Dutch Elm
Disease (DED)
Many people think that
Dutch Elm Disease is caused by the European Elm bark-beetle
(Scolytus species). But that is not strictly true. It
is actually a fungus called Ceratocystis ulmi, which
is the cause of trouble. The beetles are just one of the
means by which the fungus can be transferred from tree to
tree. Others are root contact between trees and humans
transporting infected wood around (either with fungus or beetles
in it).
If
dead Elms are not removed, they can serve as a breeding ground
for both fungi and beetles for about 2 years. After this
time the wood is no longer attractive for feeding on. During
warm spells in the summer the young beetles emerge through
circular holes and are able to fly several miles to find an
Elm tree and feed on the sap. If successful it gives off a
scent to alert its little friends to the find. Incisions are
made in the branches to reach
the sap and this is be the beginning of the end for the tree
because the beetles infect the sap with the fungus.
The fungus is similar to a yeast and this is carried along
in the tree sap. It excretes digestive substances
which are toxic to the tree. The cells in the wall of
the sap-vessels react to it by producing gum-filled
enlargements (tylosis), which then block the vessels. This
possibly may be an effort by the tree to control the spread
of fungi, but in doing so it suffocates/starves itself. The flow of
nutrients is stopped and soon the outwards signs of the
disease can be noted: yellowing leaves, drooping or curling
shoots and within a few weeks a dying branch.
There are different forms of the fungus, which may exist
side by side. One is non-aggressive, whereas the other
causes the death of the tree in a short time. |
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Elm recognition
All Elms have in
common the fact that the leaves are slightly uneven at the bottom
(near the stalk) and that their leaves are double-toothed (that
means that the zig-zaggy edges, or teeth, around the edge of the
leaves, themselves have little teeth).
There are quite a few species and sub-species of Elm in Britain and
it can be notoriously difficult to name them, unless you are an
expert.
" 'Hybrids are of
frequent occurrence and usually fertile', according to the Flora.
This is a classic scientific understatement. Hybrids are so
common as to make the species comparatively rare, and the
commonest of the hybrids are not simple crosses but trees of
multiple heredity. This may apply as much to great trees planted 300
years ago as to any elm which springs in the hedge. No wonder the
characteristics of 'true' species are hard to pick out." (Gerald
Wilkinson)
Leaves on the actual tree may differ from the leaves on its
late summer suckers, since Elms with smooth leaves tend to revert to
a hairy form.
As an introduction to various Elm Species we've used some
illustrations and information here from Gerald Wilkinson's excellent
book "Epitaph for the Elm", Hitchinson
& Co, 1978.
Native species
There have
been doubts which Elm are native to this country and no
doubt opinions differ, but following the work of Melville
at Kew, we recognise the following native Elms:
-
Wych Elm - Ulmus
glabra Hudson
-
Common or
English Elm - Ulmus procera Salisbury
-
Coritanian Elm
- Ulmus coritana Melville
-
Plot's Elm, Lock
Elm - Ulmus plotii Druce, Ulmus minor
-
Smooth-leaved
of feathered Elm - Ulmus carpinifolia, Ulmus nitens
-
Cornish Elm
- Ulmus augustifolia Weston.
Dutch Elms
(including the Huntingdon variety)are hybrids between U.glabra
and U.carpinifolia.
The Jersey, Guernsey and Wheatley Elms are
subspecies of U.augustifolia. |
