Controlling Tar Spot in Birmingham

Tar spot is a fungal disease that affects maple trees throughout Birmingham, causing distinctive black spots on the leaves. It is caused by fungi in the genus Rhytisma, with the most common species being Rhytisma acerinum and Rhytisma punctatum. The disease is primarily cosmetic and rarely threatens the overall health of infected trees. The disturbing aspect of having a tree infected with tar spot is that the leaves become discolored and start falling off in August. With less leaves on the tree the fall color display is also affected.

Symptoms and Development of Tar Spot

• Initial symptoms appear in late spring or early summer as light green or yellow-green spots on leaves.
• By mid to late summer, these spots develop into raised, black, tar-like structures on the upper leaf surface.
• Spots can range from 0.5 to 2 cm in diameter for Rhytisma acerinum, while Rhytisma punctatum causes smaller, punctate spots about 1 mm in diameter.
• In severe cases, spots may coalesce to form larger black masses up to 1½ inches in diameter.

Impact and Control of Tar Spot

While tar spot can cause premature defoliation, it generally does not significantly harm the overall health of trees in Birmingham.

Management strategies include:

1. Raking and removing fallen leaves in autumn to reduce overwintering fungal spores.
2. Mulching leaves, ensuring the mulch pile is covered or turned before spring to destroy spores.
3. Applying fungicide applications, as the leaves are developing in the spring. Two or three fungicide treatments is usually adequate to control tar spot.

Tar spot is a common occurrence in many areas of Michigan, including Birmingham, and affects various maple species such as Norway maple, red maple, silver maple, and sugar maples.