While oak wilt has been given serious attention as a tree disease, Texas A&M AgriLife experts are now warning Texans of another tree disease that’s becoming a growing problem in the state — laurel wilt.
“The laurel wilt pathogen, Raffaelea lauricola, affects any trees in the family Lauraceae,” said David Appel, Ph.D., a Texas A&M AgriLife Extension Service plant pathologist and professor in the Department of Plant Pathology and Microbiology in the College of Agriculture and Life Sciences, Bryan-College Station. “This includes red bay, sassafras, avocados, swamp bay and other related trees.”
Appel said the laurel wilt pathogen is a classic invasive species, first found in Texas in 2013 after being introduced in Georgia in 2004 and spreading across the southern U.S. and into several East Texas counties.
“Until now, laurel wilt has not gotten much attention,” he said. “But last fall, it was brought to my attention that it was killing trees in the Kingwood area of northeastern Harris County, the same county where Houston is located.”
Appel said mature red bay trees comprise a large proportion of the shade tree population in the eastern part of the state, and the pathogen can be expected to keep spreading throughout the red bay population of East Texas and downward along the Gulf Coast.
“The disease has been identified in 14 East Texas counties and is a significant disease of forest and shade trees in residential neighborhoods and urban landscapes,” he said. “Many trees will likely die due to the explosive nature of pathogen spread. I expect there will be a good amount of spread this summer, and the situation will require some serious attention.”
Appel and others, including Texas A&M Forest Service experts, are developing an educational program on laurel wilt to be held mid-June in the Houston area. The first day of the seminar will focus on arborists and other tree-care professionals; the second day will focus on homeowners.
Trees expected to be affected
“In Texas, laurel wilt has primarily killed red bay and sassafras trees,” Appel said. “However, many other trees such as swamp bay, spicebush and California laurel, also found in Texas, may eventually be hosts to the pathogen. And while we do not have any significant number of avocado trees in Texas, these are also known to be affected by the disease.”
Appel said red bay, the tree mainly affected by laurel wilt, serves as a prized shade tree in urban communities in East Texas.
“Seeds of the red bay serve as a food source for wildlife,” he explained. “This is an attractive and adaptable evergreen suitable to many landscapes, including those of coastal beachfront properties.”
Mickey Merritt, urban and community forestry program leader with Texas A&M Forest Service, Bryan-College Station, said most red bay trees in Harris County are located along the Spring Creek and San Jacinto River corridors.
“While this is a limited area of Harris County, red bays are more prevalent in other East Texas counties,” Merritt said. “It’s important that homeowners in the areas likely to be impacted identify and inspect their trees, especially the high-value trees that have sentimental worth or would be costly to replace.”
However, he said, before removing any trees, it is best to consult an International Society of Arboriculture certified arborist to assess the tree and determine if it is laurel wilt.
Recently, Susan Messmer of Kingwood said she noticed leaves on the top branches of the red bay trees on her property had begun to brown and wither. She also noticed small, compressed sawdust structures near the base of the trees.
“I looked around the neighborhood and noticed several trees in the same or worse condition,” she said. “I did an internet search in the tree and the symptoms and then contacted my local AgriLife Extension office.”
After some discussion and a visit to her property by Appel, with an arborist from the International Society of Arboriculture Texas Chapter, followed by a lab analysis by the Texas Plant Disease Diagnostic Laboratory at Texas A&M University, Bryan-College Station, it was determined Messmer’s trees were infected with laurel wilt.
“I collected tissue samples from dying trees and confirmed the presence of Raffaelea lauricola by isolating the pathogen and having the molecular analyses done to prove the identity of the fungus,” Appel said.
Messmer said it was unfortunate to see the disease affecting so many trees throughout her area.
“Three of the six trees in my front yard are dead, and dozens more trees in the neighborhood are diseased or dying,” she said. “It has been very challenging and expensive to try and keep the disease from spreading and to replace the trees that have died or we know will never recover.”
Kate Henderson, another Kingwood resident, had many of the red bay trees on her 1-acre property infected by laurel wilt.
“I began to notice some browning in my trees last summer and thought at first it was the weather,” Henderson said. “Later, I realized it was just happening to the red bay trees on my property.”
She contacted Merritt, who, along with Texas A&M Forest Service forest entomologist Demian Gomez, based in Austin, came to her property to investigate and take samples for analysis. The results confirmed her trees had been infected by laurel wilt.
“It really amazed me how quickly the trees died once I saw signs of laurel wilt,” Henderson said. “I remember seeing the initial signs on one of the red bay trees, and it was totally brown within four weeks. So far, I have lost 17 red bays on my property. I’ve seen it all around this area and have heard of it also being found in Liberty, Conroe and elsewhere.”
