During a recent tour of the Ford House latest outdoor exhibit, Vanishing Acts: Trees Under Threat (open during normal Ford House hours from 9 a.m. to 5 p.m. Admission to the exhibit is included with regular admission), a question was posed to me, “What are the black spots on my trees?” I responded by picking up a silver maple leaf (Acer saccharinum) and said, “Is this the problem?” The leaf had several dime-sized black spots all over it.
The disease is called tar spot, which is a fungus in the genus Rhytisma. This disease is purely cosmetic and doesn’t cause permanent damage to the tree. Occasionally, the disease will cause early defoliation of the tree. Tar spot and other fungi have been ever-present in nature for many years. The right conditions such as a susceptible host and proper weather can infect a tree.
The spots reveal themselves late in the growing season as the tree is preparing for winter. There is no appropriate control measure this late in the season. Tar spot can be controlled with a fungicide, but timing is everything. In my opinion, it’s not worth the money to try to prevent it. If you are concerned with the cosmetic appearance of the leaves, you may mulch the leaves in the fall to disrupt the disease cycle. Keep in mind that this will not eliminate the disease, as there are other sources (i.e. silver maple) throughout our community that can play host to tar spot.
I do not support applying fungicides to control this cosmetic issue. Instead, I encourage you to find the beauty of these spots against the beautiful fall color of the leaves.