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Common Name: Tar Spot
Scientific Name: Rhytisma acerinum
Type: Fungus
Principle Host Trees & Shrubs:
Sycamore and other Maples (Acer species).
The fungus overwinters on fallen leaves. Fresh spores are produced in Spring which have a sticky coating. The spores attach themselves to the new leaves as they form. A raised, shiny black spot appears on infected leaves. Can cause premature leaf drop but the vigour of the tree is rarely affected. Rarer in cities due probably to fewer fallen leaves left under trees.
Control is generally unnecessary. If a problem on ornamental species dispose of fallen leaves in Autumn to reduce spore levels in Spring.
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 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.
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.
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.
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