Protecting your growing assets: Dealing with Austin’s oak wilt epidemic
One of Austin’s best assets includes the lush green canopy of a wide variety of oaks in and around our city. Covering our parklands and hiking trails, shading our buildings, and surrounding our waterways, our oaks are true beauties in our urban forest. But, if you’ve managed or worked with any properties in Central Texas for more than a few weeks, you’ve no doubt heard of the ongoing tree damages caused by oak wilt. One of the most destructive and invasive tree diseases in the US, oak wilt is killing our trees in epidemic proportions. But what exactly is oak wilt and how can it be stopped? Oak wilt is an infectious disease caused by an invasive fungus (Ceratocystis fagacearum) that disables the water-conducting network in susceptible trees. Above and underground angst – how does oak wilt spread? The disease typically spreads in two ways: via sap feeding beetles carrying spores on their bodies (overland transmission) and via interconnected roots among trees. Overland transmission of oak wilt begins with an infected red oak (Shumard oak, Spanish oak, Blackjack oak, Texas red oak, etc.). When the tree dies, one or more fungal spore mats may form under the bark. The mat grows and expands causing the bark to crack open. The spore mat emits an odor that attracts Nitidulid beetles, which enter the spore mat to feed. While feeding, spores stick to the insect and are carried to other trees to feed. The insects are attracted to fresh sap-emitting wounds. Oak wilt spores can then infect these other trees, starting a new disease center. Under their own power, these beetles can travel over a mile appearing on fresh wounds in under 15 minutes. Root transmission is largely a problem for live oaks because the root system of one live oak is highly connected to neighboring trees creating a “shared root community.” Traveling through the roots from one live oak to the next, the fungus can spread at approximately 75-100 feet per year on average. With the high concentration of live oaks in our area, root system transmission destroys a large area of our live oak forest. Unfortunately, contrary to common belief, simply removing dead trees either by cutting them down or bulldozing them, won’t eliminate the root transmission of the disease. The only way to prevent root transmission is to completely sever the root system.
Identification of Oak Wilt – What does it look like?
Leaf browning in oak leaves
Confirmation that oak wilt is present in an area is often challenging. However, an experienced certified arborist with a background in oak wilt can usually make a reliable diagnosis in a timely manner while ensuring that other factors such as drought, construction damage, lightning strikes, or chemical poisoning are not misdiagnosed. There are four primary ways to identify oak wilt: foliar (leaf) symptoms, patterns of tree mortality, fungal mats and laboratory tests. The prominent symptom most commonly associated with oak wilt is the distinctive browning out of the veins in live oak leaves called veinal necrosis and/or the appearance of a burned tips of leaves. The veins in the leaf become yellow or brown and the leaf soon falls off the tree. Trees in this family lose leaves in a manner similar to autumn with the leaves turning brown from the edges in and then falling off. Leaf loss during an odd time of year should always be investigated. Major defoliation (leaf loss) of a live oak other than during spring (when they normally replace their leaves) should be investigated. Can it be managed? While oak wilt is incurable, once the disease has been positively identified, there are three possible means to deal with the disease: do nothing, sever the root system between infected trees and healthy trees, and injection of fungicide into trees. Experienced professionals can help you balance a combination of all three possibilities by explaining the costs and potential aesthetics involved with each option. Doing nothing may be an appropriate response in areas where there are a limited number of trees at risk or the value of those trees is minimal. Such a course of action might be taken where oak wilt has already claimed the high value trees in an area and the remaining specimens are in poor health or in a remote area. Root trenching is appropriate where there is sufficient space and lack of existing buried substructure(s) to correctly and economically install a trench. Fungicide treatment is appropriate for high value trees that cannot be isolated by a trench. In general, injections work very well with pre-symptomatic trees and for smaller areas.
Can it be prevented? As with many other environmental issues, new oak wilt disease zones are often caused and impacted by human activity. We can take valuable steps to reduce the chance of starting a new oak wilt center by implementing the following steps: 1.
Only allow qualified, trained professionals to prune your trees.
2.
Unless crucial, don’t prune oak trees between early February and late June.
3.
If it’s necessary to prune susceptible trees, or if trees are wounded, use a wound paint (pruning sealer) immediately. A new infection can be started quickly on a fresh wound in less than 15 minutes. Wounds are naturally sealed off by the tree after about a week, so painting old wounds is of no value. All wounds, regardless of size, should be painted.
4.
Trench around active oak wilt zones to stop the spread of the fungus through common or grafted roots.
5.
Watch trees in and around oak wilt centers for new infections. Take steps immediately to prevent additional spread.
6.
Sanitize cutting equipment with a bleach solution or Lysol® spray before cutting on an oak tree and before moving on to the next tree to avoid the possibility of cross contamination.
7.
Before using firewood from infected red oaks, wood must be thoroughly dried for at least 12 months. Cover new firewood piles with clear plastic for one year. Make sure that the edges of the plastic are buried with soil to prevent insect migration out of the wood pile.
Oak wilt in its latter stages
Written and submitted by Amy Weldon of Hamlin Tree Care. For assistance with your arbor needs, she can be reached at amy@hamlintreecare.com or 512-975-0196.