Introduction
Oak wilt is a serious disease that infects many species of oak. It is
responsible for the death of thousands of oak trees in forests, woodlots, and
home landscapes each year. Oak wilt occurs in the eastern U.S., in an area
delimited by Minnesota, Texas, Pennsylvania, and South Carolina.
Oak wilt is caused by a fungus that invades and impairs the tree's water
conducting system, resulting in branch wilting and tree death. Oak species vary
in their susceptibility to oak wilt. Red oaks are very susceptible; live oaks
in Texas are intermediate; and white and bur oaks are moderately resistant to
this disease.
Oak trees can become infected by the oak wilt fungus by root systems that
become grafted between infected and healthy trees, or by beetle vectors
carrying spores to freshly wounded trees. The biology and control of oak wilt
is discussed in a USDA Forest Service companion publication, "How To
Identify, Prevent, and Control Oak Wilt."
Accurate diagnosis of oak wilt is essential to proper management and is often
needed to justify the cost of control efforts. Trained professionals can often
diagnose oak wilt in the field based on signs, symptoms, pattern of spread,
rate of tree mortality, and site history. Laboratory testing to positively
confirm the presence of oak wilt is necessary, however, if field data are
inconclusive.
Positive confirmation of oak wilt is obtained by laboratory isolation of the
fungus from infected tree tissue. The following information, compiled from
published papers and personal communications with diagnosticians in university
plant disease clinics, provides standard and currently recommended sample
collection and laboratory isolation techniques. Written for field and
laboratory staff, it is offered as a practical guide on how to collect branch
and stem samples suitable for oak wilt testing, and how to isolate and identify
the oak wilt fungus in culture.
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