Rhytisma americanum (tar spot of maple)

Rhytisma americanum
surface of stroma

In late summer and fall Rhytisma americanum makes an appearance on the leaves of silver maples and red maples across the Midwest. It is a fungus, sometimes called "tar spot of maple," and it is a parasite on leaf tissue (especially the outer tissue layer on the upper side of the leaves). Don't freak out, however; Rhytisma americanum will not harm your red or silver maple tree—and even if it were harmful to the tree, there wouldn't be anything you could do to stop it.

The fungus actually appears in June or July, when airborne spores land on leaves of the right host. In the case of Rhytisma americanum the host leaves belong to silver and red maples, but related tar spot fungi are hosted by other maples: Rhytisma acerinum infects leaves of Norway maple (Acer platanoides), Rhytisma punctatum infects big-leaf maple (Acer macrophyllum) and mountain maple (Acer spicatum). A related species, Rhytisma salicinum, infects leaves of willows. In the first stages of infection, the fungus grows slowly and appears as a slightly yellowish or grayish spot that is easily overlooked.

In late summer and fall, the fungus develops into the stage for which it is named, becoming black and looking like a spot of tar, often with a thin yellow halo. This stage, officially called a "stroma" (plural: "stromata"), is very noticeable, especially since it occurs when leaves are turning to fall colors and receiving more attention. In Rhytisma americanum the tar spots range from about half a centimeter across to 2 centimeters, and are more or less circular in outline, or irregularly shaped.

Close inspection of the stroma reveals an interesting surface, channeled with grooves and ridges that are more or less parallel and radiate (again, more or less) from the center. The surface looks like the top of a charcoal briquette that has been strip-mined by some half-crazed ants with no decent plan.


The other tar spots of maple have different-looking stromata (at least when mature; developing stromata of all three species can look like little black pimples). Mature stromata of Rhytisma acerinum (the Norway maple disease) look similar to those of Rhytisma americanum from a distance, but lack the ridges and grooves and are instead composed of tiny bumps. Mature stromata of Rhytisma punctatum (the big-leaf and mountain maple disease) are smaller, bump-like spots that are not aggregated into larger spots.

The level of infection varies from tree to tree; some trees have only a few infected leaves, while others seem to have half a dozen spots on every leaf. As the leaves begin to fall, the fungus falls with them, maturing over winter and, in the spring, splitting open and releasing spores into the air to start the cycle over again.

Now you see why there is precious little you could do to stop the fungus, unless yours was the only silver or red maple within the range of wind currents, in which case you might have some luck if you destroyed all your leaves after they fell. But, of course, there are infected trees all across the Midwest, making even this gesture futile. And anyway, are you really that bothered by some harmless black spots on your maple leaves? If so we sure hope you're equally willing to devote a lot of worry and time to improving your town's schools and feeding the hungry in your community!

Range of Rhytisma americanum

midwestern range

Rhytisma americanum
tar spots on red maple leaves


Rhytisma americanum
close-up of stromata

Rhytisma americanum
close-up of stromata


Rhytisma americanum
tar spots on silver maple leaves

References: Hudler et al. 1998, Hsiang & Tian 2007, Beug et al. 2014.

Kuo, Michael & Melissa Kuo (September, 2017). Rhytisma americanum (tar spot of maple). Retrieved from the midwestnaturalist.com website: www.midwestnaturalist.com/rhytisma_americanum.html

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