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Tar Spot (fungus)

Tar Spot (fungus)
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The symptoms of tar spot are small about 1∕8 inch yellowish spots that form on leaves. These spots may remain small, or may enlarge over the growing season to roughly 3∕4 inch in size. As tar spot progresses, the center of the infected area becomes raised and turns black.

Tar Spot gets its name from the shiny black lesions it creates on infected foliage. The disease is caused from the fungus Rhytisma and can be found on maple trees, but can affect sycamores as well. Even though tar spot is unsightly and will cause premature leaf drop, the disease rarely impacts the long term vigor of the tree.

Infections can occur in early spring as new leaves are expanding. However, the characteristic tar-like spots normally don’t arise until mid to late summer. The first symptoms are light green to yellowish green spots which enlarge and intensify in color as the season progresses. Spots grow in diameter and thickness until by late in the season, they look like a spot of tar.

There are no fungicides available to the home gardener for the control of tar spot. The most effective management treatment is to implement good sanitation practices. Rake and remove fallen leaves in the fall to prevent fruiting bodies of rhytisma to form over the winter which can then release spores in the spring to re-infect new foliage. Do not rake leaves into the street; bag and leave them for curbside collection.