You may have noticed that some Horse Chestnut (Aesculus Hippocastanum) trees have brown blotches on the leaves and indeed the leaves drop off prematurely. This is due to an  attack by an insect known as Cameraria ohridella or the horse chestnut leaf miner (HCLM). The insect larvae tunnel into the leaves destroying tissues and stunting future growth.  Initially the leaves are covered in small brown patches like in the picture. These patches gradually spread across the whole tree making it look like autumn has arrived in mid summer. The leaves will eventually die and fall prematurely.  Research is ongoing to determine what can be done to save our horse chestnut trees from this pest. The full impact of repeated HCLM infestations is not yet known. The trees do seem to grow normal leaves the following year with no significant symptoms of decline.  One study concluded that the trees fail to feed the growing seeds (conkers) sufficiently due to the lack of leaves which in turn reduces the survival rate of the trees. The disease can be treated with the application of pesticides and has been proven to reduce the effects by 80-100%