The tar spots are due to the fungus ''Rhytisma acerinum" which survives the winter on fallen leaves. In the spring the spores of the fungus, blown by the wind, infect newly developed leaves. The fungus affects trees, especially maple trees, and appears to be sensitive to and impacted by the level of air pollution. The level of fungal infection is reflected in the number and size of tar spots on the leaves. Tar spots rarely threaten the overall health of the tree.
The concern from a human perspective is that fungal infections, especially those where the spores are carried in the air, can impact a person's health if they have a compromised immune system or if they are susceptible to respiratory conditions. Common systems associated with fungal infections include:
- increased inflammation
- chest congestion
- increased risk of chronic infections, especially chronic respiratory infections.
- muscle pain and weakness
Guidelines for cleaning up leaves with tar spots:
- The best time to clean up your leaves is after a rain. When the leaves are wet the spores are less likely to be air borne.
- If the leaves are dry, it is best to water them down before cleaning them up.
- If you are susceptible to respiratory infections, wear a mask when cleaning up the leaves and shower after you're done.
- Vacuuming the leaves into an enclosed bag may decrease your exposure.
- If you have a known respiratory infection it is best to have someone help clean up your leaves.
- Wear garden gloves when cleaning up the leaves. After you are done either discard the gloves or wash them in hot water.
Avoid the following as they increase the mobilization of fungal spores:
- Blowing the leaves
- Mulching leaves
- Raking dry leaves
- Cleaning up your leaves on a dry, windy day
Prevention is a key strategy of naturopathic medicine. There is no direct research, that I have found, that looks at the link between tar spots and the health of individuals. There are some that show a link between horses that have grazed on leaves with tar spots and muscle weakness. As I receive additional information on the health impact of these tar spots I will update this blog, but in the meantime, it is better to be safe than sorry.
To learn more about fungal infections: