This is one of the three diseases affecting horse chestnut trees at present. Unlike the other two (Bleeding Canker and Leaf miner) although Horse Chestnut leaf Blotch is not pretty, it does not significantly affect the strength or health of the infected tree.
Horse Chestnut Leaf blotch is a fungal condition caused by Guignardia aesculi. The common horse chestnut (Aesculus hippocastanum) and the Red flowered variety (Aesculus carnea) can be infected. The principal symptom is irregular brown blotching developing on the leaves, starting at the edges of the leaves, from late June. Unlike leaf miner damage, the blotches are a chestnut brown, but by the middle of September the two conditions look very similar and the end result of early de-foliation is the same. Spores of the fungus overwinter in fallen leaves, rising again into the trees in spring. The only realistic control prevention is to break the cycle by raking up and destroying (burn or take to the dump) the fallen leaves.