When to control tar spots on maples

Jan Byrne<byrnejm@msu.edu>, MSU Diagnostic Services, Department of Plant Pathology - August 26, 2011

Although tar spot symptoms are readily visible now, pause for a moment to consider the overall impact on tree health and appropriate chemical management.

Signs and symptoms of tar spot are becoming more noticeable on maples in the landscape. Initially, the lesions are yellowish in appearance, and by the end of the season they take on a shiny, black appearance, hence the name “tar spot.” The appearance of the spots is alarming to some. In some cases, severe disease will result in premature leaf drop. However, the health of large, well-established trees that are otherwise healthy will not be significantly impacted.

For those interested in chemical control recommendations, now is not the time. The lesions visible now are a result of infections that occurred earlier in the growing season. Fungicide applications made now will not limit disease severity. Fungicide recommendations are limited to spring applications of maples in commercial production, those with particular aesthetic importance, or perhaps those that were recently transplanted. Furthermore, regardless of the timing, it is typically not practical to treat large landscape trees with foliar fungicide applications.

For more details on this disease please see the MSU Extension News for Agriculture article, Why do my maple leaves have spots?!

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