Tar Spot of Norway Maple

Tar spot of Norway Maple is caused by an invasive fungus (Rhytisma acerinum) that was introduced to North America from Europe. This disease was first reported in New York by Hudler et al. 1987. Tar spot can be found on all cultivars of Norway maple including Olmstead, Emerald Queen, and Crimson King.

Typically, infection occurs in mid to late spring when temperature and moisture begins to rise. The infection appears as black masses called stroma on the leaves. After overwintering, stroma on fallen leaves from the previous year will open up to reveal an apothecium containing asci which discharge ascospores that can infect new leaves.

When infection gets severe in late summer, several spots coalesce on a single leave. These spots cause the leaves to die and fall off, leading to defoliation of the tree. In most cases, the infection is only cosmetic and will not cause any significant harm to the tree. Repeated infection over several years may reduce tree vigor.

Current best practice for management of this disease is good sanitation of plant debris i.e. to clean up, remove, and destroy infected leaves that have fallen to the ground during the autumn. Good sanitation ensures that infection does not return the following spring. In some cases fungicides can be used if infection is severe on several trees.