UW Plant Disease Facts

Tar Spot

Authors: Brian Hudelson, UW-Madison Plant Pathology
Last Revised: 12/28/2011
D-number: XHT1126

What is tar spot? Tar spot is a common, visually distinctive and primarily cosmetic fungal leaf spot disease. While tar spot can affect many species of maple including big leaf, mountain, red, Rocky mountain, sugar, and sycamore maple, in Wisconsin, this disease most commonly affects silver maple. Boxelder (also known as ash-leaved maple), willow, holly and tulip-tree can also be affected by tar spot.

Symptoms of tar spot on silver maple leaves.
Symptoms of tar spot on silver maple leaves.

What does tar spot look like? Initial symptoms of tar spot are small (approximately 18 inch) yellowish spots that form on infected leaves. These spots may remain relatively small, or may enlarge over the growing season to roughly 34 inch in diameter. As tar spot progresses, the center of the infected area becomes raised and turns black. This black area resembles a blob of tar on the leaf surface. Careful examination of the tar-like areas reveals convoluted line patterns that resemble fingerprints.

Where does tar spot come from? Several fungi in the genus Rhytisma (most commonly Rhytisma acerinum and Rhytisma punctatum) cause tar spot. These fungi commonly survive in leaf litter where they produce spores that lead to leaf infections.

How do I save a tree with tar spot? DO NOT panic. For most maples and other susceptible plants, tar spot is not a serious disease. It is primarily a cosmetic disease that makes the tree look a little ragged, but does not kill the tree or shrub, nor even cause serious defoliation. Fungicides containing copper and mancozeb are labeled for tar spot control in Wisconsin. However, fungicide treatments for this disease are rarely, if ever, warranted. Consult with your county UW-Extension horticulture professional to determine if your tree warrants treatment. If warranted, three fungicide applications will be necessary for control: one at bud break, one when leaves are half expanded, and one when leaves are fully expanded. Be sure to read and follow all label instructions of the fungicide that you select to ensure that you use the fungicide in the safest and most effective manner possible.

How do I avoid problems with tar spot in the future? You can reduce or even eliminate tar spot (and thus the need for fungicide treatments), by simply removing fallen, infected leaves from around your trees each fall. Infected leaves should be buried or composted.

For more information on tar spot: Contact your county Extension agent.

Thanks to Lis Friemoth, Ann Joy and Mike Maddox for reviewing this document.

This Fact Sheet is also available in PDF format:

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An EEO/Affirmative Action employer, University of Wisconsin-Madison Division of Extension provides equal opportunities in employment and programming, including Title IX and ADA requirements. This document can be provided in an alternative format by calling Brian Hudelson at (608) 262-2863 (711 for Wisconsin Relay).

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Thanks to Lis Friemoth, Ann Joy and Mike Maddox for reviewing this document.

A complete inventory of UW Plant Disease Facts is available at the University of Wisconsin-Madison Plant Disease Diagnostics Clinic website: https://pddc.wisc.edu.