Oak wilt threatens trees in North Texas

Photos by LOUIS DeLUCA/DMN

Oak wilt is a deadly fungal disease that threatens all red oaks and live oaks in North Texas. In rural areas across the state, particularly in the Texas Hill Country, the disease has killed entire stands of oaks, one by one, like sets of dominoes.

In residential areas heavily planted with oaks, the fungus can leave once-shady streets barren, their signature landscape specimens sad skeletons of the majestic trees. Oak wilt has been around for more than 50 years, and experts say it's becoming more prevalent in the Dallas area.

The disease can quickly kill mature trees; proper diagnosis is difficult. Consumer spending habits, furthermore, are adding to the problem. Area arborists estimate red oaks and live oaks make up 30 percent of the total tree population in the Dallas area. Local nurseries estimate 75 percent of the shade trees purchased here are Shumard red oaks and live oaks.

Homeowners can take steps to minimize the risk of oak wilt infecting their trees, but there is no guaranteed immunization. Costly and repeated fungicide injections into a tree's vascular system may protect infected trees from death, but the injections will not cure oak wilt.

The good news is experts are learning more about this disease and how to manage it. A 50-square-mile survey conducted by Texas A&M University this spring will reveal just how common oak wilt is in the Dallas area and what neighborhoods are most threatened.

In addition, new diagnostic techniques are being developed that are less damaging to the tree. The Internet site www.texasoakwilt.org was launched to help homeowners recognize the disease and urges them to contact experts if symptoms are detected.

Oak wilt was first reported in South Dallas in the 1960s, says David Appel, Texas A&M professor and associate head of the university's Department of Plant Pathology and Microbiology. Since no surveys were done in those years, there is no way of knowing how widespread oak wilt was at the time, he says.

The most recent survey of the area was conducted in 2003 and showed a congregation of infected trees in North Dallas. At that time, most infected trees were located in a triangle between Interstate 35 East, Interstate 75 and Interstate 635. Infected trees were still present in South Dallas but the disease had spread north into Collin County.

While the new survey will give current details, Appel believes oak wilt is now fairly evenly distributed across the region. "If you're in a neighborhood with a lot of red oaks and live oaks, you're going to see some oak wilt," he says.

Tree-care professionals are aware of oak wilt hot spots, says Steve Houser, certified arborist and owner of Arborilogical Services in Dallas. Houser sees a large number of oak wilt cases near the intersection of Hillcrest and Belt Line roads in Far North Dallas. There also is a hot spot near Spring Creek Parkway and Custer Road in Plano and, in McKinney, near the intersection of Custer and Eldorado Parkway.

"We don't want to mislead people. Oak wilt is not going to be as devastating as Dutch elm disease," Houser says, in reference to the fungal disease, spread by the Dutch elm beetle, that destroyed American elm populations in the previous century. "But it is a very dangerous disease. If you have it in your neighborhood, you do need to take appropriate actions, or you're going to lose your trees."

Susceptible oak species include Shumard, Texas red, Spanish red and blackjack oaks and escarpment and coastal live oaks. Species in the white oak family, however, are not affected by the disease. White oaks include bur, chinquapin, post, lacey and Monterrey oaks.

The fungus, Ceratocystis fagacearum, is spread to healthy trees in two ways. It enters trees via above-ground wounds, and it spreads from infected trees to healthy trees by their intermingling root systems.

Sap-feeding nitidulid beetles commonly carry oak wilt spores into tree wounds. Protecting trees from these insects is crucial in preventing oak wilt.

Preventive steps

There are two basic actions homeowners can take to protect their trees from oak wilt, Appel says.

Use wound paint on all cut branches and wounds of susceptible oak species from Feb. 1 to June 1. This is the period when the spore-carrying beetles are active. The paint prevents spores from entering the tree's sap.

At least one area arborist suggests avoiding pruning red oaks and live oaks between Feb. 15 and June 15, unless you are clearing storm damage.

Burn all the firewood you've stacked at your house by the end of January. Firewood can contain the fungus and is a conduit for introducing the disease to your oaks and your neighbors' oaks.

Appel believes that transporting firewood around the state has been a major cause of the spread of oak wilt in Texas.

Properly identifying oak wilt is not easy, even for experts, Houser says. There is a fairly common disease named bacterial leaf scorch that has similar symptoms and also affects red oaks.

Infected live oaks have one telltale symptom - a wavy pattern develops on the leaves called veinal necrosis. If this symptom develops, you can be fairly certain the tree has oak wilt. Not all infected live oaks develop veinal necrosis, Houser says.

"If you're dealing with red oaks or live oaks that don't have classic symptoms, then you have to take a lot of things into consideration," Houser says. "Is there oak wilt in the neighborhood? Is there reason to believe the fungus is likely to be here?"

Trunk samples can be excised and sent to a lab to detect the oak wilt fungus. This process takes at least two weeks, and it involves removing a large portion of the tree. A sample 4 to 5 inches wide and 6 to 8 inches long, cutting through the tree's cambium layer, must be removed.

"You're taking a large portion of the vascular system out, and that's just not good for the tree," Houser says.

New tests are being developed by Texas A&M that involve taking much smaller tissue samples, and they could be ready for use in the next few years.

Can trees be saved?

Once trees are infected, a homeowner must decide if it's worth the time and expense to treat them.

"Fungicide treatments don't prevent the disease or cure the tree of oak wilt," Houser says. "It's not going to solve all your problems. They will suppress symptoms in the tree for a period of time, but the disease will always be there."

Sometimes infected trees can be treated and, with time, be fully weaned off the fungicide treatments and survive, Houser says. For any treatments to work, the infection must be caught early.

"With a red oak, if we catch the disease when you've lost only 10 to 15 percent of the foliage, you can inject with fungicide and save the tree. But red oaks are harder to save," Houser says. "With live oaks, you can still save it if you're looking at a 20- to 30-percent loss of foliage."

Treatments involve using an air spade to safely remove soil from the base of the tree. The fungicide, propiconazole, is injected into the primary roots near the tree trunk. If you inject the fungicide directly into the trunk, it will not be evenly distributed throughout the tree.

At this time, there is no organic product used specifically to treat oak wilt, Houser says. Although the arborist prefers to implement organic methods when possible, he points out the fungicide goes into a tree's vascular system, not into the soil surrounding the tree.

The injections take 24 to 48 hours to complete, and technicians have to return repeatedly to ensure the injection system is properly pressurized. One treatment suppresses oak wilt symptoms for about two years, but this could be less if oak wilt is persistent in your neighborhood, Houser says.

The average treatment costs about $20 per inch of a tree's trunk diameter. This measurement is taken at 4 ½ feet above the ground. So a large oak with a 30-inch-diameter trunk would cost about $600 to treat - and that would be a recurring cost about every two years.

"If you're talking about a 10-inch tree, you might have to treat it enough that it would make more financial sense to just replace it," Houser says. "But if you're talking about a 20- or 30-inch tree, it would take a lifetime before the new tree would reach those proportions."

Todd W. Davis is a freelance writer and Texas certified nursery professional in Richardson.