Issue 9, August 24, 2020

Tar Spots on Maple

Tar spots are now evident on many maple species. I recently traveled to northern Illinois where nearly every Norway maple that I came across had tar spot symptoms. Fungi in the genus Rhytisma cause this disease.

Tar spot on Norway maple

Tar spots are appropriately named for the raised, black spots that develop on the upper surfaces of affected leaves, which resemble splattered tar. The symptoms initially appear in mid-June as small, pale yellow spots. By mid-July, the yellow spots expand and a thick, raised, black stromata starts to form within the spot. Then, by late summer, the affected leaves develop the characteristic tar spot symptoms. When severe, the disease may cause some premature defoliation. Fortunately, injury resulting from tar spot infections is mostly aesthetic and rarely affects the host tree's overall health.

Tar spot on Silver Maple

Tar spot outbreaks have been more frequent in recent years, likely due to moist spring weather with above average rainfall. Trees that are damaged on an annual basis tend to be located in moist, sheltered sites that provide an ideal environment for the pathogen. Tar spot fungi overwinter on infected leaf debris. In the spring, overwintering fungal fruiting bodies ripen and eject spores. Wind then carries the spores to nearby developing leaves of susceptible hosts where the infection occurs.

Disease management practices are rarely warranted. When necessary, the first step is to rake and destroy leaf debris in the fall or early spring. This practice will help reduce spores capable of causing new infections. Fungicides containing the active ingredient Mancozeb (Fore 80 WP or Protect DF) or Copper Hydroxide (CuPro 500) can be used to protect newly developing leaves from infection. Begin sprays when the leaf buds are opening and re-apply twice more at 10-day intervals. Results from tar spot research on Norway maples in Canada suggested that one fungicide application, just prior to full leaf expansion, may provide sufficient control for this disease on Norway maples.

Bergdahl, aaron D., and Hill, alison, tech. coords. 2016. Diseases of Trees in the
Great Plains. gen. Tech. Rep. RMRS-gTR-335. Fort Collins, CO: U.S. Department of agriculture, Forest Service, Rocky Mountain Research Station.

Hsiang, T.; Tian, X.L. 2007. Sporulation and identity of tar spot of maple in Canada. Acta Silvatica & Lignaria Hungary. Special Edition: 71–74.

Travis Cleveland

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