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Dutch Elm Disease
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Dutch elm disease
cause leaves to wilt
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Leaves then brown and
die
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Dutch
elm disease (DED) is the most devastating shade tree disease in North
America. It is a wilt disease
with an extremely high fatality rate among elms.
Dutch
elm disease (or DED) is caused by a fungus. After the disease is
contracted, spores rapidly reproduce and spread toxins throughout the
tree.
The
fungus Ophiostoma (Ceratocystis)
ulmi attacks various species of elm. It can kill a tree within a few
weeks or it can kill it gradually over a period of years. The
fungus blocks the water-conducting or vascular system of the tree
preventing water and minerals from reaching the branches and leaves. The
leaves wilt and eventually the tree dies.
There
are two strains of the fungus in North America - the non-aggressive strain
(O. ulmi) and the aggressive strain (O. novo-ulmi).
While the elm’s natural defense mechanism tries to fight off the
fungus, the aggressive strain often moves too quickly for the tree to
react without human intervention.
A
younger fast-growing tree can die quickly, but some younger trees seem to
have a degree of natural resistance to DED.
However, this resistance tends to wear off after 15-20
years.
Slow-growing older trees can linger for a year or two.
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Discolouration of the wood is also a sign of
the disease. If you peel back the bark on a wilted branch, you will
see some streaking on the wood.
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An infected elm tree usually exhibits symptoms soon after
infection. Because of the speed with which the disease attacks, detecting
symptoms as early as possible is essential for treatment. The first
sign of the disease is the sudden wilting of leaves in the upper reaches
of the tree. Next, the leaves change colour from green to yellow to
brown. They then shrivel and die.
If the infection occurs very late in the season, the
leaves will appear to fall normally. However, the following spring the new
leaves will be smaller than normal. The tree will often die before
mid-summer. In late summer, it may be difficult to distinguish
between wilting and natural fall colours.
Control efforts have focused on two areas - prevention
and treatment. The spread of Dutch elm disease can be
effectively checked with a stringent sanitation program involving
surveillance, timely pruning and proper disposal of infected wood.
Directly attacking the elm bark beetle population is less effective and
often not necessary - only a very small percentage of the beetle are
carriers of the disease.
Preparing your elm by improving the soil with mycorrhizal
fungi and fertilizer can also boost a tree's strength. Click
here for more.
Treatments have so far been based almost entirely on
fungicidal injections. There are indications that new treatments
resulting from advanced molecular biology research will become available
in the future.
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Insects and Diseases
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