What Is Oak Wilt?
Oak wilt is a fungus that colonizes the xylem within the tree, blocking water from getting to the branches and leaves. The xylems are the tubes within the tree that move water from the ground to the canopy. In response to the fungus, the tree shuts down these water conducting vessels in an effort to compartmentalize the infection. As a result, the tree is responsible for its own decline/death. Today, oak wilt is one of the top five tree pathogens in the US and a serious concern for Texas residents with oak trees on their property.
How Oak Wilt Disease Spreads
Oak wilt can be spread through two primary methods. In red oaks, fungal mats can be formed. Their fruity odor attracts the Nitidulid which then transfers spores to fresh wounds on healthy oaks. Underground, the roots of oak trees can graft together to help them thrive, but this can lead to the oak wilt spreading, too. Oak wilt can spread through the connections between roots, infecting nearby trees easily. This is responsible for most of the spread of oak wilt disease, especially within live oaks.
Diagnosis of Oak Wilt
Diagnosing oak wilt can be difficult, as there are other issues that can lead to similar symptoms. The most common symptom is the presence of fungal mats in red oak or when the tree flashes fall colors during the summer months. In red oaks, the fungal mats appear during the spring months, but the tree would have been infected during the previous summer or fall. Red Oaks infected in the spring or early summer won’t typically last long because of the extreme heat during the warmer months. Other symptoms include dropping leaves, a yellowing canopy, leaf tip burn, leaf margin burn, and a winter color change in live oaks. Any of these or other changes should lead to an inspection by an Arborist to diagnose the tree. Since there are other diseases that can have the same or similar symptoms, it is necessary to confirm that oak wilt is infecting the tree before determining the next steps.
Beware of False Symptoms
It is important to note that just because there are issues with an oak tree doesn’t mean the cause is oak wilt. With various issues that can impact a tree, oak wilt can be misdiagnosed by property owners. If any of the following false symptoms are noticed, it is likely the tree is suffering from a different issue and not oak wilt.
Toxicity Damage
Toxicity damage done by herbicides or other chemicals can resemble oak wilt, but an experienced Arborist can tell the difference. It is necessary to look at the tree carefully and do a thorough inspection to determine the source of decline. Trees are broadleaf plants and many over the counter weed killers are detrimental to them.
Damage by Frost or Lightning
Frost damage can appear similar to oak wilt disease during the winter months, especially when it doesn’t appear uniformly over the tree. Expert assistance is needed to tell the difference. If the tree is struck by lightning, there is typically an exit wound on the tree that will make the damage apparent. However, this isn’t always the case, and when there isn’t any obvious lightning damage, the symptoms the tree has can mimic those of oak wilt. An Arborist will need to inspect the tree to determine the cause of the symptoms.
Leaf Drop
If many trees in an area drop more than 30% of their leaves within a few weeks, oak wilt is definitely a concern. However, if there has been constant rainfall, too much irrigation, or flooding due to rain or irrigation near the tree, it’s more likely that the tree has received too much water and the tree can’t photosynthesize correctly. Treatment is needed to help trees with this issue, but it is different from the treatments for oak wilt.
Treatment of Oak Wilt Disease
Oak wilt will kill the tree it has infected, but there are treatments that can slow the progression of the disease or prevent it from spreading to other nearby trees. When oak wilt is suspected, confirm the diagnosis with help from an Arborist. Then, it may be possible to use chemical treatments or to cut through the roots to stop the spread.
Chemical Treatments
Propiconazole is a fungicide that has been proven effective against oak wilt. It’s most effective when used prior to infection, however it can be used therapeutically at early onset of disease in some cases. It is absolutely recommended for oaks in proximity to oak wilt diseased trees.
Cutting the Roots
Severing the roots grafted together underground can help prevent the spread of oak wilt disease. As soon as it is detected, it’s important to use specialized trenching machines to create a trench at least four feet deep around the tree. The trench should be at least 100 feet away from any trees showing symptoms.
Oak wilt is a severe disease that can impact red and live oaks within Texas. If oak wilt is suspected, always seek an official diagnosis by a trained Arborist to determine the next steps to take. Though it may not be possible to save the tree, it is likely possible to prevent the disease from spreading to other trees on the property or on nearby properties.
Oak Wilt FAQ’s
Where Is Oak Wilt A Problem?
