Tar spot

Rhytisma acerinum

also called sycamore tar spot, maple tar spot

Large glossy black blobs like spilt tar on the leaves of sycamores and maples — striking but harmless to the tree.

Fungus Disfiguring Airborne spores Spreads to neighbors

What you'll see

Big raised black spots, up to 2cm across, with a shiny, tar-like surface.
Each black blot is usually ringed by a yellow halo on the leaf.
Spots start in early summer as pale yellow patches, then darken and thicken through the season.
Fine ridged lines can be seen within the black spots on close inspection.
Affected leaves may fall a little early but the tree is otherwise unharmed.

Why it happens

A leaf-spotting fungus of maples and sycamore. It overwinters on fallen leaves on the ground, and in spring releases spores that blow up onto the new foliage and settle in to form the familiar black spots.

A wet spring that keeps the emerging leaves damp favors infection. It is notably scarce in towns and beside busy roads, as the fungus is sensitive to sulfur dioxide air pollution.

Treating it

No treatment is needed — the disease is disfiguring but does the tree no lasting harm.
On small or young trees, rake up and bin or burn fallen leaves to reduce next year’s spores.
On large trees, simply accept it as a cosmetic feature.
Do not spray; fungicides are neither practical nor warranted on established trees.
Keep the tree healthy with mulch and watering in drought so it shrugs off the leaf loss.

Keeping it away

Clear and destroy fallen leaves each autumn where practical to break the cycle.
Accept that spores blow in from neighboring trees, so control is rarely complete.
Choose less susceptible maple species if planting anew in a damp spot.
Keep trees vigorous so early leaf drop matters little.

Who gets it

Sycamore and maples, especially field maple and Norway maple.