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“Tar Spot? Not a Problem!
This year we have been seeing what looks like drops of tar on maple and some other trees. What we are finding is “tar spot” – a common fungal leaf spot disease that is not pretty to look at but is only a cosmetic problem. Although tar spot can affect several species of maple, silver maple Acer saccharinum is most commonly affected. Boxelder (also a maple – Acer negundo), willow (Salix spp.) and tulip-tree (Lirodendron tulipifera) are also by tar spot.
Initial symptoms of tar spot are small, commonly yellowish spots that form on infected leaves. These spots may remain small, or may enlarge over the growing season to roughly 3⁄4 inch in diameter. The center of the infected area becomes raised and turns black and looks like a spot of tar on the leaf surface. The tarred areas actually look like fingerprints close up.
Several fungi in the genus Rhytisma (usually Rhytisma acerinum and Rhytisma punctatum) are the culprits that cause tar spot. They survive in fallen leaves where they produce spores that lead to leaf infections.
For most maples, tar spot is not a serious disease. It is primarily a cosmetic disease that but does not kill the tree and rarely causes serious leaf loss. Some fungicides are labeled for tar spot control, however, fungicide treatments are rarely, if ever, warranted.
So unless you are a horticulture fashionista who worries about every little blemish on your tree, don’t think twice about tar spot. You can reduce tar spot by removing infected leaves from around the base of affected trees when the leaves drop in fall. Burn or compost the leaves to get rid of the fungus that will send out spores for re-infection – and your silver maple should have a clear complexion next year.
Direct comments or questions concerning this column to Marilyn Odneal via email at MarilynOdneal@missouristate.edu; write to Missouri State Fruit Experiment Station, 9740 Red Spring Road, Mountain Grove, Mo. 65711; or call (417) 547-7500. Visit our Web site at http://mtngrv.missouristate.edu.