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They are called tar spots and are caused by a microscopic fungus that develops around the end of spring or early summer when the weather is particularly damp. As the leaves reach their full size, the spots blacken and look like tar, explaining the origin of the name of this infection. The leaves affected dry up and fall to the ground prematurely in late summer. If the infected leaves remain on the ground, new spores will develop the following spring and they will spread to the young leaves of the maple tree, causing new infections. That is how the infection cycle can continue from one year to the next.
To reduce the impact of this disease, it is important to pick up the infected fallen leaves in the fall.