TAR SPOT OF MAPLE
Rhytisma acerinum and Rhytisma punctatum
Tar Spot can be found throughout the natural range of maple trees, but is most common in the eastern part of the United States. Although Tar Spot has a minimal effect on the health of a maple tree its appearance causes a certain amount of anxiety to homeowners. The disease can be prevented or reduced with fungicide treatments, but most arborists will assure you that the impact to your maple tree is mostly cosmetic and in many cases will not appear on the tree every year.
TREE INFECTED BY TAR SPOT
Tar Spot can infect the leaves of most maple species including Bigleaf Maple, Mountain Maple, Norway Maple, Red Maple, Rocky Mountain Maple, Silver Maple, Sugar Maple, Sycamore Maple, and Boxelder (which is maple species known as Ash-Leaf Maple). Tar Spot can also infect Tulip-tree and Willow trees.
SIGNS & SYMPTOMS OF TAR SPOT
The first signs of Tar Spot are small yellowish spots that appear soon after the leaves start forming in the spring. As the summer progresses many of these spots will expand and grow in a circular pattern. Eventually, the circular spot turns black and looks like a drop of tar on surface of the maple leaf.
In late summer a certain percentage of maple leaves will curl, turn brown, and fall from the tree prematurely. A high level of Tar Spot can reduce the impact of fall color due to the early leaf loss caused by the disease.
HOW TO CONTROL TAR SPOT
In most cases trying to control Tar Spot isn’t necessary. On many trees infection only occurs when there is exceptionally cool, wet weather during maple leaf development in the spring. For high value trees, or trees that are infected every year, a combination of cultural control and fungicide controls should be implemented.
Cultural Control of Tar Spot
The spores that cause Tar Spot will overwinter on fallen leaves. Thoroughly raking up infected leaves can significantly reduce the amount of innoculum around your maple tree. Ideally, this is done in the fall, but can be done in the early spring before maple trees start leafing out. Infected maple leaves can be buried, burned, or composted.
Fungicide Control of Tar Spot
Although raking and destroying infected maple leaves will reduce a good percentage of local innoculum, it’s still possible spores can blow in from surrounding yards.
If a fungicide program is implemented to control Tar Spot then two or three treatments should be applied. Several fungicides are labeled for control of Tar Spot. A typical three-treatment program could have one treatment done at bud break, a second Tar Spot treatment when the leaves are half expanded, and the final treatment when the leaves are fully expanded.