STEM AND BRANCH DISEASES

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the form of American elms and are susceptible to winter injury. Hybrids between
American and Siberian elms incorporating the resistance of Siberian elms have
been produced. Biological control has some possibility, including parasitic wasps
on bark beetles and parasitic fungi that lower bark beetle reproduction.
Oak Wilt
Oak wilt is extremely important in the north-central United States where oaks are
predominant, but it also occurs in the southern states and across to Texas (Tainter
and Baker 1996). It is of great concern internationally (Nair 1996). Most oak
species and varieties are susceptible, along with three species of Castanea, and a
species of Castanospis and Lithocarpus. Red oaks are very susceptible, white oaks
range from susceptible to highly resistant (e.g., Quercus alba).
The fungus causing oak wilt is Ceratocystis fagacearum. Like Dutch elm disease
it is transmitted by insects and through root grafts. Ascospores and conidia
are spread by sap-feeding and bark beetles. Spores are apparently introduced to
healthy trees through wounds. The fungus spreads rapidly through the vascular
system of the tree, degrading the cell wall of infected sapwood cells. Tyloses plug
the water-conducting tissues of the tree, resulting in wilt symptoms. Brown to
black discoloration occurs in the sapwood, usually in the most recent ring. After
the tree dies the fungus produces spore mats between the bark and wood. The fermenting
odor attracts insects.
The fungus spreads rapidly through root grafts, but grafting occurs only
between trees of the same species. The best method of management is the disruption
of root grafts either chemically or mechanically. Complete bark removal
of infected trees prevents spore mat formation and spread by insects. Systemic
fungicides also have been used successfully.
STAINS
Stains produced by fungi are common in many conifers, especially pines and
Douglas-fir, and usually are brown, black, or blue in color (Boyce 1961). This
topic is covered in more detail in Chapter 16. The fungi involved are mostly
Ascomycetes or Deuteromycetes. Blue-stain fungi usually are wedge-shaped
because the fungus moves preferentially through the tangential ray cells (Hansen
and Lewis 1997) (see Figure 16.5). Stains and wetwood also are common in whitewood
species such as spruces, firs, and hemlocks. Wetwood is wood that is high
in moisture content.
R EFERENCES
Agrios, G. N. 1997. Plant pathology, 4th ed. Academic Press, New York.
Allen, E., D. Morrison, and G. W. Wallis. 1996. Common tree diseases of British Columbia. Pacific
Forestry Research Centre, Canadian Forestry Service, Victoria, B.C., Canada.
Boyce, J. S. 1961. Forest pathology, 3rd ed. McGraw-Hill, New York.