A group of dead sassafras trees with brown leaves infected with laurel wilt caused by Raffaelea lauricola. Below, discoloration of sapwood of a sassafras tree infected with laurel wilt. Photos by Raj Singh.
Laurel wilt is a devastating disease of woody trees in the Lauraceae family. Trees currently susceptible to laurel wilt include avocado, California laurel, camphor tree, pondberry, pondspice, redbay, sassafras, swampbay and spicebush. Laurel wilt was first confirmed in Louisiana in 2014 on mature sassafras trees in Union Parish. Since then, the disease has spread to Beauregard, Bienville, Claiborne, Grant, LaSalle, Lincoln, Natchitoches, Ouachita, Rapides, Sabine, Vernon and Winn parishes.
The disease is caused by a fungus called Raffaelea lauricola that clogs the vascular (xylem channels) system of the tree and interrupts the water supply. As a result, the affected tree wilts and eventually dies. Initial symptoms of laurel wilt are rapid wilting and drooping (flagging) of leaves. Leaves on affected twigs may exhibit marginal necrosis due to lack of water. As the disease progresses, infected trees exhibit reddish to purplish-brown discoloration of foliage until the entire canopy turns brown (Figure 2). Brown leaves do not defoliate immediately and tend to remain attached to the branches for a period of one year or more in the case of redbay trees, but brown leaves drop readily in other host trees. Removal of bark from infected trees reveals discoloration of sapwood (Figure 2).
The fungus is carried from infected to healthy trees by the invasive redbay ambrosia beetle (Xyleborus glabratus). The pathogen may also spread from infected trees to neighboring healthy trees through grafting roots. In addition, both the beetle and the fungus may spread to new locations indirectly when people move infested firewood from areas where laurel wilt and redbay ambrosia beetles are prevalent.
Redbay ambrosia beetles are brown to black in color and very small (2 mm) in size. Initially, the redbay ambrosia beetles may attack the branches, and the infested trees may not look wilted. Later, the trees start to wilt, and tubes of fine sawdust that look like toothpicks and are produced by ambrosia beetles can be seen on the diseased tree trunks. The sawdust toothpicklike tubes may easily wash away with rainwater and may not be present on infected trees after a downpour. Tiny entrance holes created by redbay ambrosia beetles are present on small branches as well as the trunk of diseased tree.
Rapid and early disease detection and removal of infective trees is the most effective management strategy to combat laurel wilt. After removal, burn the diseased trees or dispose of them properly to prevent further disease and beetle spread. Avoid moving firewood from areas where laurel wilt and redbay ambrosia beetles are prevalent or known to occur. When it comes to firewood, remember to buy locally and burn locally!
Please note that early symptoms of laurel wilt can be easily misdiagnosed with the damage caused by the black twig borer (Xylosandrus compactus). The black twig borer attacks small diameter branches and causes death (flagging) of infested branches. If you notice symptoms of laurel wilt on susceptible host trees listed above, please contact Raj Singh at 225-578-4562 or by email at email@example.com.
Raj Singh is the director of the LSU AgCenter Plant Diagnostic Center.