Guignardia leaf blotch occurs on horsechestnut and buckeye. This disease gives the appearance, at least from a distance, of a tree with a leaf scorch (see Two Types of Tree Leaf Scorch in issue No. 10). On closer inspection, reddish brown leaf spots with clear, bright yellow margins are apparent. These blotches will enlarge and may cover the entire leaf surface by late summer. Leaves then become dry and brittle, and premature defoliation may occur.
Guignardia leaf blotch is caused by a fungus that forms fruiting bodies (pycnidia) in the lesions. Pycnidia are black, pinhead-sized specks when observed with the naked eye. They are not too difficult to see, but a hand lens may be helpful. Guignardia leaf blotch can be distinguished from environmental scorch by the presence of these fruiting bodies. Another characteristic is that the fungal disease affects most leaves, whereas scorch affects new leaves on the sun- or wind-exposed side of the tree.
Guignardia leaf blotch is serious yet treatable in nursery stock. Mature landscape trees usually are not harmed because the leaves remain until late summer and next year's buds are retained. Chemical treatments are not recommended. Prune surrounding vegetation to allow better airflow for more rapid drying of foliage. Removing fallen leaves may help to reduce the number of fungal inoculum that survive the winter.