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 and tulip-tree can also be affected by tar spot.
What does tar spot look like?
Initial symptoms of tar spot are small (approximately 1∕8 inch) yellowish spots that form on infected leaves. These spots may remain relatively small, or may enlarge over the growing season to roughly 3∕4 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?
Don’t panic! For most maples, 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, nor even cause serious defoliation. Some copper-containing fungicides are labeled for tar spot control. However, fungicide treatments for tar spot 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 are 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 insure 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 the base of your trees each fall. Infected leaves can be burned, buried or even composted.
If you think you might have Tar Spot, bring a sample to our office and we can help identify and treat the problem.
By: Brian Hudelson, UW-Madison Plant Pathology