San Antonio Express News
GARDENING, Etc
Sunday, March 7, 2004

By Lynn Rawe

Photo:plantpathology.tamu.edu/Texlab

Many people in the Texas Hill Country have experienced devastating results of oak wilt. Oak wilt is a fungus that causes the disease that is a major problem on live oaks, Shumard oaks, Spanish oaks, and other members of the red oak family.

Being knowledgeable about oak wilt is a great beginning to controlling a serious problem. The goal is to reduce or prevent the loss of oaks in your landscape. Protecting your trees is protecting your investment. Extension carries an informative publication that includes color photos of oak leaves infected with this disease. Call us at (210) 467-6575 or send a self-addressed stamped envelope to “Oak Wilt” at 3355 Cherry Ridge Dr., Suite 212, San Antonio, TX 78230.

Taking preventative steps can help reduce the spread of oak wilt. Pruning at the proper time is important. The Nitidulid beetle is responsible for transmitting the disease and is most active in early spring. Avoid pruning all oak trees between February 15 and June 15. Pruning should be done between December 1 and February 1.Protect cuts and wounds by painting them with latex paint. All cuts greater than ½ inch should be painted within a few minutes of pruning. This step is especially important if oak wilt is within 3 miles of an area that is known to be infested with oak wilt.

The Nitidulid beetle is attracted to the plant sap that forms on the surface of a cut or wound. These insects can transmit fungus spores from an infected tree to the open cuts or wounds of another oak, endangering that tree.

Typical oak leaf pattern and coloration from tree diagnosed with Oak Wilt.

Photo: http://dallas.tamu.edu/oakwilt

Another way to help prevent the spread of oak wilt is to take precautions with firewood. Firewood cut from infected trees with fungus mats can s pread oak wilt. These fungus mats only form on red oaks–not live oaks. If you question whether or not oak firewood has fungus mats, it should be covered with clear plastic. Be sure the edges of the plastic are tucked into the soil to prevent insect spread. Do not use black plastic because the beetles can see any holes through it and escape.

Oak wilt can be spread in other ways also. Live oak roots can graft together underground, spreading the fungus from infected trees to healthy ones through tree sap. If there is oak wilt in an area, one method of prevention is trenching between the healthy trees and the ones with oak wilt. The 48-inch deep trench should be at least 100 feet from the trees that are showing symptoms, and filled immediately after it is dug. Severing the joined roots will prevent the fungus from spreading.

Sometimes a systemic fungicide can be used to control oak wilt. Alamo is a product labeled for oak wilt control. The product is injected into the root flares under low pressure. Trees that are symptomatic have less chance of a complete recovery. If trees have lost more than 30% of their canopy, they should not be treated. Oaks can be replanted in areas where oak wilt previously occurred and caution should be taken to prevent wounding the new trees.

Check out this informative online publicaiton: http://dallas.tamu.edu/oakwilt/oak_wilt_series_02.html

This article was written by Lynn Rawe, Extension Agent-Horticulture, with Texas Cooperative Extension in Bexar County.