Oak Trees and Heart Rot

• Laetiporus sulfureus, or heart rot, is a fungal disease of oak trees that produces few external symptoms when it is present. This disease enters an oak tree through wounds, as from improper pruning or weather damage, and rots out the heart of the oak tree. The first external sign of heart rot is the presence of bright yellow shelf- or conk-like fungal growths that appear on the trunk of the oak tree. Remove oak trees with heart rot, as they are at risk of falling.
  
  

Oak Wilt

• Another disease of oak trees is Ceratocystis fagacearum, or oak wilt. The disease is found only in the United States, according to the Iowa State University Extension. Oak wilt causes the leaves of the tree to use its own defenses to block xylem vessels, leading to the dieback of branches and premature defoliation of the tree. Do not prune oak trees between early April and late June, to avoid causing an oak tree to contract the disease; injections of fungicide help control the spread of oak wilt.
  
  

Oak Leaf Blister

• Some fungal disease affect the appearance of the leaves without doing excess damage to the tree itself. Taphrina caerulescens, or oak leaf blister, is a fungal disease that causes light green spots or blisters to form on the leaves of oak trees. According to the Clemson Cooperative Extension website, fungicides applied to the oak tree before bud break help control the spread of the disease, but fungicides are usually ineffective if applied after the disease is present.
  
  

Powdery Mildew on Oaks

• Powdery mildew is caused by a variety of fungi that cause a white or gray powder to cover the leaves of the oak tree. Powdery mildew prevents the tree from photosynthesizing properly, leading to weak growth. Severe infestations lead to leaves on oak trees that become curled or drop prematurely from the tree. Minor infections of oak trees do not require chemical control, but fungicides are effective at killing the fungi that cause powdery mildew.