Tar spot of maple

Tar spot of maple is a common fungal disease caused by the ascomycete fungus Rhytisma acerinum. It is widely recognized by the distinct, raised black spots that appear on the leaves of maple trees, particularly species such as Acer platanoides and Acer pseudoplatanus.

The symptoms typically begin in early summer as small, pale yellow-green spots on the upper leaf surface. By late summer and early autumn, these spots expand and develop into shiny, tar-like black crusts. These structures, known as stromata, are where the fungus produces its spores.

The disease cycle is heavily influenced by environmental conditions, particularly high humidity. The fungus overwinters in the leaf litter on the ground. In the following spring, mature spores are discharged from the overwintered leaves, spreading via wind or rain to infect the newly emerging foliage.

While tar spot is generally considered more of a cosmetic issue than a fatal one, heavy infections can lead to premature leaf drop. This premature defoliation can stress the tree, potentially reducing its overall vigor if the infection persists over several consecutive years.

Management of tar spot focuses on breaking the disease cycle through cultural practices rather than reliance on chemical fungicides. Effective control measures include:

• Raking and disposing of all infected fallen leaves in the autumn to remove the primary inoculum source.
• Deep burial of leaves if composting is not an option, to prevent spore release in the spring.
• Maintaining optimal tree health through proper watering and fertilization.
• Thinning out the tree canopy to promote air circulation and decrease leaf wetness duration.