Not too long after you leave the city limits of Fredericksburg, Texas, heading west toward the town of Harper, you see it—long stretches of trees devastated by oak wilt. Completely barren of leaves, with once sturdy branches now drooping to the ground, the impacted oak trees line U.S. Highway 290 for miles. Most landowners, particularly in the central part of Texas, have witnessed oak wilt to some degree. Whether it stretches for miles or infects just an acre, oak wilt’s presence can be alarming and heartbreaking. Understanding some basic facts about oak wilt can help prevent its spread and, hopefully in some instances, give you some peace of mind. Here are 10 things to know about oak wilt in Texas. The fact sources for the following information are found, below.
1) Oak wilt is an infectious disease caused by the fungus Ceratocystis fagacearum. The fungus infects a tree’s vascular system. In short, the fungus invades and disables the water conducting system in susceptible trees resulting in severe die-back or tree death.
2) The origin of the oak wilt pathogen is not known and its presence has been reported only within the borders of the U.S. Per Texas Invasive Species Institute: “Reports of widespread oak death that resemble oak wilt date back to the 1930’s. The disease was first described by a scientist in 1942 where oak wilt was killing red oaks in the upper Mississippi river valley of Wisconsin. However, it is believed that oak wilt was likely killing trees as early as the 1890’s.” Oak wilt has been found in 22 states and 76 Texas counties. Increase in oak wilt has been linked to increased tree wounding due to home construction in oak woods.
3) All oaks are susceptible to oak wilt but some much more than others. Red oaks, particularly Spanish oak, along with Shumard oak and Blackjack oak, are the most susceptible. Red oaks also help spread the disease (more on that, below). Live oaks are less susceptible but their interconnected root systems allow the disease to more easily spread to adjacent trees. White oaks, such as post oak and bur oak, are the least susceptible, are resistant to the fungus, and rarely die.
4) Oak wilt is spread in two ways: long distances with the aid of sap-feeding (nitidulid) beetles and locally through interconnected roots. During the spring, oak wilt fungus forms spore-producing fungal mats on red oaks. Tiny—1/8-inch long—nitidulid beetles are attracted to these fungal mats that produce a sweet, “fruity” smell. The fungal mat spores cling to the bodies of the beetles. The beetles are also attracted to the tree sap of an oak tree’s fresh wound—created by man or nature (think lightning strikes). If a contaminated beetle lands on the fresh wound of a healthy tree, that tree can become infected. Once infected, it can then spread locally via interconnected roots—the most common way oak wilt is spread.
5) The appearance of a tree infected with oak wilt will vary depending on the type of tree, but “veinal necrosis” is one tip-off in live oaks as the leaves exhibit yellow to brown veins and eventually fall from the tree. Trees typically die within two to four months. Per Texas A&M Forest Service: “Infected red oak trees don’t exhibit distinct symptoms. Leaves often turn pale green then brown and usually remain attached to the tree for a period of time. This can begin on one branch and quickly engulf the entire tree. Trees generally die within two to four weeks.”
6) Other diseases may resemble oak wilt such as oak leaf blister and oak rust. Professional arborists experienced with oak wilt can often diagnose the problem in the field using host species, symptoms, and mortality patterns. In some cases, suspect trees may need to be sampled and sent to a laboratory.
7) Landowner actions play a key role in preventing the spread of oak wilt. The following is a summary of recommendations by Texas A&M Forest Service. It is suggested to read the steps in detail (see How Can I Prevent My Trees From Getting Oak Wilt):
- Avoid pruning or wounding oaks between February 1 and July 1. This is just a general rule of thumb for the time of year when oak wilt fungal mats are most likely to form and nitidulid beetles are active. However, because Texas weather is so unpredictable, be advised that the least hazardous periods for pruning are during the coldest days of midwinter or extended periods of hot weather in mid- to late summer.
- Sterilize/sanitize all pruning equipment between trees.
- Regardless of the time of year, immediately paint all wounds on oaks to prevent contact with contaminated beetles.
- Do not transport or buy unseasoned firewood. Fungal mats may form on unseasoned red oak firewood infected with oak wilt making it possible to spread oak wilt to uninfected areas.
- Promptly remove and either burn or bury all red oaks that are dying or have been recently killed by oak wilt. This will prevent nitidulid beetles from spreading spores from fungal mats that may form on the trees in the fall or the following spring.
8) If you have oak wilt, there are two main ways to treat: trenching and fungicide. Breaking the root systems can reduce or stop transmission. The most common technique for this is by trenching at least four feet deep. The fungicide propiconazole (Alamo) can be used as a preventative to reduce oak wilt symptoms in live oaks before infection and, for those in early stages of infection, some success may be achieved through therapeutic injections. Fungicidal treatment should be done by trained professionals.
9) To help improve your property, plant resistant trees. Go for a diversity of trees that are not only resistant to oak wilt, but also well-adapted to your part of Texas. Texas A&M Forest Service has a handy “Texas Tree Planting Guide” that lets you select your county and the type of tree you want. Under “Option 4: I want a tree that…” be sure to select “Texas native.”
10) Burning wood from infected trees will not spread oak wilt. The fungus is destroyed by heat and will not even survive in dry firewood. But, again, as described above, fungal mats may form on unseasoned red oak firewood infected with oak wilt making it possible to spread oak wilt to uninfected areas. That’s your firewood concern, not the burning of the wood.
SOURCES:
“Forest Health: Oak Wilt: Frequently Asked Questions” (Texas A&M Forest Service)
“Oak Wilt 101” (City of Austin)
“Oak Wilt” (Texas Invasive Species Institute)
“How To Identify, Prevent, and Control Oak Wilt” (U.S. Department of Agriculture)
“Texas Tree Planting Guide” (Texas A&M Forest Service)
