Laurel Wilt
Laurel Wilt is a fungal disease caused by Raffaelea lauricola and transmitted by the redbay ambrosia beetle, Xyleborus glabratus (pictured at right). At present, the only plant species that are attacked by this fungus are members of the Laurel (Lauraceae) family which include, of course, the various laurel trees as well as avocado, sassafras and pondberry, an endangered species found in coastal southeastern United States.
This fungal species can be transmitted in at least two methods. The first is via its normal life cycle, including either sexual reproduction or fragmentation. The second is via the redbay ambrosia beetle. The latter occurs when the beetle chews through the bark of one of the laurel species. In doing so, the fungus, which resides in pouches near the mouth parts of the beetle, is deposited in the plant. If the tree selected by the beetle is not a member of the laurel family, the fungus eventually dies and no damage is done. If the target species happens to be a member of the laurel family, the fungus quickly begins to reproduce and migrates to the vascular tissue of the tree. The water-transmitting cells of the tree, the xylem, are clogged and the tree soon dies, usually within 3 – 5 years, sometimes even more rapidly.
(at left) Ambrosia Beetle size
The laurel wilt was inadvertently discovered in 2002 in Port Wentworth, Georgia, near Savannah. Wildlife inspectors discovered a new beetle species, later named the laurel ambrosia beetle, that found its way to the U.S in wood shipments. After a series of tests were performed on the new beetle, the fungal species was identified, too. Inspectors suspect that the beetles and their mutualistic fungus had been in the United States for many years before its discovery and probably was introduced to the US via shipments of lumber products and wooden packing materials from Asia, since that is where these beetles are native. We now know that the beetle and fungus are responsible for the deaths of millions of laurel trees, especially the redbay found on many of the barrier islands along the southeastern coast of the US. These infection sites are easily identified by the brown, dead trees seen still standing in an otherwise healthy maritime forest.
While the impact on the avocado industry is measured in the millions of dollars, a homeowner who has a number of redbays on his land will sooner or later find these are at risk and may be quickly killed. At the current time, there is no treatment for this disease, and apparently none will be available in the near future.
The laurel wilt disease has spread throughout the southeastern US, but in spite of several sassafras trees being located at Reflection Riding Arboretum and Nature Center, we have not yet seen any indication of this pest. Counties northwest and southeast of Chattanooga have been infested with this pathogen but not the Chattanooga area, however data indicate that the species has been identified in parts of Hamilton County.
Indications that the disease is present include drooping leaves during the growing season which eventually turn brown. These leaves remain on the tree for upwards of a year revealing a brown mass or leaves in an otherwise green forest. In addition, the chewing of the beetle often produces a plug of sawdust that extends out from the bark of the tree signifying the presence of the beetle and, often, the fungus.
(at right) Vascular streaking in a Sassafrass branch; indicative of Laurel Wilt
The laurel wilt disease has spread throughout the southeastern US, but in spite of several sassafras trees being located at Reflection Riding Arboretum and Nature Center, we have not yet seen any indication of this pest. Counties northwest and southeast of Chattanooga have been infested with this pathogen but not the Chattanooga area, however data indicate that the species has been identified in parts of Hamilton County.
Indications that the disease is present include drooping leaves during the growing season which eventually turn brown. These leaves remain on the tree for upwards of a year revealing a brown mass or leaves in an otherwise green forest. In addition, the chewing of the beetle often produces a plug of sawdust that extends out from the bark of the tree signifying the presence of the beetle and, often, the fungus.
This pathogen is a surprising invader and one that many people will find damaging, both environmentally as well as financially on their property. The laurel wilt will have devastating impacts on the southeastern US for many years to come until some type of cure or resistance screening is developed.
