Bur Oak Blight

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Bur oak (Quercus macrocarpa) is a majestic tree species native to North America, cherished for its grandeur, longevity, and ecological importance.  It is one of our most ecologically significant trees here in Rochester, Minnesota, having been a key species to our native savanna ecosystem that was once widespread here on the edge of the mesic hardwood forest.  However, in recent years, an emerging threat has been looming over these magnificent trees - Bur Oak Blight (BOB). Bur Oak Blight is a fungal disease that has been increasingly observed in several regions, causing significant concerns among arborists, conservationists, and property owners.  Here, we will delve into the intricacies of Bur Oak Blight, exploring its causes, symptoms, and effective management strategies.

Understanding Bur Oak Blight:

Bur Oak Blight is caused by the fungal pathogen Tubakia iowensis (previously known as Tubakia dryina), which primarily affects bur oak trees. The disease was first identified in Iowa in the late 1990s and has since been documented in various states across the Midwest and into eastern North America. While the exact mechanisms of its spread are still being studied, it is believed that environmental conditions and tree stress play crucial roles in its development and proliferation.

Causes of Bur Oak Blight:

1. Fungal Pathogen: The fungus Tubakia iowensis is the primary causal agent of Bur Oak Blight. It primarily infects the leaves of bur oak trees, leading to characteristic symptoms such as browning and premature leaf drop.
2. Environmental Factors: Certain environmental conditions, including high humidity, prolonged leaf wetness, and warm temperatures, create favorable conditions for the growth and spread of the fungal pathogen. These conditions are often prevalent during the spring and early summer months, coinciding with the period of active growth and leaf emergence in bur oak trees.
3. Tree Stress: Bur oak trees that are stressed due to various factors such as drought, poor soil conditions, or mechanical damage are more susceptible to infection by Tubakia iowensis. Stress weakens the tree's natural defenses, making it easier for pathogens to invade and colonize the host tissues.
4. Susceptibility:  Within the native population that is ubiquitous here in Rochester, Minnesota (var. oliviaform) genetic susceptibility appears to vary widely between individuals and play an important role.

Symptoms of Bur Oak Blight:

1. Leaf Lesions: One of the most characteristic symptoms of Bur Oak Blight is the presence of irregularly shaped, necrotic lesions on the leaves. These lesions typically appear as brown or purple blotches, often surrounded by a yellow halo. As the disease progresses, the lesions may coalesce, leading to extensive browning of the leaf tissue.
2. Premature Leaf Drop: Infected leaves may exhibit premature senescence and drop prematurely from the tree. This early defoliation can weaken the tree and affect its overall health and vigor, especially if the infection occurs repeatedly over multiple growing seasons.
3. Dieback: In severe cases of Bur Oak Blight, stressed trees attract populations of the native two-lined chestnut borer (TLCB).  These boring insects create heavy damage to the cambial layer of oak trees in their larval stage, causing progressive die-back and in some cases tree mortality.

Management Strategies for Bur Oak Blight:

1. Cultural Practices: ArborWise recommends implementing cultural practices that promote tree health and vigor that can help reduce the risk of Bur Oak Blight. This includes proper watering and mulching to ensure optimal growing conditions for the tree. Additionally, minimizing mechanical damage to the tree during pruning or construction activities can help prevent stress-induced susceptibility to fungal infection.  
2. Fungicide Applications:  ArborWise will apply a systemic fungicide and growth regulator to control symptoms and reduce tree stress.  Treatments provide two years of control.
3. Pruning and Sanitation: Removing and properly disposing of infected leaves and twigs can help reduce the inoculum levels of Tubakia iowensis in the vicinity of the tree. Pruning out dead or diseased branches can also improve air circulation within the canopy, reducing humidity and inhibiting fungal growth.
4. Genetic Resistance: Selecting and planting bur oak cultivars that exhibit resistance or tolerance to Bur Oak Blight can be an effective long-term management strategy. Researchers are actively studying the genetic mechanisms underlying resistance to identify and breed cultivars with improved disease resistance traits.

Characteristic leaf necrosis pattern of BOB

A bur oak heavily stressed by BOB.  This disease tends to evenly distribute throughout the canopy.