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Photo Credit: Dr. Dave Appel

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

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.

While the disease is found more commonly in red oak species than in white oaks, no oaks (Quercus sp.) 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).

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Photo Credit: David L. Morgan

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.

In the Southwest, the most susceptible species are Texas red oak (Quercus buckleyi), Shumard oak (Q. shumardii) and blackjack oak (Q. 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 (Q. virginiana) and the escarpment live oak (Q. 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.

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Photo Credit: Dr. Dave Appel

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

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?