Tar spot of maple

Tar spot of maple is a common fungal disease caused by the ascomycete fungus Rhytisma punctatum. This disease is widely recognized in urban and forest landscapes. Unlike its close relative Rhytisma acerinum, which causes large, singular tar-like blotches, Rhytisma punctatum produces smaller, numerous spots, giving the maple leaves a distinct speckled appearance.

The disease primarily affects various maple species, including Norway maple and field maple. The fungus specifically attacks the foliage, causing aesthetic damage rather than systemic harm to the tree's vascular system. It is prevalent in regions with high humidity and moderate temperatures, which are essential for the survival and dispersal of the fungal spores during the growing season.

Symptoms typically manifest in mid-summer. Initially, small, pale yellow chlorotic spots appear on the upper surface of the leaves. As the season progresses, these spots develop small, raised, shiny black structures. These black spots are often surrounded by a persistent yellow halo, making the infection easily identifiable even from a distance.

The development of the fungus is strictly dependent on environmental factors. The pathogen overwinters in the fallen, infected leaves on the ground. In the spring, when temperatures rise and humidity is high, the fungus releases ascospores that are wind-borne to the newly emerging leaves. Dense canopies with poor air circulation significantly increase the risk of secondary infections throughout the summer.

While the disease is rarely lethal, its impact on tree health should not be ignored. Severe infection can lead to premature leaf drop, which reduces the tree's photosynthetic capacity and weakens it over several consecutive years. To effectively protect and manage the disease, consider the following practices:

• Sanitation: Raking and destroying all fallen leaves in autumn to eliminate the primary inoculum source.
• Pruning: Truning trees to improve airflow within the canopy, which helps dry out the leaves faster after rain.
• Fungicides: Applying copper-based sprays in early spring if the disease history is significant in the area.
• Monitoring: Assessing tree health and ensuring proper site conditions to prevent water-logging and high humidity.