Tar Spots

Now that we've reached late autumn, many of the leaves on our trees have dropped off and are scattered across the lawns. This makes the distinctive "tar spots" of Rhytisma acerinum a whole lot easier to spot. The fungus is common on maples and sycamores across Europe; a very similar and closely-related species occurs in North America. Fortunately for the infected trees, the fungus is a cosmetic problem which impacts only the leaves and seldom harms the tree itself. Next spring, when the weather warms up again, sticky spores released from these "tar spots" will travel on the wind and infect new leaves. While it doesn't generally harm trees or humans, the fungus isn't entirely benign. It has been associated with "equine atypical myopathy" and can cause death in horses if they consume too many infected leaves.