BP-50-W
Purdue University
Department of Botany and Plant Pathology
Ornamental Diseases
Dutch Elm Disease
Paul C. Pecknold, Extension Plant Pathologist
Despite the destructiveness of Dutch elm disease
(DED) many American elms continue to survive, adding grace and beauty
to the Indiana landscape. Knowledge of the disease and proper
management practices will hopefully enable you to save your elm for
another season, or possibly, even another generation.
Cause
DED is caused by a fungus (Ophiostoma ulmi )
that is carried from diseased trees to healthy trees via two species
of elm bark beetles: the smaller European elm bark beetle and the
native elm bark beetle. Spores of the fungus are produced in the wood
and bark of dead or dying trees; the beetles also inhabit dead or
dying elm trees. When the beetles emerge from DED-infested trees,
they carry spores of the fungus on their bodies and migrate to
vigorous elms to feed. After feeding in healthy trees, the beetles
then move to dying or dead elms where the females lay their eggs;
thus, the disease is perpetuated through this linking of fungus and
insect. A second way the fungus can be spread is by root
grafts. Roots of adjacent elms often make contact and grow together,
thus allowing the fungus to travel from an infected tree to a healthy
adjacent tree.
Symptoms
The disease results in wilting and yellowing of the
foliage, followed by leaf death, defoliation and death of the affected
branches. Wilting and yellowing of the leaves usually becomes visible
about mid June and are most evident during July and August. Brown
streaks develop under the bark in the sapwood of infected branches.
This may be seen as a ring of discoloration when a diseased branch is
cut or as dark streaks when the bark is peeled back from the infected
branch.
Management
The most important step in control of DED is good
sanitation. If proper sanitation practices are not followed, other
control measures are of little use.
- Sanitation. Remove and dispose of all diseased elms
and all elms killed or seriously weakened regardless of cause. Elm
wood may be chipped so none remains with sufficient bark to serve as
brood wood for beetles. If chipping is not possible, diseased elm
should be burned (where permitted) or buried in a landfill. It is
also important to keep elm trees pruned so that large dead or weakened
branches in otherwise healthy trees do not become beetle brood wood
sites. Prune in the winter while trees are dormant. Do not
stockpile diseased wood for firewood!
- Insect Control. An insecticide to control the beetles that
spread DED is an essential part of any management program. Proper application
and timing are essential to effectively reduce the population of elm bark
beetles. For recommended insecticides to control elm bark beetles see publication
E-41, "Recommendations for Managing Insects and Mites on Shade Trees
and Shrubs," available from your local county extension office.
- Preventing Spread through Roots. Spread of DED through root
grafts can be prevented by chemical soil fumigation, or by mechanical separation
(cutting of roots by digging a narrow trench 18 to 24 inches deep between
diseased and healthy elms). Simply removing infected trees promptly does not
necessarily prevent spread of the disease to adjacent trees through connecting
roots.
- Eradicant Pruning. Removing diseased branches, well
below the point where sapwood discoloration is evident, may help rid
the tree of infection. Such eradicant pruning must be done early, for
trees that are showing 10-20% of the crown infected it is too late for
such pruning. Prune back at least 10 feet into healthy wood, usually
to a major limb. Eradicant pruning may result in the loss of a major
limb. Nevertheless, many American elms can be saved by this pruning
method. Carefully watch the tree for any signs of further
wilting.
- Injection of Systemic Fungicides. The injection of
systemic fungicides can be done on a preventative basis for selected
high-value trees located in high disease risk areas, or they can be
applied to help cure infected elms showing early stages of disease
development (less than 5-10% of crown showing symptoms). Alamo and
Arbotect 20-S are trade names of two systemic fungicides registered
for control of DED. These fungicides are generally effective for 2 to
3 years, however trees in high disease risk areas or high value trees
should be evaluated for possible retreatment 12 months after
treatment. Repeated injections are discouraged due to the physical
damage (woodstain and decay) that results from the injection
process. The pesticide labels recommend that the fungicides ‘be
administered by trained arborists or others trained in injection
techniques and in the identification of DED’. Systemic fungicides
are most effective when used in conjunction with those management
practices described above.
Reference to products in this publication is not intended to be an endorsement
to the exclusion of others which may be similar. Persons using such products assume responsibility for their use in accordance with current label directions of the manufacturer.
REVISED 5/96
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