Oak wilt has been found in over 76 counties and in almost every city in Central Texas, as well as Abilene, Midland, Lubbock, Dallas, Ft. Worth, College Station, Houston, and San Antonio. It can be a problem wherever live oaks tend to be the predominate tree. It does not matter whether they are transplanted or naturally grown. An individual tree’s age, size or previous health status does not make it more or less likely to contract or die from oak wilt.
Can My Trees Get Oak Wilt?
Oak trees in central Texas are placed into one of two groups – red oaks or white oaks. Red oaks are the most susceptible to oak wilt, will typically die within two to four weeks of symptom appearance, and can play a unique role in spreading the disease. Common red oaks in central Texas include: Spanish oak (Quercus buckleyi), Shumard oak (Q. shumardii), and blackjack oak (Q. marilandica).
Oaks in the white oak group are the most resistant of the disease, with variations among the group depending on species. Post oak (Q. stellata), bur oak (Q. marilandica), and Mexican white oak (Q. polymorpha) are very resistant of the disease. These species may exhibit some canopy loss, but rarely die when infected. White shin oak (Q. sinuata var. breviloba), chinquapin oak (Q. muehlenbergii), and Lacey oak (Q. laceyi) can grow in stands with interconnected root systems, enabling the fungus to infect adjacent trees. These species are more resistant of the disease than red or live oaks and usually survive infection with moderate to severe canopy loss, but they can die in large numbers from the disease.
Live oak (Q. fusiformis) is a member of the white oak group and is the most common and abundant oak in central Texas. While live oaks succumb to oak wilt in the greatest numbers, they are intermediate in susceptibility to the disease with a small percentage (< 20%) surviving infection. Live oaks share a common root system. All live oaks in a stand are likely connected to each other and other live oaks as far as 200 feet away. If oak wilt infects one of the trees in the stand, the disease spreads through the common root system to adjacent trees and an oak wilt infection center begins.
How Does Oak Wilt Spread?
Oak wilt spreads to other oak trees in two ways – long distances with the aid of certain beetles or locally through common or grafted roots. Sap-feeding (nitidulid) beetles are believed to be responsible for much of the long distance spread of oak wilt. During the spring, the oak wilt fungus forms special spore-producing structures called fungal mats on red oaks. Nitidulid beetles are small (about 1/8-inch long) and are attracted to oak wilt fungal mats because the mats have a sweet, “fruity” smell. Mats form underneath the bark of diseased red oaks and are not known to occur on live oak trees. The fungal mats apply pressure under the bark causing a tiny crack to form.
These mats can be found on the trunk and major branches of red oaks. When a nitidulid beetle feeds on an oak wilt fungal mat, spores of the oak wilt fungus will cling to the body of the beetle. Nitidulid beetles also feed on tree sap associated with fresh wounds. If a beetle contaminated with oak wilt spores lands on a fresh wound on a healthy oak, then that tree can become infected. Tree wounds can be made by man or nature, but nitidulid beetles are attracted to both.
Once established, the fungus moves from one tree to the next through common or grafted roots.
How Quickly Can Oak Wilt Spread?
Because live oaks tend to grow from root spouts and can form root grafts very readily, all or most of the live oaks within a given area share a common root system. If one tree in a group of live oak trees becomes infected with oak wilt, the pathogen can spread through the common root system at an average rate of about 75 feet per year. An aerial photograph (below) shows how oak wilt has spread through live oak trees around the house on this ranch. Trees killed by oak wilt are gray. The dark green trees are juniper (cedar) and are not impacted by oak wilt.
How Can I Tell If My Trees Have Oak Wilt?
The most reliable characteristic for diagnosing oak wilt in the field is what is called “veinal necrosis” of the live oak leaves. Apparently veinal necrosis only occurs on live oak. Symptoms on other infected oak trees are less distinct. Laboratory culture of the fungus is another way to confirm the presence of oak wilt. Veinal necrosis is characterized by chlorotic (yellow) veins that eventually turn brown. The leaves will fall from the tree and dead leaves with brown veins can be found on the ground under the tree.
Live oak trees infected with oak wilt usually die within two to four months. Adjacent trees begin to drop their leaves and die a few months later as the disease progresses from tree to tree through the interconnected root system.
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.
Read more about the oakwilt suppression project of 2013 (PDF, 9MB).
Download an information sheet about identification and management of oak wilt (PDF, 654 KB)
Visit the Oak Wilt Partnership website at http://texasoakwilt.org/