Henderson said she also has a number of oaks, yaupons and magnolia trees on her property, as well as sassafras trees.
“I was told sassafras was also susceptible to laurel wilt, but fortunately, I haven’t seen any signs of it yet on my sassafras trees,” she said. “But I know this disease is serious, and people need to be aware of it.”
Henderson said she believes in reusing natural materials in the landscape, so she has had her dead red bays chipped into small chunks for lining the pathways on her property and for mulch.
How is it spread, and what are its symptoms?
“The way laurel wilt spreads is similar to that of Dutch elm disease or oak wilt,” Appel said. “It can spread by insect or through the roots of one tree to another if the root systems comingle.”
He said part of the reason for the rapid spread of the laurel wilt pathogen is the ambrosia beetle. It can carry the pathogen from diseased to healthy trees, sometimes for considerable distances.
“The red bay ambrosia beetle was introduced simultaneously with the pathogen,” Appel explained. “These beetles are uniquely suited to acquire the fungus from dead trees and transmit it to healthy trees.”
Female ambrosia beetles carrying the fungus will bore tunnels into healthy trees and lay eggs. As the eggs hatch, the juveniles feed on the pathogen as it grows in the original tunnels. They then emerge as adults to visit new trees and spread the pathogen.
“This is a very virulent disease that colonizes in the water-conducting vessels of the tree’s vascular system,” he said. “This starts with an attack of a small number of beetles. Eventually, more are attracted to the tree, and they overwhelm its natural defenses. Once the tree is overwhelmed like this, the likely result will be death.”
Appel said the initial symptoms of an infected tree are yellowing and wilting of leaves in random branches.
“These symptoms will spread to larger limbs so that increasingly more of the tree canopy will be affected, displaying splotchy patterns of yellow and brown leaves throughout the crown,” he said. “Although this may not always kill the tree, it will negatively affect its appearance and overall health.”
Appel said that, internally, the tree will have distinctive streaking in the sapwood that is revealed when bark is removed from the trunk or larger limbs. The ambrosia beetles also cause small holes on the bark surface that will extend beneath the bark into the sapwood.
“Externally, the beetles cause tiny ‘sticks’ comprised of compacted wood dust protruding from the bark,” he said. “The sawdust-like substance will usually accumulate in bark furrows or around the base of the tree.”
How can I find out if my tree has laurel wilt?
Appel said an initial assessment of whether a tree has laurel wilt can be made by direct observation of any symptoms known to be associated with the disease.
“However, different plant diseases or environmental factors may account for one or more of those symptoms, so the most accurate way to determine if laurel wilt is present is to send a sample to a plant disease diagnostic laboratory for assessment,” he said.
For example, samples from diseased trees may be submitted to the Texas Plant Disease Diagnostic Laboratory of AgriLife Extension. For information, go to https://plantclinic.tamu.edu.
“Submission involves collecting the appropriate sample,” he said. “In this case, it should be a branch or trunk sample consisting of symptomatic sapwood from the infected tree or trees. Further instructions will depend on the diagnostic protocol being used by the clinic running the samples and should be obtained prior to submission.”
Laurel wilt control, management
“Proper diagnosis of laurel wilt is always the first step in planning an effective management program,” Appel said. “The second step is to prevent the pathogen from spreading to new, healthy trees.”
He said all precautions should be taken to prevent the wood from diseased trees from being transported into areas where the pathogen is not known to occur.
“If the pathogen is introduced into an area, then diseased and dead trees should be promptly identified and removed,” he said. “The wood should be destroyed, buried or otherwise treated appropriately to eliminate the threat of spreading contaminated beetles.”
Merritt noted that although burning the infected trees has also been proposed as an option for disposal, drought conditions should exclude this option for the time being.
“While red bay trees are not typically used for firewood, it’s important not to transport them for this or any other use,” he said. “But it’s generally safe to have the trees chipped into small pieces that can eventually be used in compost or as mulch, once the chips have sufficiently dried and start to decompose.”
Appel said any wounds on uninfected trees should be treated with pruning paint as they are likely to attract beetles.
“Weakened or stressed trees also attract beetles, so keep susceptible host trees healthy and free of stresses that might compromise their health,” Appel said.
He said the intravascular injection of trees with a fungicide may also effectively manage the pathogen.
“Propiconazole, a fungicide sold under the trade names Alamo and Propizol, have been shown to have at least some short-term benefit in protecting trees when properly applied,” Appel said.
He noted there are many injection methods and types of equipment used in this process, so it is best to have a trained commercial arborist perform any such procedure.
Merritt noted that another means of reducing the spread of tree diseases is to plant a diverse selection of trees.
“Having a diverse mixture of species will help keep diseases that affect the same tree genera and families from spreading and infecting a significant amount of your landscaping.” he said.