I have a young horse chestnut tree in the garden, about twelve feet tall. Most of the leaves are discoloured by leaf miner. Can I do anything? I am in London UK.
I'm afraid there is no treatment at all - as I write this (here in London, UK), I look out the window at a 40 foot horse chestnut in the gardens here which is dropping brown leaves all over the place, backed up by an avenue of horse chestnut in the local park, some half a mile away, which even from this distance, I can see is in the same state. This has happened every year for at least ten years - some years, the trees look okay up till August, other years they look bad by July.
The Forestry Commission advises that collecting fallen leaves, particularly during autumn, helps to reduce the infection next year, but frankly, though the leaves are collected weekly here, never left on the ground for longer than 7 days, it makes no apparent difference that I can see.
There are a couple of varieties of horse chestnut which are less affected, and these are mentioned here https://www.rhs.org.uk/advice/profile?PID=533