Rockinon: the blog. Lately, I've been noticing these tar spots on fallen maple leaves in London and so I thought a reprint was in order.
A lot of the information came directly from a report from Guelph University by W.A. Attwater. I think it is best to ensure accuracy and so I am leaving this report essentially untouched.
Whenever I saw round, black dots on maple leaves, I used to wonder what the black dots were and what measures should we be taking to protect our trees. If you have thought the same thing, read on.
These distinctive round to irregular black, spots on infected maple leaves are known as tar spots. Not noticeable until late summer, tar spots are caused by two species of Rhytisma fungus.
The first, 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 fungal tissue called stroma. Red, silver, Norway (including the varieties with red leaves) sugar and Manitoba maples as well as others are affected.
Both fungi 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 develop within these spots in mid to late summer. Severely infected leaves may be shed.
Although tar spots are conspicuous, they are seldom so injurious in home gardens to justify spraying with a fungicide. As the strong visual appearance develops late in the growing season, the overall health of trees is rarely affected.
To reduce the amount of disease overwintering, rake up fallen leaves in the autumn and destroy or remove them from the yard.