Oak Wilt: How to Help Control the Spread
December 19, 2017
Oak trees make up a large portion of forest and urban trees in Texas. They are and have been threatened by a serious disease, Oak Wilt. With so many oak trees in the DFW area, it is important to familiarize yourself with the disease.
Oak Wilt is a vascular wilt disease caused by the fungus Bretziella fagacearum. It disrupts the flow of water and nutrients by plugging the vessels in the vascular system of the tree. According to the latest 2016 update, the pathogen has been found in 26 states, with spread in Texas mostly affecting Central and North Texas.
All Oaks are susceptible on some level. Red and Shumard oaks are the most susceptible, along with live oak. White oaks do show some tolerance, but can still be infected.
The disease is most frequently spread by root to root contact or root grafts. This means your neighbor’s tree can easily infect your tree. Another path of transmission is by an insect vector—a sap feeding Nitidulid beetle—that is unique to red oaks. This happens less frequently because the fungal mats are only present during cooler times of the year and spores are viable for an even shorter period of time. Also, there must be a fresh wound on the healthy tree in order for the beetle to transfer the fungus.
Symptoms of Oak Wilt differ for each type of oak, but the symptom most commonly noticed is dieback in the crown of infected trees. The foliar symptoms on live oak include chlorotic veins that eventually turn brown, which is known as veinal necrosis. Death of entire branches within the canopy is common and is known as "flagging.” Defoliation happens quickly. Total death of a live oak can take up to 6 months or even longer, in some cases.
Red oak foliar symptoms are different in appearance. In early spring, leaves wilt, turn pale green and brown. Mature leaves turn bronze from the leaf tip inwards. This will start on one branch and move throughout the entire tree quickly. Besides the foliar symptoms, fungal mats will appear under the bark in cooler months and sometimes are easily noticed by cracks in the bark. Red oaks generally die within 4-6 weeks. Unfortunately, red oaks rarely, if ever, survive after the appearance of Oak Wilt symptoms.
Methods of Control
An important method to help control the spread of Oak Wilt is to avoid pruning oaks from February through June in Texas, when the fungal mats are present and beetle transmission is most likely. If pruning must be done during this time of year, make sure you work with oak wilt certified arborists who will use special preventative techniques to help prevent infection. In certain areas of the Metroplex, the timing of pruning is really not a concern, and depending on what the tree is truly in need of; controlled pruning is a better option than letting the tree suffer structural damage from not pruning. A big x-factor in Texas that unfortunately cannot be controlled and normally results in a lot of fresh wounds in the spring, are hail storms.
Do not leave dead, diseased red oak trees in your landscape. Red oaks are the source of fungal mats, and dead or diseased trees should be removed as soon as possible. Make sure to cut down diseased wood and dispose of it immediately. Do not move fresh firewood from diseased trees—wood needs to be dried under plastic for a year to make sure the fungus is dead. It is even better to remove the wood from diseased trees and chip, burn, or bury it.
Another method to stop the spread is through roots by trenching. This method is used in heavily infected areas (Central Texas) when an infected tree is already present. Oak Wilt has been found to spread 75 feet or more each year. Trenches—at least 4-foot-deep—would need to be dug at a minimum 100 feet out from the center of the symptomatic tree. Unfortunately, in most urban settings, trenching is not a viable option.
There is a fungicide treatment of Propiconazole that has been successful as a preventative for the disease. Fungicide may also be used in addition to trenching if you already have a diseased tree in the area. This highly specific tree injection application should only be carried out by a trained professional, such as our technicians at Preservation Tree, to ensure it is done correctly. Fall into winter is an ideal time to perform these injections to protect your oak trees moving into spring.
Always contact certified professionals immediately when suspecting a possible infection. While you may not be able to save the “patient zero” tree, it’s imperative to help get in front of the disease and control the spread of disease to other, healthy oak trees, and in turn, save Texas’ forest and city trees.