[Illustration: Mimosa branches showing wilt symptoms.]
A vascular wilt of the mimosa (silktree) is caused by the fungus
_Fusarium oxysporum forma perniciosum_. The fungus is known only to
attack the mimosa, a tree imported from eastern Asia and grown
throughout the southeast as an ornamental. Since the discovery of the
disease in North Carolina in 1935, the fungus has spread north to
Maryland, south to Florida, and west to Texas.
The fungus causing mimosa wilt is soil-borne and gains entrance into the
tree by attacking the roots. Once entrance is gained by the fungus, the
pathogen enters the outer water-conductive system in the sapwood. As the
fungus grows throughout the system, it hinders or completely inhibits
the water movement from the roots to the aerial portion of the tree. The
first outward symptom of disease is the wilting of leaflets, usually in
the upper portion of the crown. The wilted leaflets turn yellow, then
brown, and die. Often a branch or two will succumb at a time until the
entire crown is dead. A second symptom of the disease is found in the
outer sapwood of the tree. A brown discoloration, appearing as spots or
a ring, is observed by cutting into the outer sapwood of the infected
stem or branch.
Control of the fungus is very difficult, since it is soil-borne and
enters through the roots. However, resistant varieties of mimosa trees,
developed by the U.S. Department of Agriculture, are now available at
most commercial nurseries.
[Illustration: Branch sample showing brown discoloration of
sapwood.]