Tar Spot
Rhytisma americanum; Rhytisma acerinum; Rhytisma punctatum
A common, very visible but harmless leaf disease of maples (including sugar maple) and a few other trees, in which the fungus forms large, raised, shiny black spots that look like drops of tar on the leaves. It alarms gardeners every late summer but does not hurt the health of the tree.
🔎 How to spot it
Spots begin in early summer as small, pale yellow-green flecks on the leaves that are easy to miss. Through the summer they enlarge and a black, raised, tar-like crust builds up in the center, ringed by the yellow halo. By late summer the spots can be small and clustered (on Norway maple) or single and large, up to about three-quarters of an inch and visibly raised with a fingerprint-like surface (on silver maple). Besides maples it also turns up on boxelder, willow, holly, and tulip-tree.
🥀 Damage it causes
The damage is almost entirely cosmetic. Tar spot does not kill trees or shrubs and does not usually cause meaningful defoliation, though a heavy year can make leaves look spotty and drop a little early. It is a disease of appearance, not of tree health.
🔬 What causes it
Fungi in the genus Rhytisma, mainly Rhytisma americanum and Rhytisma acerinum and less often Rhytisma punctatum. The fungi overwinter in fallen infected leaves on the ground and produce spores in spring that blow up onto the new leaves and start fresh infections. Wet springs and lots of leaf litter left under the tree make for more spots the next year.
🛡️ Prevent it
Because it is harmless, control is optional and is all about sanitation: rake up and remove or destroy the fallen leaves each autumn to take away the overwintering spores, so fewer infections start the next spring. Burning, burying, or hot composting the leaves works; ordinary cool home compost does not get hot enough to kill the fungus.
🧯 If it is already here
No treatment is needed for the health of the tree. Fungicides are rarely warranted and are impractical on a large tree, so raking and removing infected leaves in fall is the only sensible step, and even that is for looks more than for the trees sake.
💡 Good to know
The big, shiny, raised black spots with yellow halos are unmistakable and, despite how dramatic they look, are not a threat to the tree. Cleaning up fallen leaves in autumn is the simplest way to cut down on the spots the following year.
🌱 Plants it affects
4 plants in the library can be affected by this problem
For educational and informational purposes only. Disease management advice is general guidance drawn from university cooperative extension sources; always identify a problem positively and read and follow the label on any product before use, especially around food crops, children, and pets.