Tree Diseases

Oak Wilt Disease

Oak wilt disease, common in the Texas Hill Country, affects a variety of oak trees. This disease is caused by a fungus, Ceratocysris fagacearum, that clogs the water conducting vessels of infected trees, causing them to wilt and die within a few weeks or months.

The fungus spreads from infected to healthy trees in two ways:

  • First, long range transmission is accomplished with the help of a small insect. Sap feeding beetles called nitidulids, can carry fungal spores from infected oaks to healthy oaks, inoculating them with the fungus when feeding on sap from fresh wounds. Although these beetles generally fly short distances, greater distances are covered when beetles and spores hitchhike on unseasoned firewood.
  • The second path of transmission is from tree to tree through root grafts. This is possible because similar species of oak growing close together often form interconnecting root systems. These grafted roots allow the fungus to move from an infected tree to a healthy tree

Preventative Measures To Safeguard Against Oak Wilt Disease

In the Houston area, emphasis should be placed on disease prevention. An obvious source of both the fungal spores and the beetles is firewood cut from infected red oaks Firewood cut from infected live oak trees, while not harboring the spores that are produced on infected red oaks, can still transport contaminated beetles For these reasons, it is best to avoid unseasoned firewood cut from known oak wilt centers. Well-seasoned firewood, exhibiting loose bark and checked wood, would have become inhospitable to the oak wilt fungus during the previous hot, dry summer If the firewood has a tight bark and the place of origin is unknown, it is best to cover the wood pile with clear plastic, making sure the edges are tucked into the soil to prevent insect spread.

Preventative measures commonly used in areas affected by oak wilt include minimizing the pruning of live and red oaks between February 1 and June 1 (times when beetle activity and fungal spores are most prevalent) and applying pruning paint immediately on fresh wounds.

Planting a diversity of tree species is another way to safeguard against oak wilt disease. To protect oaks from becoming vulnerable to insect and disease attack. no single species should compose greater than 10% of the total tree population in a given area. When planting oak trees in areas already populated with red and live oaks, special consideration should be given to oak wilt resistant species such as bur, overcup, swamp chestnut and white.

Although oak will is presently not a problem iu the Houston area, outbreaks are possible. Any multiple: oak tree loss that cannot be explained by a known cause should be inspected by a county extension agent, Texas Forest Service forester, or trained arborist.

For more information on oak wilt disease contact the Texas Forest Service at 713/688-8931 or call the Wilt Hotline at 512/ 473-3517.

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