Scientists with the USDA Forest Service Southern Research Station
(SRS), Iowa State University, and the Florida Division of Forestry have
provided the first description of a fungus responsible for the wilt of
redbay trees along the coasts of South Carolina, Georgia, and Florida.
In the February issue of Plant Disease, SRS plant pathologist Stephen
Fraedrich and fellow researchers provide results from their assessment of
the fungus, the beetle that carries it, and their combined effect on redbay
and other members of the laurel family, including sassafras, spicebush, and
avocado.

Extensive mortality of redbay, an attractive evergreen tree common
along the coasts of the southeastern United States, has been observed in
South Carolina and Georgia since 2003. Though the wilt was at first
attributed to drought, the cause was soon found to be a fungal pathogen and
the exotic redbay ambrosia beetle, Xyleborus glabratus, a native to
Southeast Asia that was first found in the area in 2002. Many ambrosia
beetles carry species of fungi as food for their larvae; a previously
undescribed fungus in the genus Raffaelea is a fungal symbiont of this
ambrosia beetle.

To determine if the fungus was the cause of the wilt, Fraedrich and
his colleagues inoculated redbay trees and containerized seedlings with the
Raffaelea fungus; the plants died within 5 to 12 weeks. To connect fungus
and beetle, they also exposed redbay seedlings to X. glabratus beetles; the
ambrosia beetles tunneled into almost all of the plants, causing 70 percent
of them to die. The researchers found the fungus in 91 percent of the
beetle-attacked plants.

"These experiments showed that the Raffaelea species we isolated from
wilted trees and from the redbay ambrosia beetle is the cause of redbay
wilt," says Fraedrich. "The fungus, which is routinely isolated from the
heads of X. glabratus ambrosia beetles, is apparently introduced into
healthy redbay during beetle attacks on stems and branches."

Redbays are common along Southeastern coast, and both residents and
visitors are disturbed by the massive mortality. Deer browse on the
evergreen foliage of the tree, and the fruit is eaten by songbirds, wild
turkeys, and other animals. Redbay is also the primary host for the larvae
of the palamedes swallowtail butterfly. But it?s not just the redbays that
plant pathologists are worried about.

"The fungus we isolated has also been associated with the death of
other trees in the laurel family, and the Raffaelea sp. has been isolated
from wilted sassafras, pondberry and pondspice," says Fraedrich. "Our
inoculation studies have shown that the fungus is deadly to these species as
well as to spicebush, and avocado, but not to red maple."

The researchers concluded that there is reason to be concerned about
the spread of the wilt to other members of the laurel family, which are
common components in forests across the United States and other areas of the
Americas. Recent studies have shown that California laurel, a West Coast
species in the Lauraceae, is also susceptible to laurel wilt.

"We are also very concerned about avocado, a species indigenous to
Central America which is grown commercially in Florida and alifornia," says
Fraedrich. "Our evaluation of avocado indicates that it is also susceptible
to laurel wilt, and the wilt has been found recently in avocado trees
growing in a residential area of Jacksonville, Florida."
[www.srs.fs.usda.gov]