If you have a Horse Chestnut, you are going to have this disease very often - sometimes each year. Leaf Blotch starts a series of small yellowish to yellow orange blotches that quickly turn to  a series of brown dead  blotches that  appear on the leaves as early as June that get progressively worse as the season goes on.  The blotches are caused by a fungus called Guignardia aesculi.   As with many leaf invading fungi, it appears and does best in wet environments such as sustained rains . Usually if Apple Scab is bad in any given year, odds are Horse Chestnut Leaf Blotch will also be bad

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As the year progresses the blotches progress and result in early leaf drop (starting as early as late July).  Severe infections can make the tree look almost dead because all the leaves look brown. 

 

 Fortunately, over eons, Chestnut and this disease have found a balance and despite its awful appearance, it very rarely kills the tree . There are treatment chemicals but   we don't recommend it because by the time you notice, it  it is usually too late (the leaves are going to look awful regardless); Also, it would likely require repeated treatments. 

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Just to complicate things, there is also Chestnut Leaf Scorch which looks almost identical and is also quite common.  The main difference is leaf scorch is caused by hot, dry/drought conditions and the brown areas are more on the entire margins and not as blotchy but even we have a hard time telling them apart  (We have to look for the little sporophores of Leaf Blotch later in the year)