Our much loved horse chestnut trees are once again looking under
the weather this year- instead of glorious green, the leaves are
very much brown. Trees in the south east appear to have been hit
the worst.
The main cause is the horse chestnut leaf miner- a small
caterpillar which lives in the leaf and munches through it until
it has eaten enough to pupate and fall to the ground where it
will spend the winter and emerge next spring as a moth.
As the caterpillar munches through the leaf, it dies and turns
brown and crispy. This is what can be seen on many of our horse
chestnuts however the following year the tree still puts out new
fresh leaves.
Fortunately there is a way to help, because the caterpillar
overwinters among the dead leaves at the base of the tree,
collecting and removing these leaves means fewer moths with
emerge next year to lay their eggs and become the next
generation of caterpillars. Alternatively the leaves could be
covered with a thick layer of soil which should prevent the
moths emerging. We are unlikely to ever solve the problem
completely but we can certainly reduce the impact.
However the horse chestnut does suffer from other illnesses and
another disease that has received much attention is the bleeding
canker. Unlike the leaf miner, which will not kill the tree, the
bleeding canker can result in the death of the tree.
The obvious symptom of bleeding canker is a run of
blood/rust-red sap down the bark of the tree. If you look up it
is also likely that parts of the canopy of the tree are dying
back with withered and dead leaves. Over time the bleeding
canker can spread through the tree and once it has got all the
way around the living wood in the outer area of the trunk, the
tree is throttled and will slowly die, over a few years.
Research is underway to pinpoint the cause of the bleeding
canker and hopefully find a cure but at the moment there is
little that can be done. Only remove the dead wood in the crown
if it is a risk as it provides very valuable habitat for
wildlife.
Together both of these afflictions are hitting our horse
chestnuts hard but trees can be surprisingly resilient and may
survive against great odds. Looking after our trees and reducing
other stresses on them can help to make them more healthy and
able to withstand diseases. Things which can help include:
-
Avoid using chemicals and pesticides near
trees- these can affect the roots and the ability of the
tree to take up nutrients
-
Avoid compacting the roots by driving over
them or letting livestock gather under the tree, for the
same reasons
-
Major tree surgery may seriously affect the
long term survival of the rest of the tree and should be
avoided unless there is a risk to people or property.
This information
has been collated from Forest Research and for more information
please try the following links:
Leaf miner
Bleeding canker
September
2008
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