Download Dutch elm disease

CASE STUDY
Elms in England
English Elm (Ulmus
procera)
• Thought of as a native tree
but most likely brought to
Britain by the Romans
about 2,000 years ago.
• English elm naturally
Propagates by root suckers
• Produces a clonal patch as
young trees are produced in
an ever increasing radius
from the parent tree.
English elm (Ulmus procera).
Public domain image, Peter Bourne.
• English elm was widely planted in new
hedgerows during the Enclosures (1550–1850).
– Twigs planted by foresters strike readily
– Produce roots and grow quickly
Why was the elm a sensible choice for
hedgerows and why did this make the
species especially vulnerable to disease? [3]
Answer
• All English elm trees are virtually genetically
identical; clones of the original introduction.
• Ability to grow from suckers is an advantage;
Regrowth is possible/ maintains hedge
• Genetic uniformity (identical/clones) is a
problem due to susceptibility to Dutch elm
disease
• Close proximity aids spread of disease
Dutch Elm Disease UK History
• There have been two waves of Dutch elm
disease in Europe:
– First epidemic 1910 to 1940s
• Killed between 10–40% of elms
• Caused by the fungus Ophiostoma ulmi
– Second epidemic started in the late 1960s
• More destructive
– Within 10 years it had killed 70% of elms in Britain
– By the 1990s it had killed 85%
– Virtually wiped out the English elm
• Caused by the more pathogenic Ophiostoma novo-ulmi
• The fungus is transmitted to the elm by the elm
bark beetle, Scolytus scolytus
– The fungus spreads through the tree
– Blocks xylem, restricting water transport
– So that leaves wither and the tree dies
• Once most of the available breeding inner bark of
the elm has been used and the mature trees are
dead, the disease almost disappears.
• This allows regeneration by root suckering (basal
sprouts) from the surviving roots.
Elm bark beetle (Scolytus scolytus).
Elm bark beetle gallery.
Question
Why has English elm not been wiped out
by the disease? [2]
• New suckers not susceptible OWTTE
• Therefore can grow once parent dies
• Only effected once a certain size
• By the 1980s large numbers of young elm found growing.
• However:
– Become attractive to the elm bark beetle when regenerated
elms reach a certain size
• > 10 cm diameter and > 3 m height
• Dutch elm infection reappears because the newly
regenerated trees are not resistant to Dutch elm disease
– The regenerated trees are genetically identical to the parent
plants
– Have exactly the same susceptibility to the disease
• Ecological role of English elm is changing:
– From being a dominant tree in hedgerows
– To one more like a shrub or small tree
Wych elm (Ulmus glabra):
tree, bark, leaves, flowers and fruit
Wych Elm
All native elms are susceptible to Dutch elm disease. The Wych elm
(U. glabra) is very susceptible to the fungus, but does not seem to be
as affected by it as the English elm.
Question: Suggest why the following might explain this.
• The Wych elm:
– Produces fertile seed
– Does not reproduce by root suckering
– Produces diterpenes, which deter the bark beetles from
feeding
Produces fertile seed
• Genetic variability from sexual reproduction
• May be different susceptibility levels
Does not reproduce by root suckering
• Suckering is a means of vegetative propagation; resulting in
genetically identical offspring.
• Produces diterpenes, which deter the bark beetles from feeding.
• Diterpenes put off the bark beetle
• They do not feed so do not infect the tree with the Dutch elm
fungus