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Guignardia Leaf Blotch

Horsechestnut and buckeye trees that appear from a distance to be severely scorched may actually be infected with this fungal disease. On closer inspection, reddish brown leaf spots with bright yellow margins are obvious. The spots will enlarge and cover most of the leaf surface. Leaves then become dry and brittle and drop early. You can distinguish this disease from environmental scorch by the presence of fruiting bodies formed by the fungus (Guignardia aesculi) in the leaf lesions in moist weather. These black, pinhead-sized structures are called pycnidia. With Guignardia leaf blotch, all leaves will be affected, unlike with scorch, which first affects newest leaves on the sun or wind side of the tree.

This disease is serious yet treatable in nursery stock. Mature trees usually retain live buds and lose leaves late in the season, so they are not significantly harmed. 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 airflow through the area for more rapid drying of foliage.

Author: Nancy Pataky


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