Autumn colour hasn’t really started to show itself as of yet on much of Devon’s tree stock, but a very obvious exception has been our Horse Chestnut trees. In fact many have been showing signs of premature autumn colour since mid summer! The reason is not that autumn has come early for Horse chestnuts but is mainly due to the effects of the Horse Chestnut Leaf Miner and we have had many calls from concerned tree owners regarding the effects of this insect pest.

The Horse Chestnut Leaf Miner (Cameraria ohridella) is a moth with caterpillars which feed on the leaf tissue of the European white-flowered Horse Chestnut (Aesculus hippocastanum). The leaf miner was first recorded in the UK in south-west London in 2002 and quickly spread to many parts of England and Wales. Red Horse Chestnut trees are rarely attacked and only when growing close to heavily infested white-flowered Horse Chestnut trees. Affected trees can be rapidly colonised by the insect, with elongated blotches, at first white but later turning brown, developing on the foliage from mid-June onwards. By August, most of the leaf area may be occupied by leaf mines, giving the impression that the tree is dying, although it will survive. Heavily infested trees can drop their leaves early.

Although looking dramatic, the effect on the physiology of the tree is thought to be minimal and studies undertaken by the Forestry Commission show that the harm caused is mainly aesthetic. This pest does not appear to lead to a decline in the health of the tree nor does it pre-dispose the tree to attack by other organisms, in particular by Bleeding Canker Disease.

Control of the leaf miner via insecticides is not warranted. Control has been attempted by carefully collecting all the leaf fall in the autumn and burning it or composting in sealed bags until the next summer, but this is I suspect a futile effort unless perhaps you have a prized, isolated specimen.

Bleeding Canker Disease (BCD) is a bacterial disease with the unfeasibly long scientific name of Pseudomonas syringae pathovar aesculi. This is a much more serious disease of Horse Chestnuts, characterised by bleeding lesions on the trunk or limbs of trees. This disease can lead to the death or dieback of affected trees.

Control of BCD may be possible in some situations, though sanitation felling (i.e. the removal of badly infected trees so as to reduce the spread of infection through the remaining population) may be a more realistic option at times.

If you are concerned about the health of your Horse Chestnut trees then please call us and we will be happy to advise.

By Graham Joyce | October 4th, 2013