Figure 210. Callus folds and flaring dead bark caused by Ceratocystis fimbriata
Symptoms/Signs:
Cankers are target shaped with concentric ridges of dead callus.
Older cankers typically have a central area of dead wood surrounded
by a series of bark calluses. These callused areas may be concentric
in outline, but usually are irregularly shaped and ragged in appearance
because of the massive callus folds and flaring dead bark. Bark
and exposed wood darken, resulting in a black canker.
Biology:Ceratocystis fimbriata
can infect through the epidermis of leaf blades, petioles, and young
stems; but trunk wounds are considered to be the primary places
of infection. Several species of beetles have been found to carry
the fungus and are believed to vector it. Infection first appears
as a circular necrotic area on the trunk around a fresh wound or
branch junction. During cambial growth in the spring, the tree forms
a callus at the margins of the canker, which temporarily walls off
infection. The fungus invades the new cambium and
inner bark during the tree’s next dormant season and kills
a new zone of tissue. This process is repeated each year until the
canker, consisting of successive rings of dead bark and wood, is
formed.
Figure 211. Bark and callus removed to expose target canker.
The
fruiting structures, perithecia, are formed in the spring along
the border of the canker on tissues dead at least a year. Spores
ooze from the perithecia in sticky masses and are often vectored
to other wounded trees by insects. Boring insects are often found
in cankered areas. There appears to be a genetic predisposition
to infection, as some aspen clones are more susceptible to infection
than others.
Effects:Ceratocystis fimbriata seldom kills
large trees since trees generally grow faster in circumference than
black cankers enlarge. However, it is possible for several cankers
to coalesce and girdle a tree. The greatest damage from this disease
is stem deformation and subsequent decay that can result in bole
breakage.
Similar Insects and Diseases:Sooty
bark (Encoelia pruinosa) is also black in color but does
not form a target canker like C. fimbriata.