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About Oak Wilt

Oak wilt is a deadly infectious disease caused by the fungus Ceratocytis fagacearum, which invades and disables the water-conducting system in susceptible trees. Live oaks and red oaks (including shumard oaks, Spanish oaks and blackjack oaks) are the most susceptible to the disease and almost always die when infected. In a cruel irony, the biggest and healthiest trees are thought to be the most vulnerable to the disease.

How Does It Spread?

There are two main pathways for the spread of the disease:

  1. A disease center spreads outward as the fungus travels through the vast interconnected root systems of live oaks. The Texas Forest Service estimates that over 90% of trees with oak wilt are infected this way.
  2. New disease centers are started by flying insects that come in contact with the sap of an infected tree and then transmit the fungus to a healthy tree. Tree sap on the surface of a cut and fungal mats that form on infected red oaks attract sap-feeding beetles. Fungus spores can attach to these beetles. The spores are then transported by the beetles and dislodged on the surface of a healthy tree, where they germinate and infect the new host.

People can cause the spread of the disease by moving firewood from infected red oaks bearing fungal mats to new locations. It is also speculated, though not proven, that the use of contaminated tools to prune healthy trees can spread the disease. The Texas Forest Service estimates that fewer than 10% of trees with oak wilt are infected through above-ground vectors.

How Do I Tell if a Tree Has Oak Wilt?

Leaf symptoms, pattern of spread and rate of tree mortality are all means of identifying an oak wilt infection. Out-of-season browning and leaf drop is a telltale sign. Trees will typically drop most of their leaves within a few months of being infected. Examine fallen leaves of live oaks for veinal necrosis (yellowing or browning of the veins on the leaves), one of the most common symptoms of oak wilt. Here’s a photo. Healthy trees within 200 feet of infected trees are at immediate risk of becoming infected through root transmission. Contact the Texas Forest Service or an experienced arborist if you’re in doubt.

How Do We Stop Existing Oak Wilt Infections?

Oak wilt infections can be contained using the following steps:

  • Stop the spread through live oak roots by installing a trench at least 100 feet beyond the perimeter of the infection center to sever root connections. Streets may not be effective barriers unless utility trenching has been conducted recently at sufficient depth.
  • Inject high-value threatened oaks with Alamo fungicide. The fungicide will not stop root transmission of oak wilt, but if applied properly and regularly, it may be effective in saving individual infected trees.
  • Destroy infected red oaks

For more information on oak wilt, see the following websites: