Bur Oak Blight

What Is Bur Oak Blight?

Bur Oak Blight, or BOB, is a fungal disease caused by Tubakia iowensis. It primarily affects bur oak trees, particularly the smaller-acorn variety known as Quercus macrocarpa var. oliviformis, which is common throughout Missouri and the St. Louis region. This disease has become increasingly problematic over the past two decades, especially in areas that experience wet springs and summers.

The fungus targets the leaves and petioles, causing noticeable browning, leaf drop, and long-term weakening of the tree. While it may begin with minor symptoms, Bur Oak Blight can worsen with each passing year if left unmanaged. The disease spreads when fungal spores overwinter on infected leaves and petioles that remain on the tree into spring, making it more severe year after year.

Bur Oak Blight is more than just a seasonal leaf issue. It can cause significant canopy loss, dieback of major limbs, and increased vulnerability to other pests and diseases like the two-lined chestnut borer or root rot. In some cases, untreated trees may eventually die.

Signs Your Tree Might Have Bur Oak Blight

Bur Oak Blight symptoms typically appear in mid to late summer and can sometimes be mistaken for drought stress or other leaf diseases. However, there are several distinct indicators that can help you spot BOB early and take action:

• Dark, wedge-shaped brown lesions that begin at the tips of leaves and progress inward along the veins.
  
  
• Vein discoloration, where major veins turn brown or black, often with sharply defined edges.
  
  
• Premature leaf drop, usually beginning in the lower crown and spreading upward throughout the canopy.
  
  
• Black fungal fruiting structures (called pycnothyria) visible on the underside of leaves and petioles—often crusty or speckled in appearance.
  
  
• Retention of infected leaves and petioles through the winter, instead of falling off normally.
  
  
• Repeated leaf loss year after year, leading to thinning canopies, reduced growth, and weakened tree vigor.

How Bur Oak Blight Spreads to Other Trees

Bur Oak Blight is a slow-spreading but persistent disease. While it doesn’t move rapidly from tree to tree like some pests or viruses, it builds year after year and spreads under the right environmental conditions. Here’s how the disease typically spreads:

• Overwinters on infected petioles and leaves that remain attached to the tree through winter. These tissues harbor fungal spores that will be released in spring.
  
  
• Rain splash disperses spores during wet weather, spreading them from infected leaves to healthy new growth on the same or nearby trees.
  
  
• Wind and insects can carry spores to nearby bur oaks, though short-distance transmission is more common than long-range infection.
  
  
• Contaminated pruning tools or equipment can occasionally spread spores if proper sanitation isn’t followed.
  
  
• High humidity and prolonged leaf wetness promote spore germination and infection, especially in dense urban canopies or shaded areas with poor air circulation.

Common Trees in Missouri at Risk from Bur Oak Blight

Bur Oak Blight is highly specific to bur oaks, but not all bur oaks are affected equally. While Bur Oak Blight is not known to infect red oaks or other unrelated hardwoods, proximity to infected trees and repeated exposure to spores can still stress nearby trees or create conditions favorable for other diseases. The following tree types are most vulnerable to infection and long-term damage in Missouri and the greater St. Louis area:

• Bur Oak (Quercus macrocarpa)
  The primary host of Bur Oak Blight. This species is widespread across Missouri, both in natural woodlands and urban landscapes.
  
  
• Bur Oak (small-acorn variety – Q. macrocarpa var. oliviformis)
  This variety, common in the Midwest, is especially susceptible. It tends to grow in urban and suburban areas where conditions (like poor airflow and compacted soils) can accelerate disease spread.
  
  
• Hybrid White Oaks
  Though less commonly affected, hybrids between bur oak and white oak can occasionally show symptoms if environmental conditions are favorable for the fungus.
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• Other Native White Oaks (rarely)
  White oak species like swamp white oak or overcup oak are generally resistant, but isolated cases of infection have been reported under prolonged exposure.
  
  

Treatment and Prevention for Bur Oak Blight

Bur Oak Blight can be managed effectively with a combination of proper care, proactive monitoring, and targeted treatment. While it’s not always possible to eliminate the disease entirely, these steps can greatly reduce its impact and slow its progression:

• Fungicide Injections
  Trunk injections of fungicides such as propiconazole (applied by a certified arborist) have shown success in reducing symptoms. These are typically done in late spring or early summer and may need to be repeated annually or biannually.
  
  
• Annual Monitoring
  Keep a close eye on trees each summer for signs of infection. Early detection allows for timely intervention and better long-term outcomes.
  
  
• Sanitation and Leaf Removal
  Remove and dispose of fallen leaves and twigs in autumn to reduce overwintering spores. Raking around the tree’s base helps break the disease cycle.
  
  
• Prune Dead or Infected Limbs
  Prune in late winter or early spring to remove deadwood and improve air circulation. Avoid pruning during wet seasons to reduce the risk of spreading spores.
  
  
• Reduce Tree Stress
  Water deeply during dry periods, mulch to maintain soil moisture, and avoid compacting soil around the root zone. A healthy tree is more resilient to infection.
  
  
• Avoid Wounding the Tree
  Injuries to the bark and limbs can provide an entry point for secondary pests and pathogens. Be careful with lawn equipment and construction near the root zone.