Texas A&M Forest Service’s pruning tips to prevent the spread of oak wilt

While winter is the ideal time to prune trees, pruning after recent droughts and freezes may cause more damage than good this year.

Oak wilt is one of the deadliest tree diseases in the U.S., killing millions of trees across 76 counties of Texas.

“Pruning always adds stress to trees because it opens wounds that need to seal, so if a tree is already stressed, it’s best to avoid unnecessary pruning this year,” said Demian Gomez, Texas A&M Forest Service regional forest health coordinator. “Especially avoid pruning for aesthetics and controlling growth if a tree is already showing signs of stress.”

For oak trees specifically, all wounds, including those produced by pruning, construction activities, livestock, land clearing, lawnmowers, string trimmers and storms should be painted over to produce a physical barrier and prevent the spread of oak wilt.

Know the best time to prune

Oak wilt is caused by the fungus Bretziella fagacearum. The fungus invades the xylem, the water-conducting vessels of the trees, and the tree responds by plugging the tissues, resulting in a lack of water to the leaves, slowly killing the infected tree.

Any new wound on oaks can be an entry point for oak wilt infection. Small insects, called sap beetles, move spores of oak wilt fungus to new trees and are very active in late winter and early spring.

“Prevention is key to stopping the spread of oak wilt,” said Gomez. “With wounds being the best entry point for the pathogen, landowners should avoid pruning or wounding trees from February through June.”

For oak trees specifically, all wounding, including those produced by pruning, construction activities, livestock, land clearing, lawnmowers, string trimmers and storms should be painted over to produce a physical barrier and prevent the spread of oak wilt.

“Pruning always adds stress to trees because it opens wounds that need to seal, so if a tree is already stressed, it’s best to avoid unnecessary pruning this year,” Gomez said. “Especially avoid pruning for aesthetics and controlling growth if a tree is already showing signs of stress.”

Recognizing and understanding oak wilt

Oak wilt can spread two ways – above ground by sap-feeding beetles or underground through connected roots.

The disease spreads above ground by sap beetles more rapidly in late winter and spring because of high fungal mat production and high insect populations. During this time, red oaks that died of the disease last year may produce spore mats under the bark. With a fruity smell, these mats attract small, sap-feeding beetles that can later fly to a fresh wound of any oak tree and infect it, starting a new oak wilt center.

The second way oak wilt can spread is underground by traveling through interconnected root systems from tree to tree. Oak wilt spreads an average of 75 feet per year by the root system. This occurs primarily in live oaks and is responsible for the majority of spread and tree deaths in Central Texas.

Oak wilt is often recognized in live oaks by yellow and brown veins showing in leaves of infected trees, known as veinal necrosis. It may be difficult to diagnose oak wilt due to seasonal transitioning of oak leaves in the spring  when evergreen oak trees shed their old leaves while simultaneously growing new leaves. The signs of oak wilt can be seen on most leaves when a tree is fully infected. Landowners should contact a certified arborist if they are unsure if their tree is infected.

“For red oaks particularly, one of the first symptoms of oak wilt is leaves turning red or brown during the summer,” Gomez said. “While red oaks play a key role in the establishment of new disease centers, all oaks can move oak wilt through root grafts, particularly live oaks.”

How to stop the spread of oak wilt

To stop the spread of oak wilt through the root system, trenches can be placed around a group of trees — at least 100 feet away from the dripline of infected trees and at least 4 feet deep, or deeper — to sever all root connections.

Another common management method for oak wilt is through fungicide injection. The injections only protect individual trees injected, and the best candidates for this treatment are healthy, non-symptomatic oaks adjacent to and up to 75-100 feet away from symptomatic trees.

Other ways to help prevent oak wilt include planting other tree species to create diversity in the area, avoiding moving firewood and talking with neighbors about creating a community prevention plan for oak wilt. Infected red oaks that die should be cut down and burned, buried or chipped soon after discovery to prevent fungal mats that may form.

Not only is saving oak trees important for our ecosystem and health, but also for property values and community aesthetics. Loss of trees due to oak wilt can reduce property values by 15-20%.

Some cities and municipalities, including Austin, the City of Lakeway, Dallas, Houston, San Antonio and Round Rock, have oak wilt programs in place with municipal foresters dedicated to managing the disease. Texans can also contact their local Texas A&M Forest Service representative with any questions about this devastating disease.

For more information on oak wilt identification and management, visit https://texasoakwilt.org/ or Texas A&M Forest Service’s website at https://tfsweb.tamu.edu/.