Answer: Tar spot is a very distinctive disease of maples. It causes round to irregular black, tar-like spots on infected leaves. These spots are not noticeable until late summer. Tar spots of maple are caused by two species of the fungus Rhytisma which produce different symptoms. The first species, Rhytisma acerinum, produces black, tar-like spots about 1.25 cm or more in diameter on the upper surface of infected leaves. The second species, R. punctatum, produces patches of small, 1mm wide spots and is often called speckled tar spot. The thickened black spots are actually fungal tissue called stroma. Red, silver, Norway (including the varieties with red leaves) sugar and Manitoba maples as well as others are affected by tarspot diseases.
Both fungi, R acerinum and R. punctatum survive between seasons on the fallen diseased leaves. In the spring, spores are produced within the black stroma and are carried by air currents to young maple leaves where they start new infections. Unlike many other foliar diseases, Rhytisma spp. do not continue to cause new infections throughout the summer. Infections first show up as yellow or pale green spots on the leaf surface in the early spring or summer. The black, raised tar-like spots of R. punctatum and R. acerinum develop within these spots by mid to late summer. Leaves, which are severely infected, may be shed.
Although the spots are conspicuous and detract from the appearance of the leaves, tar spot seldom is so injurious in home gardens to justify spraying with a fungicide. As symptoms develop late in the growing season, the overall health of trees is usually not affected.
To reduce the amount of diseases overwintering, rake up fallen leaves in the autumn and destroy or remove them from the yard.
Hope this answers all your questions about the black spots on your maple leaves.
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