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.
Signs and symptoms of tar spots
Early infections appear as yellowish spots on the upper leaf surface. Later in the summer, black tar-like spots form and may be one-half inch in diameter. These black spots have a ridged appearance if examined closely. The undersides of the leaves appear cupped directly beneath the tar spots.
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.