Maple tar spot

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.    

Symptoms of tar spot are now becoming more evident on maple foliage. I am receiving samples and numerous phone calls from folks concerned and down right angry about this disease. Some are very relieved to hear that the disease does not generally cause significant harm to well established trees. One client in particular, who called in frustration, was not consoled by this fact, because his wife was complaining to him that it made the tree ugly. Tar spot certainly does affect the aesthetics of the affected maple tree.

Tar spot gets it name from the shiny black, tar-looking lesions it creates on the leaves of several maple species. Several species of Rhytisma cause tar spot. Susceptible maples include Bigleaf, Mountain, Red, Rocky Mountain, Silver and Sugar. The maples do vary in their susceptibility to infection.

Tissue is infected in the late spring and early summer after the leaves are fully expanded. Initially the infected tissue turns light green or yellowish green. At this stage, the disease can not be easily diagnosed. The pathogen grows slowly in culture; meanwhile more diagnostic symptoms will develop on the host. The diagnostic symptoms include the development of black, shiny, tar-like tissue on the surface of the foliage.

Plant pathologist used to say that significant outbreaks of tar spot occurred infrequently, however, over the past few years this has not been the case. For reasons unknown, tar spot has become a yearly occurrence and with seemingly increased severity. Generally, severely infected trees are in moist, sheltered locations which allow the pathogen to easily survive the winter. This disease is usually a cosmetic problem and does not affect the long-term health of the tree. The recommended fungicide applications for tar spot are at bud break and twice thereafter at 7- to 14-day intervals. Once the tree has leafed out, applications are ineffective. Fungicides recommended for disease prevention include triadimefon, mancozeb or Junction.

Reference: Sinclair, W. and Lyon, H. 2005 Diseases of Trees and Shrubs 2nd edition. Comstock Publishing Associates, Ithaca, NY.

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