Maple Tar Spot

Need to know

• In early infections yellowish spots appear on upper leaf, later during infection black tar-like, ridged spots appear.
• Transmission occurs in spring by mature spores being released and blown by wind to newly emerging leaves.
• Tar spots do not cause serious harm to established trees, but raking and removal of fallen leaves prevent transmission.

Overview of tar spots

Tar spot is a fungal disease that doesn't require a fancy microscope for diagnosis. As you might guess, the disease is characterized by raised, black spots on leaves. Tar spot occurs primarily on silver maple.

Symptoms of tar spots   

Symptoms of maple tar spot begin with small greenish-yellow spots that begin to form on the upper leaf surface. As the season progresses the spots grow becoming black and tarlike surrounded by a yellow chlorotic area. These black tar-like spots form and may be one-half inch in diameter and have a ridged appearance if examined closely. The opposite side of the leave will show slightly lighter black spots that are cupped. 

Signs of tar spots

Black fungal structures (called stromata) on leaves, inner fungal body, and spores can only be seen with a very thin cut and under higher magnification

Disease cycle of tar spots

Tar spot is caused by the fungus Rhytisma acerinum. The tar-like spot is a fruiting structure of the fungus that survives the winter on fallen leaves. In the spring, mature spores of the fungus are released and blown by wind to newly emerging leaves.

Type of Sample Needed for Diagnosis and Confirmation

The Iowa State University Plant & Insect Diagnostic Clinic can help you to investigate and confirm if you plant has this disease. Please see our website for current forms, fees, and instructions on collecting and packing samples. Contact information for each states diagnostic laboratory for U.S. residents.  If your sample is from outside of Iowa please do not submit it to the Plant & Insect Diagnostic Clinic without contacting us

Management of tar spots

Fortunately, tar spot does not cause serious harm to established trees. Some early leaf drop may occur. Raking and removing fallen leaves can help to destroy overwintering fungal inoculum. Although fungicides can be applied in the spring to protect newly emerging leaves, their use is seldom warranted.

Fungicide applications may be avoided by following good Integrated Pest Management practices like those listed in this encyclopedia article. Often, the only preventative application is effective to manage plant diseases. If the problem requires a fungicide, state law requires the user to read and follow all labels accordingly. For more information, read Proper fungicide use.