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Leaf Minor
Guernsey Donkey2
Posts: 6,713
Is this leaf minor and is it harmful to the tree which I think is a Chestnut? 
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It is leaf miner damage. The only way to deal with it is to remove affected leaves and burn them. There is no chemical control because trees are too big to treat. The larvae overwinter in the tunnels in the leaves so collecting up leaves and burning them is a good way of reducing the moth population.
It's more likely horse chestnut leaf blotch. https://www.rhs.org.uk/advice/profile?PID=200
Literally every chestnut around here is affected with it.
Last edited: 28 July 2016 21:07:55
Nearly all Horse chestnuts trees in the UK are now infested with the Horse chestnut leaf mining moth which first appered in about 2002. If the tree is on it's own, you can try gathering up all fallen leaves in autumn and burning them which may mean less damage the following year but if there are a lot of other conker trees in the area it will make little difference. It is thought that they won't kill the trees but I personally think they will be weakened over a number of years and this may lead to other diseases and pests causing more serious damage and early death. See here for a survey you can take part in:
http://www.conkertreescience.org.uk/
So it could be either leaf blotch or leaf minor, but either way the future may be bleak for this lovely, old tree and it's younger sibling standing next to the larger damaged tree. Leaf minor appears to be a well known problem among Chestnut trees, and it still has many conkers forming on the trees - fingers crossed that the tree will last many more years yet. Thanks Bob, Mark 56 & Ladybird for helping me to identify the problem/cause of the weakened leaves.