Figure 4. A bleeding canker in
a cultivar of red maple (Acer
rubrum 'Scanlon'), showing that
from initial bleeding, a large open
wound has been created which
this tree has failed to occlude. The
culprit for this problem is a water
mould—Phytophthora in this case.
Lancashire, England has a rather
wet climate, and both fungi and
water moulds are common parasitic
diseases on our trees, as both enjoy
the high humidity and rainfall levels
here. The initial bleeds mark areas
where the inner bark is under attack:
very often, the bleeding episode
occurs for only one or two seasons,
killing a discrete area of inner bark
and, over several years, the outer
bark detaches to show the full
extent of the damage done by the
pathogen. This sort of wound is far
more problematic for an old tree—
and for species that are more prone
to decay—as the tree's main defence
is to occlude the wound (Fig. 1).
When occlusion takes many years,
these bleeding cankers are often
sites of secondary decay organisms
(esp. Cerioporus, Flammulina,
Pleurotus and Trametes fungi in
Lancashire).
Figure 5. A basal shear crack, caused by excess bending
at the base of this mature ash (Fraxinus excelsior), which
led to the failure of the tree within three years. The TOT
images I have of the failure of trees—an aspect of a tree's
life cycle that is quite difficult to capture, as you have
to get to the failed tree before it is cleared away—show
that damage to a tree has to be quite extreme for a tree
to fail: whole tree failure is often related to large cracks,
major root decay, or stem decay. In this case, the tree
was originally growing on a slope, but this was terraced
in 2004, leading to a raise of ground level on one side
of its trunk by about .3 m (1 foot). The subsequent decay
of the roots on that side caused abnormal bending at
the base of the tree, and the shear crack formed. This
defect puts the tree in an unstable state and it is unlikely
to recover (Dunster et al, 2017)—such a crack through the
main stem of the tree is a major court for decay agents—
and the crack is likely to propagate when the tree is
swayed further by the next set of winter storms. >>
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