Black spots on an October glory maple generally indicate the presence of a fungus.The culprit may be one of two species of Rhytisma fungi or the Kabatiella apocrypta fungus, which causes maple anthracnose, a serious condition sometimes resulting in the complete defoliation of the tree.
Fungi of the Rhytisma species cause a condition known as tar spot, which first surfaces in spring as yellowish freckling on the upper portion of the leaves. By late summer, these spots coalesce into black spots as large as 1 inch across. Diseased leaves may drop, but the fungus usually does not cause serious harm. Diseased leaves should be raked up promptly to prevent the spread of the fungus to healthy leaves. Fungicides do not control the spread of the disease.
Maple anthracnose is a more serious infection that only occurs in Mediterranean climates following an exceptionally wet spring. Red, sunken areas appear on the leaves, which quickly wither, blacken and die. To help an infected tree survive, the canopy needs to be trimmed to allow more light and air circulation, which inhibits fungal growth. Additionally, all fallen leaves must be cleared quickly, and the tree must be fertilized and watered appropriately to help it survive an attack. Fungicides are used only if the tree suffers repeated, severe attacks.