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Trees diseased with laurel wilt initially exhibit drooping foliage with a dark olive green discoloration. This discoloration may occur in a portion the crown at first, but gradually the entire crown wilts and reddens. The foliage eventually turns brow, tan, or gray and may remain on the branches for up to a year or more.
Left: Redbay with partially wilted crown. Right: Same tree eight months later (Photos by A. Mayfield , Florida DACS Division of Forestry).
Stem cross sections and removal of bark from wilted trees reveals a dark, blackish discoloration in the sapwood. The extent of this discoloration or stain (a response of the tree to the fungal infection) varies depending on how long the tree has been infected.
Left: Bark removed to show black discoloration on the sapwood surface of a redbay diseased with laurel wilt. Right: Sapwood discoloration shown in a stem cross-section (Photos by A. Mayfield, Florida DACS Division of Forestry).
In the early stages of disease, a wilting redbay may not show any obvious signs of ambrosia beetle attack. Early attacks are inconspicuous and may happen on branches in the crown or on the stem. Eventually, as the tree dies from fungal infection and is colonized by more ambrosia beetles, toothpick-like tubes or piles of fine sawdust may be observed on the bark. This dust is produced by the redbay ambrosia beetle as well as other species of ambrosia beetles that colonize the dead tree.
String of compacted ambrosia beetle sawdust on redbay (Photo by A. Mayfield , Florida DACS Division of Forestry).
Laurel wilt symptom expression in some other host species may vary somewhat from those observed in redbay. In avocado and sassafras, wilted leaves do not remain attached to the host for an extended period of time as in redbay. The vascular discoloration can vary and may appear brownish or bluish in some individual plants or species. In camphor, infections with the laurel wilt pathogen may result only in isolated branch flagging or dieback, from which the tree may recover (Cameron et al. 2008).
The black twig borer ( Xylosandrus compactus (Eichhoff)) can cause damage that is commonly misdiagnosed as early symptoms of laurel wilt. The black twig borer is a small ambrosia beetle that attacks small diameter twigs of healthy trees of a large number of host species and is very common on redbay. The female black twig borer produces a tiny entrance hole (0.8 mm) on the underside of a twig, creates a brood chamber in the pith, and cultivates its symbiotic ambrosial fungus therein (Dixon and Woodruff 1982). The twig may also display dark discoloration in the wood around the entrance hole and brood chamber. These attacks cause death and “flagging” of the isolated infested twigs, which may retain their brown leaves, but the black twig borer does not kill large branches or entire sections of the crown as does laurel wilt. Isolated or scattered branch flagging characterized by tiny pinholes in the twigs and no progression in severity over a matter of a few weeks can commonly be attributed to black twig borer (Mayfield et al. 2009).
The black twig borer (Xylosandrus compactus) creates pinholes in small diameter twigs (A) and causes isolated branch flagging (B) on redbay that can be incorrectly diagnosed as early stages of laurel wilt. Photos by A.E. Mayfield III, Florida DACS Division of Forestry.
USDA Forest Service - Forest Health Protection, Southern