You may be aware that our conker trees (Aesculus hippocastanum and its varieties) are under threat. The horse chestnut leaf miner grub eats the tissue between the outer layers of the leaves, reducing their ability to breathe and ultimately causing them to fall off. At the same time Horse Chestnut Leaf Blotch - a fungal condition is having exactly the same effect. If your chestnut is shedding its leaves now, you know why.
In turn, this causes the conkers to fall early, which ought to be good news for conkerers except that they will be a bit smaller this year.... because the trees have not been able to take full advantage of that lovely sunny summer you have all been enjoying.
The activities of the leaf miner and the leaf blotch weaken the tree and make it more susceptible to bleeding canker (which may well kill it). One in ten of the UK's horse chestnut trees have bleeding canker and there is no treatment available. It is of greatest danger to young chestnut trees - up to about 35 years old. Beyond that age, the tree is more likely to survive, and there are recorded instances of older trees actually curing themselves.
The horse chestnut leaf miner seems to be pretty unstoppable as well - the adult moth which is 5mm long has no natural predators. It was first seen in South-Eastern Europe in the 1970's and reached the UK in 2002. Since then it has spread from London (where it was first identified) into the Midlands, to Kent and down to Devon.
Our (albeit pessimistic view) is that until either the miner has been stopped or there is a treatment for the canker, it is probably best to avoid planting horse chestnuts in clumps. Keep them away from one another. The one photographed is in a village where every horse chestnut is in some way diseased (except this one). It is just over 100 metres from the nearest infected tree...