Oak wilt is one of the most destructive diseases of trees ever identified in America. It attacks many species of oaks without regard to tree age or health, and it’s almost always deadly. Sadly, researchers have yet to discover a satisfactory treatment, so the pestilence has gone virtually unchecked in 21 states from the Upper Midwest all the way south into Texas.

Diseased oak tree

Homeowners have lost trees that are hundreds of years old to oak wilt.

Photo Credit: Dr. Dave Appel

Open tree wound

Sap-feeding beetles can pick up the oak wilt fungus from “fungal mats” on red oaks and deposit the organism into wounds on another tree.

Photo Credit: David L. Morgan

Diseased leaves - oak tree wilt

The leaves on diseased trees often yellow, then eventually turn brown and die.

Photo Credit: Dr. Dave Appel

Chemical injector

Chemical injection has been effective in treating oak wilt in young trees when the disease is identified early.

Photo Credit: David L. Morgan

Small oak trees

Oak wilt has wiped out thousands of native red and live oaks in central Texas.

Photo Credit: David L. Morgan

While the disease is found more commonly in red oak species than in white oaks, no oaks (Quercus) are immune. In fact, according to the US Forest Service, 35 native and exotic species are susceptible, as well as American and European chestnuts (Castanea), species of chinkapin (Castanopsis), tanoak (Lithocarpus) and several varieties of apple (Malus).

In the Southwest, the most susceptible species are Texas red oak (Quercus buckleyi), Shumard oak (Quercus shumardii) and blackjack oak (Quercus marilandica). The live oaks, generally considered neither white nor red, are also very susceptible.

There are 42 species and two varieties of live oaks in Texas, the most prominent being the coastal live oak (Quercus virginiana) and the escarpment live oak (Quercus fusiformis). Both are treasured in Southwestern landscapes, with the escarpment live oak being a favored native tree in the Texas Hill Country. Austin, San Antonio and Fort Worth all have been hit hard by oak wilt.

So what causes this devastating disease? The fungus Ceratocystis fagacearum is the culprit, impairing the xylem (or water-conducting) vessels of a tree. The damage can be so great, a tree can die in one growing season. And unfortunately, the fungus is easily spread. One way it makes its way around is by a sap-feeding beetle called a nitidulid. The insect commonly picks up the fungus when it’s attracted to the sweet-smelling “fungal mats” (or mushroom-like structures) located just beneath the bark on red oaks. Once the beetle picks the fungus up, it can spread the problem by depositing the fungus into the wounds of another tree.

But the most common method of oat wilt transmittal is from one infected tree to another through frequently occurring root grafts between trees. Through these connections, the fungus is able to move freely from one tree to another tree’s xylem, spreading the infection from diseased oaks to healthy ones.

What’s a concerned home gardener to do?

The best way to save your tree is to identify any symptoms early. Spring is when the disease appears most active – when trees are actively growing and new vessel wood is being formed. (Symptoms may occur as early as May.)

Here’s what to look for: Leaves on diseased trees often develop chlorotic (or yellow) veins that eventually turn necrotic (brown and dead), a symptom called veinal necrosis. Defoliation may be rapid, and dead leaves with brown veins can often be found under the tree. As defoliation occurs, the tree canopy becomes thin, with few older leaves remaining and no new ones emerging. For accurate diagnosis, you should contact your local Extension agent or city forester for assistance as soon as you notice anything irregular with your tree.

Treatment options aren’t for the everyday homeowner. While there’s at least one chemical treatment approved for oak wilt, it must be applied by a licensed tree specialist and may be ineffective in older trees. Unfortunately, by the time an infection is detected, the tree may be too far gone to be helped. Trees with advanced stages of infection should be removed and destroyed.

At present, the most effective means of interrupting the spread of oak wilt is through trenching deep into the ground to break up the root grafts. Belowground root barriers placed between trees are also useful to limit the spread of oak roots. These are expensive processes, however, that call for cooperation among homeowners, city governments and trained arborists.

As deadly as this disease is, the good news is that there are several important measures you can take to help prevent the spread of oak wilt into your landscape. First, don’t prune your trees in spring, when the nitidulid insects are active. Wait until at least October, then treat all pruning tree wounds with wound paint. Replace your dead or infected trees with white oaks or other resistant tree species, and be sure your landscape trees remain adequately watered and aren’t stressed during summers or dry, hot autumns. Finally, keep a continual close eye out for any signs of disease so you can treat the problem early, inexpensively and effectively.

Tree disease specialists and foresters have waged war against oak wilt for more than 20 years, and infection sites continue to spring up each summer. But if you pay close attention to the health of your trees and understand the symptoms of the disease, you’ll be able to stage a strong defense against this dreaded killer of oaks.