight?

Consider contacting HH Accurate Arborists if your bur oak develops extensive browning, experiences symptoms repeatedly, begins losing canopy density, or shows other signs of declining health.  

Let HH Accurate Arborists Help Protect Your Oaks

Bur oak blight is certainly an important threat to our local trees, but help is available. 

The ISA-certified arborists at HH Accurate Arborists will be happy to evaluate your trees, determine what’s causing the problem, and recommend a treatment or management plan when necessary. With decades of experience caring for trees throughout the Greater Madison area, we understand the diseases and environmental stresses Wisconsin’s trees face.

Contact us today to schedule an evaluation.

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