These remarkable patterns on leaves of the Horse Chestnut tree (Aesculus hippocastanum) result from a combination of the natural changing of the leaf colour in autumn and the effects of infestation by tiny caterpillars of the Horse Chestnut Leaf Miner moth (Cameraria ohridella). This Spanish species has invaded Britain in recent years and is having a very noticeable impact on the appearance of this one type of tree but it does not seem to have an effect on the wood of the tree. I have noticed a decline in the numbers of conkers this year, which might have had something to do with the loss of photosynthetic capability in trees that have been bady affected, but could be attributed to a number of other causes as well.
The tiny caterpillars live in the shallow space between the upper and lower layers of the leaf, and eat away systematically between parallel veins, killing that area of the leaf and leaving narrow bands of dead tissue that cover the leaf in irregular stripes. These can be seen increasingly throughout the summer in infected leaves but in autumn, as the leaves change colour and naturally die back for the winter, the damage caused by the caterpillars becomes more pronounced. It looks almost as if some artist has been having fun decorating the foliage to celebrate Halloween.