This disease is also known as Guignardia leaf blotch because the causal pathogen is a fungus named Guignardia aesculi. We see it most commonly on horsechestnuts, but buckeye trees also host the disease. From a distance, infected trees appear to be severely scorched. On closer inspection, however, reddish brown leaf spots with bright yellow margins are apparent. The spots become large and cover most of the leaf surface. Leaves then become dry and brittle and drop early. You can distinguish this disease from environmental scorch (discussed in issue No. 5 of this newsletter) by the fruiting bodies formed by the fungus in the leaf lesions in moist weather. These structures are called pycnidia. They appear black and are about the size of a pinhead. All leaves are affected, unlike scorch, which affects newest leaves first on the side of the tree that is exposed to sun or wind.
This disease may be serious and treatable with fungicides (starting at bud break) in nursery stock, but mature trees usually retain live buds and lose leaves late in the season, so they are not significantly harmed. Most of the seasonís growth has already occurred before infection. Removing fallen leaves may be helpful in reducing the amount of fungal inoculum living through the winter on these leaves. Also, try to prune surrounding vegetation to allow better air flow through the area for more rapid drying of foliage. This disease is one more example of why you should not plant trees too close together when they are young. Consider mature size and spread when you select planting sites.