by Shirley D. Smith, Somervell County Master Gardener
One of the main reasons we bought our property in Somervell County was because of a stand of live oaks. We built our house close to these trees and they have given us a lot of pleasure as we sit under them on a summer day and enjoy their shade or just admire their beauty. Grandkids have spent hours climbing their twisted branches. However, after becoming a Texas Master Gardener and attending some classes on how to keep trees healthy, I learned of a destructive disease that can take down every one of these huge, majestic trees! In layman’s terms, it’s called Oak wilt. The scientific name for the fungus is Ceratocystis fagacearum and it is one of the most destructive diseases affecting live oaks and red oaks in Central Texas.
Because live oaks tend to grow from root sprouts and can form root grafts very readily, all or most of the live oaks within a given area share a common root system. Most of the tree mortality results from tree-to-tree spread of the pathogen by these interconnected or grafted root systems.
Oak wilt has been found in over 76 counties and in almost every city in Central Texas, as well as Abilene, Midland, Lubbock, Dallas, Fort Worth, College Station, Houston and San Antonio. In can be a problem wherever live oaks tend to be the predominate tree. It does not matter whether they are transplanted or naturally grown and an individual tree’s age, size or previous health status does not make it more or less likely to die from oak wilt.
Oak wilt spreads to other oak trees in two ways: long distances with the aid of certain beetles or locally through common or grafted roots. Sap-feeding (nitidulid) beetles are believed to be responsible for much of the long distance spread of oak wilt. During the spring, the oak wilt fungus forms special spore-producing structures called fungal mats on red oaks. Nitidulid beetles are small (about 1/8-inch long) and are attracted to oak wilt fungal mats because the mats have a sweet, “fruity” smell. Mats form underneath the bark of diseased red oaks and are not known to occur on live oak trees. The fungal mats apply pressure under the bark causing a tiny crack to form.
These mats can be found on the trunk and major branches of red oaks. When a nitidulid beetle feeds on an oak wilt fungal mat, spores of the oak wilt fungus will cling to the body of the beetle. Nitidulid beetles also feed on tree sap associated with fresh wounds. If a beetle contaminated with oak wilt spores lands on a fresh wound on a healthy oak, then that tree can become infected. Tree wounds can be made by man or nature, but nitidulid beetles are attracted to both.
Once established, the fungus moves from one tree to the next through common or grafted roots.
Interestingly, live oaks are somewhat intermediate between the two groups of oaks but are still very susceptible to the disease.
Prevention plays an important role in the management of oak wilt. Landowners and homeowners can take an active role in oak wilt prevention by taking the following steps:
- Avoid pruning or wounding oaks between February 1 and July 1. This is the time of the year when oak wilt fungal mats are most likely to form and the beetles are active.
- Sterilize/Sanitize all pruning equipment between trees.
- Immediately paint all wounds.
- Do not transport or buy unseasoned firewood.
- Promptly remove and either burn or bury all red oaks that are dying or have been recently killed by oak wilt.
Regardless of the reasons or time of year, proper pruning techniques should be used. These techniques include making proper pruning cuts and avoiding injurious practices such as topping or excessive crown thinning. If you are uncertain about any of this information, you should consult with a Texas Oak Wilt Certified arborist, ISA Certified Arborist, or an oak wilt specialist from a city, county or state government agency such as the Texas A&M Forest Service or Texas A&M AgriLife Extension Service.
Sources: Texas A&M Forest Service; Texasoakwilt.org