Maple tar spot
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EmailEditor’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.
