Laboratory Services Division, University of Guelph
95 Stone Rd. West
Guelph, Ontario N1H 8J7
Phone: (519) 767-6256 Fax: (519) 767-6240
E-mail: pdc@lsd.uoguelph.ca
TAR SPOT OF MAPLE
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.
Control
To reduce the amount of diseases overwintering, rake up fallen leaves
in the autumn and destroy or remove them from the yard.
W.A. Attwater
PDCF-023 (Revised 2003/01/14)