American Tar Spot

American Tar Spot is one of three species of parasitic fungi that infect several species of maple. Collectively the group is known by the common name Tar Spot of Maple. American Tar Spot infects only red maple, silver maple, and sugar maple in Minnesota. In other areas it also infects American Sycamore.

In the spring the infection appears as a yellow spot on the leaf. These turn first brownish-black with a yellow border, then, in late summer, black with a yellow border. The black spots resemble tar. They are raised and solid, not a cluster of small spots. They are usually 3 ⁄16″ to ½″ in diameter but can coalesce into spots up to 1½″ in diameter. On close examination a serpentine pattern of alternating ridges and depressions can be seen.

Speckled Tar Spot (Rhytisma punctatum) causes dense clusters of very small black spots.

Tar Spot (Rhytisma acerinum) in Minnesota infects only Norway maple.

Red maple, silver maple, and sugar maple

Spring to fall

Sources

7, 24, 26, 29, 30, 77.

Biodiversity occurrence data published by: Minnesota Biodiversity Atlas (accessed through the Minnesota Biodiversity Atlas Portal, bellatlas.umn.edu, 7/23/2025).

Until 1998 this species was thought to be the European species Rhytisma acerinum. Some or all of the records in North America reported as Rhytisma acerinum may be Rhytisma americanum. The map at left does not include those records.

Widespread and very common

This fungus was formerly thought to be the same as the European species and went by the name Rhytisma acerinum. A study published in 1998 (Hudler and Banik) showed differences in host specificity, tar spot diameter, and spore-producing stages. The authors proposed a new name for the North American species, Rhytisma americanum.

Location: Southern Michigan

I have had this problem for three years on my tree. I am hoping to find a solution so I can save my tree. As the years have passed it is getting worse.