What you’ll see… Initial symptoms of tar spot are small (approximately 1/8 inch) yellowish spots that form on infected leaves. These spots may remain relatively small, or may enlarge over the growing season to roughly 3/4 inch in diameter. As tar spot progresses, the center of the infected area becomes raised and turns black. This black area resembles a blob of tar on the leaf surface. Careful examination of the tar-like areas will reveal convoluted line patterns that resemble fingerprints.
If you see very distinct yellow spots on your Maple leaves in late spring that later turn black, contact us now!
The reason for the problem… Several fungi in the genus Rhytisma (most commonly Rhytisma acerinum and Rhytisma punctatum) cause tar spot. These fungi commonly survive in leaf litter where they produce spores that lead to leaf infections.
You should… not panic. You can reduce tar spot by simply removing fallen and infected leaves from around the base of your trees each fall. Infected leaves can be burned, buried or even composted. For most Maples, Tar Spot is not a serious disease. It is primarily a cosmetic disease that makes the tree look a little ragged, but does not kill the tree.
We can help by…applying a fungicide in the spring to help sterilize the spores. If spring has already passed we can still apply a fungicide to minimize the disease threshhold especially when forecasts predict cool and wet summer conditions.