Why Texas Post Oak Trees Are Dying

Texas is home to many types of shade trees, including the post oak.

Common to the dry, sandy or rocky upland areas of East and Central Texas, the post oak is a medium-sized member of the white oak group, typically reaching 50 to 60 feet. For the past 50 years, oak tree mortality has become a rising problem throughout the state. The post oak in particular is susceptible to environmental stresses and several disease-causing organisms.

Hypoxylon Canker

Hypoxylon canker is an often fatal disease caused by the fungus Hypoxylon atropunctatum. This disease affects numerous types of oak trees, but the post oak is perhaps the most susceptible. Affected trees first display sparse, yellow leaves and dead branches. As the disease spreads, bark sloughs off and crusty, dark-brown fungal structures become evident on the bare trunk. Eventually these dark spots turn light gray. The disease may take more than a year to kill a tree. Hypoxylon canker most often affects trees weakened by stress factors such as drought or insect damage. Chemical fungicides are usually ineffective. Maintaining healthy trees is the best prevention for this disease.

Oak Tree Decline

Several factors cause oak tree decline, including drought, age, insect damage and presence of other diseases, particularly hypoxylon canker. Trees more than 70 years old and those located on dry, rocky ridgelines are especially vulnerable. Although no single, causative pathogen has been identified, the disease is associated with the fungus Cephalosporium diospyri. Oak tree decline is a slow-progressing disease that begins with the dying back of leaves on the crown of the tree. As the disease spreads, twigs and branches die until only the main scaffolding branches remain. Mortality typically occurs within two to five years from onset of disease. Prevention of oak tree decline requires removal of old, weak or diseased trees.

Anthracnose

Common to the white oak group, anthracnose is a fungal disease that particularly affects the post oak. Anthracnose attacks leaves, leaf buds and new shoots. Although not always fatal, it causes extreme defoliation and stunted growth and can severely weaken trees over time. It spreads rapidly in moist, warm springtime weather. Irregular, brown, necrotic spots, appearing on leaves in early summer, characterize the disease. Eventually pustules form along the veins and midribs of infected leaves. If left untreated, the disease overwinters on twigs and branches and spreads again in the spring. Treatment requires application of fungicide and removal of dead or diseased branches. Also, rake and remove diseased leaves as soon as they fall.

Ball Moss

Ball moss (Tillandsia recuruata) is an epiphyte that grows on the bark of many Texas shade trees, including the post oak. It spreads quickly in grayish-green clusters, containing numerous plants. Roots of the ball moss plant attach to the tree's bark. Ball moss kills new buds and branches by enveloping and smothering tree limbs. Apply fungicide, especially before anticipated rainfall, to control ball moss.

References

• Texas A&M University System Cooperative Extension; Major Oak Diseases and Their Control; Jerral D. Johnson, et al.; March 2006
• University of Tennessee Extension; Managing Oak Decline; Wayne K. Clatterbuck, et al.
• Neighborhood Association of Southwestern Williamson County: Oak Tree Diseases
• Texas Forest Service: Trees of Texas, Post Oak