Tar spot (Rhytisma acerinum)

The microscopic image shows an infected leaf with sclerotia. A consequence of monoculture.

Just like buildup of mold on pine needles and on the leaves of the willow, an ascomycete (Rhytisma) is the cause of tar spot. From late summer to autumn, round black mold deposits are growing on the leaves of several species of maple, which overwinter on the ground after the falling of the leaves. In spring distribution
follows via the light and far away floating mold spores. If maples are standing close together in species-poor mixed forests, the fungus can spread excessively. The photosynthesis of infected leaves is severely hampered by the non-translucent mold deposits. The consequences are that the formation of wood will slow down and that the tree will weaken. The parasitic ascomycete gets its energy from the vascular bundles of the leaf, via its string-like hyphae, which can be recognized in the cross section of an affected leaf.
Mold class: Ascomycetes
Sclerotia: Hardened form of tissue

Source: http://bibliotecabiologia.usal.es/imagenes/picture.php?/1397/category/12